Relay and how to use term

Share |

Elliott Sound Products Relays & How To Use Them - Part 1

Relays, Selection & Usage (Part 1)Rod Elliott (ESP)December 2014

Contents

IntroductionRelay BasicsContactsRelay 'Forms'Relay Coils

AC Relay CoilsRelay Drive Circuits

Relay LogicPull-In And Release Voltages

Relay 'Efficiency' or 'Economy' CircuitsReed RelaysLatching RelaysSemiconductor (Solid State) RelaysMiscellaneous Relay Info & Circuits

Adhesives And RelaysCleaning Relay ContactsRelay Current DetectorRelay Polarity ProtectionOther Solenoid Actuators

ConclusionsReferencesPart 2 - Contacts, Arcing & Arc Suppression

Introduction

Relays (and in particular the electro-mechanical types) might seem so-o-o last century, but thereare countless places where it simply doesn't make sense to even consider anything else. Althoughone could be forgiven for thinking that there must be a better way to switch things on and off, inmany cases a relay is the simplest, cheapest and most reliable way to do it. Relays are electro-mechanical devices, in which an electromagnet is used to attract a moveable piece of steel (thearmature), which activates one or more sets of contacts. The relay as we know it was invented byJoseph Henry in 1835.

This article mainly covers 'conventional' (i.e. electro-mechanical) relays, but there are also severaldifferent types of solid-state relays. We'll look at some of those later, but very few are suitable for

Celem power capacitorWater/Conduction cooled capacitors forHF and MF induction heating

Relays http://sound.westhost.com/articles/relays.htm

1 of 35 5/21/2015 9:09 PM

use in audio circuits. Some shouldn't even be used to turn on transformers, even though theirspecifications may lead you to think that they would be ideal.

Relays are not well understood by many DIY people, and there are many misconceptions. Thepurpose of this article is to give a primer - what the Americans might call "Relays 101". It's notpossible (or necessary) to describe every different relay type, because they all operate in a similarmanner and have more points of similarity than differences. Relays are used in nearly allautomation systems, both for industrial controllers and in home automation systems. One of theirgreat benefits is that when off, no power is drawn by the relay itself or the load. There is virtually no'leakage' current via the contacts, and the insulation materials will normally have a resistanceseveral gigohms (G).Many websites discuss relays, but the intention here is not just to provide a primer, but to look atideas that will be new to many, and possible pitfalls as well. There are places where relays are usedwhere you might expect them to last forever, but they don't. Since relays are normally so reliable,we need to examine the things that can go wrong, and learn how to specify a relay for what weneed to do.

There are thousands of different relays on the market. They range from miniature PCB mountingtypes intended for switching signal or other low voltage signals, up to very large industrial types thatare used to start big electric motors and other industrial loads. These are usually referred to as'contactors', but that's nothing more than a different name for a really big relay.

Being electro-mechanical devices, this means that there are both electrical and mechanicalcomponents within a relay. The electrical part (not counting the contacts) is the actuating coil, whichis an electromagnet. When current passes through the coil winding, a magnetic field is createdwhich attracts the armature (i.e. a solenoid). Provided there is enough current (known as the pull-inor 'must operate' current), the armature will be pulled from its rest position so that it makes contactwith the remainder of the magnetic circuit. In so doing, the relay contacts change from their 'normal','rest' or 'reset' position to the activated or 'set' position.

A single electromagnet can activate several sets of contacts, but in most relays the number isgenerally no more than four sets. More may cause problems, because the armature will have to beable to move too many parts, so the return spring needs to be more powerful as does theelectromagnet. The contact alignment also becomes critical, to ensure that every set of contactsopens and closes and has sufficient clearance for the intended voltage. Some of the things thatmake relays so popular are ...

Isolation between coil and contacts allows low voltage circuits to safely control mains powerRelays are easily driven by microcontrollers, and at most need a transistor, a resistor and adiode as 'support' componentsA small coil current can control a very much larger current through the contactsMany different types available, some of which don't even need power to 'remember' a settingThere's a relay made for just about every need in electrical or electronic engineeringRelays are (usually) incredibly reliable, and many are rated for 1,000,000 cycles (but will oftenlast even longer)

It should be noted that automotive relays are a special case, are specifically designed for use withlow voltage (12 or 24V) use, and one end of the coil is often connected to internal parts of the relay.Automotive relays must never be used with mains voltages, or where there is a significant voltagedifference between the coil and contacts. The insulation is not rated for high voltages, even if the


2 of 35 5/21/2015 9:09 PM

coil is not connected to anything internally. Most also draw significantly more coil current (typically200mA or more) than 'general purpose' types (40-50mA). However, automotive relays are alsorated to handle up to 150A or more at 12V DC.

It's quite easy for a microcontroller to activate a small relay, which activates a bigger relay, which inturn activates a contactor to power a large motor in an industrial process. This can be thought of asa crude form of amplification, where a very small current may ultimately result in a huge machinestarting or shutting down. There's even something called 'relay logic', where relays are literally usedto implement logic functions (see Relay Logic for a bit more info on this seemingly odd usage).

The references have more information and for some very detailed explanations, reference [ 1 ] isworth a read.

Relay Basics

The essential parts of a simplified relay are shown below. In most relays, the coil is wound on aformer (or bobbin), and is fully insulated from everything else. The coil (solenoid) along with the restof the magnetic circuit is an electromagnet. Most relay specifications will tell you how much voltageyou can have between the two sections, and it's not uncommon for relays to be rated for 2kVisolation or more. Don't expect miniature relays to withstand high voltages unless you get one that'sspecifically designed for a high isolation voltage. We'll look at this in more detail later.

The relay is shown as de-energised (A) and energised (B). The coil is usually not polarity sensitive,and can be connected either way. Be aware that there are some relays where the polarity isimportant, either because they have an in-built diode, they use a permanent magnet to increasesensitivity (uncommon), or because they are latching types. Latching relays are a special case thatwill be looked at separately. The contact assembly is made from phosphor-bronze or some similarmaterial that is both a good electrical conductor and is flexible enough to withstand a million or moreflexing (bending) movements without failure. The contacts are welded or riveted into the contactsupports/ arms and can be made from widely different materials, depending on the intended use.

The contact 'arms' are typically fastened to the body of the relay mechanism, sometimes with rivets,occasionally with screws. Each contact is separated by a layer of insulation, and the contacts areusually also insulated from the magnetic circuit (the yoke and/or armature). The separate parts ofthe contact assembly are insulated from each other. Not all relays have a physical spring to returnthe armature to the rest position. In some cases, the contact arms are designed to act as springs aswell. You will also see relays that have the moving contacts attached directly to the armature - theoctal base relay shown in Figure 2 uses this method.


3 of 35 5/21/2015 9:09 PM

Figure 1 - The Parts Of A Relay

The relay shown has contacts that are most commonly called 'SPDT', meaning single-pole, double-throw. The term 'double-throw' means that one contact is normally open ('NO') with respect to thecommon, and the other is normally closed ('NC'). The 'normal' state is with the coil de-energised.When the rated voltage is applied to the coil, enough current flows so that the armature is pulled into close the magnetic circuit, the 'NO' terminal is now connected to common, and the 'NC' terminalis open circuit.

This allows you to disconnect one signal or load of some kind, and connect a different one.Alternatively, a circuit may be operational only if the relay is de-energised, and is disconnectedwhen power is supplied to the coil. Another very common configuration is called DPDT -double-pole, double-throw. This provides two completely separate sets of contacts, with both havingnormally open and normally closed contacts. 4PDT is now easily decoded - it means 4-pole double-throw. You will also find SPST relays - a single set of (usually) normally open contacts.


4 of 35 5/21/2015 9:09 PM

Figure 2 - A Selection Of Relays

The photo shows a very, very small sample of relays, picked to show the diversity and the internalsof some typical components. There are many others, including many different styles of reed relaysas well as several intermediate sizes of conventional relays. You can see that one relay has an octalbase - exactly the same as used for many thermionic valves ('tubes' if you must). Although the relayI have shown is many years old, this style is still available, because it makes it easy to replacerelays in industrial control systems.

In fact, there are very few relays that have been discontinued. There may be changes to the contactmaterials (see below for more) and cases might change from metal to plastic, but the basic stylesand contact configurations have remained. There are so many controllers that rely on relays used inindustrial processors that replacement relays tend to be made available for an eternity compared to'consumer' goods. Relays are not an audio product - they belong to a different class of equipmentwhere failure may mean the loss of $thousands an hour.

However, it should be remembered that relays were used in early telephone systems (and beforethat, in telegraphy), so they are actually the product of the first ever branch of 'audio' and thecatalyst for most electronic equipment - the telephone. Like so many of the things we take forgranted these days, the telephone system has been the originator of a vast array of products thatare now part of almost everything we use.

Contacts

For any given relay, there are specifications that describe the maximum rated contact voltage and


5 of 35 5/21/2015 9:09 PM

current. Relays for high voltages need contacts that are further apart when open, or may beoperated in a vacuum. Those for high current need a contact assembly and contact faces that havelow resistance and can handle the current without overheating or welding the contacts. Themaximum contact ratings must never be exceeded, or the life of the relay may be seriously affected.In particular, make sure that the relay you use can handle the peak inrush current of the load.

There are many factors that influence inrush, but be aware that it can be as much as 50 times thenormal full-load current. With inductive loads (transformers and motors for example) the worst caseinrush current is limited only by the winding resistance plus the external mains wiring impedance.Note that zero-voltage switching (with solid state relays in particular) should never be used withthese loads - ever! Capacitive loads and electronic power supplies present challenges, and arealso generally not appropriate for solid state relays, but for different (and complex) reasons.

Some heavy duty relays (contactors) only have a single pair of contacts, typically normally open.There are also 3-phase contactors that have three sets of contacts - one for each phase, and theseare very common in industrial control systems. They are used to switch heavy current and/or higherthan normal voltage, and have greater contact clearance and arc suppression features so that anarc cannot be maintained across the contacts when they are open. For particularly large currents(or for DC which is a potential relay contact killer), there may be a magnet or even a forced airsystem to direct the arc away from the contact area. These are not common with normal relays.

Contact faces are made from various metals or alloys that are designed for the intended use. Somecommon materials and their applications are shown below [ 2 ]. This is not an exhaustive list, andyou may see other metals or alloys referenced in relay specifications.

Material(s) Symbol(s) CommentsHard Silver Ag, Cu, Ni A standard contact material used in many

general purpose relays, the copper and nickeladd the hardness. Single contact minimum20V/50mA. Long contact life, but tends tooxidise at higher temperatures.

Silver Nickel Ag, Ni More resistant to welding at high loads than hardsilver, with high burn out resistance. A goodstandard contact material. Minimum contactload, 20V/50mA

Silver Cadmium Oxide Ag, CdO Used for high current AC loads because it ismore resistant to welding at high switchingcurrent peaks. Material erodes evenly across thesurface. Not recommended for breaking strongDC arcs because of the wear this creates (oneside reductions). Minimum contact load20V/50mA. Note that Cadmium was originallyincluded in the list of materials prohibited underthe European RoHS Directive, but is nowexempt for this purpose (although this maychange again at any time).

Silver Tin Oxide Ag, SnO2 The tin oxide makes the material more resistantto welding at high making current peaks. It has avery high burn out resistance when switchinghigh power loads. Low material migration underDC loads. Minimum contact load 20V/50mA.Useful where very high inrush currents occur,


6 of 35 5/21/2015 9:09 PM

such as lamp loads or transformers. Silver TinOxide is frequently chosen as the replacementrelay contact material for Silver Cadmium Oxide.

Silver Tin Indium Ag, SnO, InO Similar to Silver Tin Oxide but more resistant toinrush. Minimum contact load 12V/100mA.

Tungsten W More resistant to welding at high loads than hardsilver, with high burn out resistance. A goodstandard contact material. Minimum contact load20V/50mA single contact. Used for some heavyduty relays.

Gold Plating - 10m Au Used for switching low loads > 1mA/100mV. Thisplating will be removed by friction and erosionafter around 1 million switching cycles even in'dry' circuits (i.e. those with no DC and/ornegligible AC). Used in single and twin contactforms (twin contact is useful in dustyenvironments).

Gold Plating / Flash - 3m Au Has the same qualities as 10m Au but is lessdurable. It is generally used to prevent corrosion/ oxidation of relay contacts during storage.

Ruthenium Ru A rare element that is highly resistant totarnishing, and used primarily in reed switches/relays and other wear resistant electricalcontacts.

Rhodium Rh A rare, silvery-white, hard, and chemically inerttransition metal. Like Ruthenium, it is a memberof the platinum group of elements. Used in reedswitches

Table 1 - Common Contact Materials

From the above, you'll see that some contact materials require a minimum voltage and/or current.At lower voltages and currents (such as 'dry' signal switching circuits) there isn't enough current toensure that the contacts will make a reliable closure, which may result in noise, distortion orintermittent loss of signal.

Where good contact is needed with very low voltages and currents, gold or gold plating is a goodchoice. Note that gold is not a particularly good conductor, but it has the advantage that it doesn'ttarnish easily, so there's rarely a problem with oxides that may be an insulator at normal signalvoltages. Where silver (or many of its alloys) is used, relays may be hermetically sealed to preventoxidation. The black tarnish (silver sulphide) is an insulator. It's not a good insulator, but it canwithstand a few hundred millivolts (typical signal level) with ease. Some reed relays have thecontacts in a vacuum, and this is common with high voltage types. An arc is difficult to create in avacuum because there is no gas.

A common term you will hear is "contact bounce". When the contacts close, it's more common thannot that there will be periods of connection and disconnection for anything up to a few millisecondsor so. The time depends on the mass of the contacts, the resilience of the contact arms and thecontact closing pressure. A good example is shown below, taken from the reed relay shown inFigure 2. This is significantly better than most others, but shows clearly that even the 'best' relayshave contact bounce. A certain amount of 'disturbance' can also be created when contacts open,but this is a different effect.


7 of 35 5/21/2015 9:09 PM

Figure 3 - Reed Relay Contact Bounce

The horizontal scale is 50s per division, so you can see that the contacts make and break severaltimes in the first 150s. After that, the closure is 'solid', with no further unwanted disconnections.Sometimes you can minimise bounce effects by operating two or more sets of contacts in parallel,but that's not a guaranteed reliable method. Once one could purchase a mercury-wetted relay - the'contacts' were based on a small quantity of mercury which formed an instant contact with nobounce at all. There are (were) many different types at one stage.

Mercury-wetted relays used to be common for laboratory use to obtain test waveforms withpico-second risetimes, but of course the European Union's RoHS legislation has caused them to bebanned completely. Mercury? Oh, no - you can't use that! Strangely, the EU still allows fluorescentlamps (both compact and full size) a few of which probably have as much mercury as a smalllaboratory mercury wetted relay. One gets thrown away after a few thousand (or hundred) hoursand the other will be kept forever. I'll let you guess which is which.

The vast majority of relays have break-before-make contacts. This means that one circuit isdisconnected before the other is connected. Make-before-break relays also exist, but they areuncommon and were mainly used with telephony systems where a disconnection might result in adropped phone call. If you really need make-before-break I expect that finding one that's bothavailable and sensibly priced will be a challenge.

One area where electro-mechanical relays have real problems is switching DC. A relay that canhandle 250V AC at 10A can generally be expected to handle a maximum of 30V or so with DC,because the voltage and current are continuous. With AC, both voltage and current fall to zero 100or 120 times each second (for mains frequency applications), so the arc is (comparatively) easilyquenched as the contacts open. With DC, there is no interruption, and an arc may be maintainedacross the contacts - even when they are fully open.

This is a very serious issue, and is something that is overlooked by a great many people. Even ifthe relay contact voltage and current are such that the arc extinguishes each and every time, the


8 of 35 5/21/2015 9:09 PM

mere fact that there is an arc means that the contacts are under constant attack. With an arc,material is typically moved from one contact to the other. With AC, the polarity is usually random, socontact material is moved back and forth, but with DC it's unidirectional. It takes a long time withvery robust contact materials like tungsten, but it still happens, and eventually the relay will fail dueto contact erosion. The manufacturer's ratings are the maximum AC or DC voltage and current thatwill give the claimed number of operations. If either the rated voltage or current is exceeded, therelay will probably have a short life. DC is the worst, and DC fault conditions are often catastrophicfor a relay that's intended to provide any protective function.

In some cases a magnet can be used to help quench the arc created as the contacts open.Because the arc is conducting an electric current, it both generates and can be deflected by amagnetic field. Magnetic arc quenching (or 'blow-out') is rarely provided in relays, but it may bepossible to add it later on provided you know what you are doing and can position the magnet(s) inexactly the right place. You might see this technique used in high current circuit breakers, and evenin some relays (although they are more likely to be classified as contactors).

There are countless 'speaker protection' circuits on the Net that may not actually work when theyare most needed. To see how it should be done, have a look at the way the relay contacts are wiredfor Project 33. When the relay opens it puts a short across the speaker, so even if there is an arc, itpasses to ground until a fuse blows. Any speaker 'protection' circuit that doesn't short the speakercould leave you well out of pocket, because not only is the amplifier probably fried, but so is therelay and the speaker it was meant to protect. A relay that can actually break 100V DC at perhaps25A or more is a rare and expensive beast, but that's what might be needed for a high poweramplifier.

The subject of relay contact materials, arc voltages and currents, metal migration during make andbreak operations (etc., etc.) is truly vast. It's the subject of academic papers, application notes andlarge portions of books, and it's simply not possible to cover everything here. Suffice to say thatmanufacturer's recommendations and ratings are usually a good place to start, and the maximashould never be exceeded. The number of electrical operations can be extended significantly byde-rating the contacts (using 10A relays for 5A circuits for example), and AC is nearly always muchless troublesome than DC.

This discussion covers snubbing networks and other measures that may be needed to protect thecontacts from the load in Part 2. This is a very complex topic, and depends a great deal on theexact nature of the load. In many cases nothing needs to be done if the voltage and current areboth well inside the maker's ratings. In other cases extreme measures may be needed to preventthe contacts from being destroyed. DC is the worst, and high voltage and/or high current will requirevery specialised relay contacts and arc-breaking techniques. If possible, consider solid state relaysfor DC, because they don't use contacts so can't create an arc.

This really is a science unto itself, and thanks to the InterWeb you can find a lot of really good data.Unfortunately, it can be very difficult to find information that is both relevant and factual, so don'texpect to find what you need on the first page of the search results, and in general ignore forum orusenet posts. There's a great deal of disinformation out there, and whether it's by accident, design,or just people claiming to know far more than they really do is open to debate. Suffice to say that agreat deal of such 'information' is just plain wrong.

In a great many cases, the only way to get a solution that works is by trial and error. This isespecially true if you have a difficult load - whether because the supply is DC, the load is highly


9 of 35 5/21/2015 9:09 PM

inductive, or high currents and voltages are involved. For large-scale manufacturing, getting acustom design is viable, but the costs will be high and can't be justified for small runs or one-offprojects. I've covered a very small subset of possible failure modes and contact erosion - there is somuch more to learn if you have the inclination.

Relay 'Forms'

A common way to designate a relay's contact arrangements is to use the 'form' terminology. Forexample, you will see relays described as '1 Form C' in datasheets, catalogues and even in webpages on the ESP site. This terminology is roughly equivalent to referring to SPST or DPDT forexample.

Form A Normally open (NO) contacts onlyForm B Normally closed (NC) contacts onlyForm C Changeover contacts (normally open, normally closed and common)

So a 1-Form-C relay has a single set of changeover contacts, 2-Form-A has two sets of normallyopen contacts, etc.

Relay Coils

One would think that this is too simple to even discuss, but it's definitely otherwise. The coil is aninductor, and because it's wound around a magnetic material (usually soft iron or mild steel) theinductance is increased. It's also non-linear. When the coil is not energised there's a large air-gap inthe magnetic circuit, and this means the inductance is reduced. Once the relay is energised, themagnetic circuit is completed, or at least the air-gap is a great deal smaller, so now the inductanceis higher.

I used an inductance meter to get the values shown below, but if you need an accuratemeasurement you'll have to use another method. The inductance is in conjunction with the coil's DCresistance, and that changes the reading so there's a significant error. True inductance can bemeasured by using a series or parallel tuned circuit with a capacitor to get a low frequencyresonance (< 100Hz if possible) if you really want the real value. It's not often needed and yourarely need great accuracy, and although an inductance meter has a fairly large error used this way,but it's fine for the purpose.

Inductance meter measurements taken from two of the relays pictured above gave readings of ...

Octal Base 10R open 335 mH 186 Coil Resistanceclosed 373 mH

STC 4PDT open 283 mH 248 Coil Resistanceclosed 303 mH

How large is the error? I checked the octal based relay using a series 5.18F capacitor, andmeasured the peak voltage across the cap (indicating resonance) at 61Hz with the armature openand 37Hz with it closed. This gives an inductance of 1.3H open, 3.6H closed, so the error issubstantial. There's plenty of scope to get the frequency measurement wrong too, because the'tuned circuit' created has low Q and the frequency range is quite broad - expect the result to be25% at least, depending on how closely you can get an accurate peak voltage while varying the


10 of 35 5/21/2015 9:09 PM

frequency. The formula is ...

L = 1 / (( 2 * * f ) * C )L = 1 / (( 2 * * 61 ) * 5.18 )L = 1.3H

Although the error is large, the simple fact of the matter is that we don't really care. I included theinductance purely to demonstrate that it changes depending on the armature's position, but the coilinductance isn't provided by most relay manufacturers because you don't need it. These data areprovided purely for interest's sake. Since inductance is part of the relay's 'being' (as it were), youcan't do anything about it.

However, because the coil is an inductor, it stores a 'charge' as a magnetic field. When voltage isremoved, the magnetic field collapses very quickly, and this generates a large voltage across thecoil. The standard fix is to include a diode, wired as shown below (Figure 4A). However, adding thediode means that the relay will release slower than without it, because the back-EMF generates acurrent that holds the relay closed until it dissipates as heat in the winding and diode. The flybackvoltage will attempt to maintain the same current flowing in the coil as existed when the current wasbeing applied. Of course it can't do so because of losses within the circuit.

Because the coil is an inductor, the operating current is not reached as soon as power is applied.For example, with a 280mH coil, it may take up to 2ms before there's enough current to attract thearmature. This delay isn't usually a problem, but it does mean that you can't expect anelectromechanical relay to provide accurate timing or instantaneous connections. If you needsomething to happen at a very precise time, then you'll have to use a solid state relay (see below formore information).

A relay coil's magnetic strength is defined by the ampere turns, and the current is defined by thecoil's resistance. Let's assume as an example that a relay needs 50A/T (ampere turns) to activatereliably. A single turn with 50A will provide 50A/T, as will 10 turns with 5A, but they are impracticalunless the relay is intended to sense an over-current condition (used for electric motor startswitches for example). It will be more useful to have a larger number of turns with less current, sowe might wind 1,000 turns onto the bobbin. The wire will be fairly fine, and may have a resistance ofaround 240 ohms. Now we only need 50mA to get the 50A/T needed, so applying 12V will produce50mA through the 240 ohm winding. Since there are 1,000 turns at 50mA, that works out to 50A/Tagain, so we have the required magnet strength and a sensible voltage and current.

Please note that this info is an example only, and the actual ampere turns needed for a typical relayis fiendishly difficult to find on the Net. If you really need to know, you'll have to test it yourself byadding a winding with a known number of turns. If you add 50 turns and the relay pulls in at 600mA,that's 30A/T. Since you always need to allow for coil self-heating and/or a lower than normal supplyvoltage, you'd need to use more turns or a higher current. Most relays are designed to act withbetween half to three-quarters of the rated voltage. A 12V relay should activate with a voltagebetween 6 and 9 volts.

A pretty much standard circuit for a relay is shown below, along with a useful modification. A voltageis applied to the input (typically 5V from a microcontroller), and that turns on Q1 and activates therelay. Without D1, the voltage across Q1 will rise to over 400V (measured, but it can easily exceed1kV) when the transistor is turned off, which would cause instant failure of Q1. D1 (sometimesreferred to as a 'freewheeling' or 'catch' diode) acts as a short circuit to the back-EMF from the coil,so the voltage across Q1 can only rise to about 12.6V. However, as long as enough current flows


11 of 35 5/21/2015 9:09 PM

between the relay coil and D1, the relay will not release. It may take several milliseconds before thearmature starts to move back to the rest position after Q1 is turned off.

Figure 4 - 'Standard' & Modified Relay Switching Circuit

I tested a relay with a 270 ohm coil having 380mH of inductance - although the latter is not aspecified characteristic in most cases. If you need to know the inductance you will probably need tomeasure it. With just the diode in circuit, there is enough coil current maintained to keep the relayenergised for some time after Q1 turns off. The release time is a combination of electrical andmechanical effects. If the resistor (R2) is the same as the coil resistance, the 'flyback' voltage will belimited to double the supply voltage, easily handled by the transistor I used.

You can also use a zener and a diode, typically using a 12V zener. It can be rated for up to twice theapplied voltage, in which case the peak voltage will be about 3 times the supply voltage. A zener isslightly better than the diode/ resistor combination shown, and is seen in more detail below. Thegraphs below show the behaviour of the circuit with and without the resistor and diode. Themeasured 400V or more is quite typical of all relays, which is why the diode is always included.Voltage peaks that large will destroy most transistors instantly, and while a high voltage transistorcould be used that simply adds cost. The flyback voltage is created by exactly the same processused in the standard Kettering ignition system used in cars, but without the secondary winding. It'salso the principle behind the 'flyback' transformer used in the horizontal output section of a CRT TVset (remember those?) or flyback switchmode power supplies.

Workshop tests were done to see just how much voltage is created, and how quickly a fairly typicalrelay could be operated. I used the 'Low Cost SPDT' relay shown in Figure 2 for the tests. Theresults were something of an eye-opener (and I already knew about the added delay caused by adiode!). The relay I used has a 12V, 270 ohm coil and has substantial contacts (rated for 10A at250V AC). With no back-EMF protection, the relay closed the normally closed contacts in 1.12ms -this is much faster than I expected, but the back-EMF was over 400V - it varied somewhat as theswitch contacts arced on several tests. When a diode was added, the drop-out time dragged out to6ms, which is a considerable increase, but of course there was no back-EMF (Ok, there was 0.65V,but we can ignore that). Using the diode/ resistor method shown above, release time was 4ms, andthe maximum back-EMF was 24V (double the supply voltage). This is a reasonable compromise,since there are many transistors with voltage ratings that are suitable for the purpose.


12 of 35 5/21/2015 9:09 PM

Figure 5 - Relay Flyback Voltages

The blue trace shows when the NC contact is made as the relay releases, and is from zero to 12V.The peak relay voltage ((A) - No Diode) measured over 400V on my oscilloscope, and due to thevoltage range little detail about the voltage collapse is visible. In both cases, the relays were wiredin the same way shown in Figure 4, but using a switch instead of a transistor. The second traceshows the release time and voltage spike when a diode and 270 ohm resistor are used to get ahigher release speed. The diode isn't essential, but without it the relay circuit will draw twice asmuch current as it needs because of the current through the resistor. Note that the horizontal scaleis 1ms/ division in (A) and 2ms/ division in (B), and the vertical scale for the relay back-EMF (yellowtraces) is also changed from 100V/ division (A) down to 10V/ division in (B).

The kink in the relay voltage curve is caused by the armature moving away from the relay polepiece and reducing the inductance. The 'NC' contacts close as the relay releases. As you can see,this is 4ms after the relay is disconnected (with the resistor + diode in place). With no form offlyback (back-EMF) suppression, the relay will drop out faster because the current is interruptedalmost instantly (excluding switch arcing of course).

These graphs are representative only, as different relays will have different characteristics. You canrun your own tests, and I encourage you to do so, but in all cases the behaviour will be similar tothat shown. Upon contact closure of the normally open contacts, I measured 2.5ms of contactbounce (not shown in the above oscilloscope traces). These tests might be a little tedious, but arevery instructive.

When the resistor has the same value as the coil's internal resistance, the back-EMF will always bedouble the applied voltage. If the resistor is 10 times the coil's resistance, the peak voltage will be10 times the applied voltage (both are plus one diode voltage drop of 0.7V). This relationship iscompletely predictable, and works for almost any value of coil and external resistor. It's simplybased on the relay's current. If the relay draws 44mA, the collapsing magnetic field will attempt tomaintain the same current. 44mA across the external 270 ohm resistor will generate 12V, and if theresistor is 2.7k the voltage must be 120V (close enough).

While this trick was common with early electric clocks (but without the diode because they hadn'tbeen invented at the time), it seems that few people use it any more. That's is a shame because itworks well, limits the peak voltage to something sensible, and reduces the relay release timecompared to using only a diode.


13 of 35 5/21/2015 9:09 PM

If you search hard enough, you will find it mentioned in a few places, and it's been pointed out [ 8 ]that simply using a diode can cause the relay to release too slowly to break 'tack welding' that canoccur if the contacts have to make with high inrush currents. This can happen because thearmature's physical movement is slowed down, and it doesn't develop enough sudden force tobreak a weld. It's far more complex than just an additional delay when a diode is placed in parallelwith the coil.

Figure 6 - Flyback Voltage With Diode+Zener

The zener diode scheme shown above may be a bit more expensive than a resistor, but it allowsthe relay to deactivate much faster. The most common arrangement will be to use a zener rated forthe same voltage as the relay's coil and supply. In the example, the release time was 2.6ms, andthat's significantly faster than obtained using a resistor and diode (4ms). A higher voltage zener willbe faster again, with a 24V zener giving a drop-out time of 1.84ms. If the voltage is too high youmay end up needing a more expensive drive transistor to get the voltage rating, but using more thandouble the supply voltage won't improve matters by very much. Overall, this arrangement isprobably be best compromise. It's faster than a resistor for not a great deal of extra cost, anddoesn't require you to try to purchase parts that may not be readily available at your localelectronics shop.

I also tested the circuit shown with a 100nF ceramic capacitor in parallel with the coil. The flybackvoltage measured 86V, and the relay released in 1.23ms. That's a good result, but the voltage ishigher than desirable and the cap needs to be a high-reliability type to ensure a long life. Thismakes it more expensive than other options, but there may be situations where this turns out to bethe best choice for the application, with or without a series resistor.

Other transient suppression techniques can be used that don't affect the armature release speedgreatly, including using a carefully selected TVS diode, a low voltage MOV or a resistor/ capacitorsnubber network. The latter is generally not cost effective and is rarely used now, but was fairlycommon in early systems and is still useful with AC relay coils. If relays are to be used towards theirmaximum contact ratings, be aware that these are often specified with no form of back-EMFsuppression, which ensures the fastest possible opening time for the contacts. If you decide to usea TVS, you either need a bidirectional type, or add a diode in series. MOVs will work well, but theirclamping voltage is something of a lottery so you need to allow a safety margin for the switchingtransistor's peak voltage rating that accommodates the voltage range of the MOV (or TVS - theyaren't precision devices either).

What about the diode ratings? The diode must be rated for the full supply voltage as an absolute


14 of 35 5/21/2015 9:09 PM

minimum. That part is easy, because the 1N4004 diode is not only ubiquitous, but it's as cheap aschips. There aren't many applications where you need more than 400V relay coils. It can betempting to use 1N4148 diodes, and although their voltage rating is usually fine, they are ratherflimsy and their current rating is only 200mA continuous or 1A peak (1 second, non-repetitive). Idon't really trust them for anything other than signal rectifiers, but a lot of commercial products usethem across relays.

The diode current rating should ideally be at least the same as the relay coil current, not becauseit's needed but to ensure reliability and longevity. For most general purpose relays, the 1N4004 is agood choice - 1A continuous, 30A non-repetitive surge (8.3ms) and a 400V breakdown voltage.Remember that the peak current through the diode will be the same as the relay coil current, so ifyou have a (big) relay that pulls 2A coil current, you need a diode rated for at least 2A, preferablymore. You can rely on the rated surge current for the diode, but it's better to allow a generous safetymargin. The cost is negligible.

So, you may have thought that relay coils were simple, and you only need to add a diode so thedrive transistor isn't destroyed when it turns off. Now you know that this is actually a surprisinglycomplex area, and there are many things that must be considered to ensure reliability and longevity.It's only by research and testing that you know the effects of different suppression techniques andthe limitation that each imposes.

AC Relay Coils

To confuse matters more, some relays are designed so that the coils can be run from AC, withoutany noticeable 'chatter' (vibration that causes noise - often very audible) and possibly continuouscontact bounce. AC relays can usually be operated from DC with several caveats, but a DC relaycoil should never be used with AC. Larger AC relays use a laminated steel polepiece, yoke andarmature to reduce eddy current losses that would cause overheating, but this is not generally aproblem with comparatively small relays. The current flow in a DC relay coil is determined by itsresistance, but when AC is used there is a combination of resistance and inductive reactance -covered by the term 'impedance'. If the maker doesn't tell you the coil's current, it will have to bemeasured, as it can't be determined by measuring the coil's resistance.

There's a little secret to making the coil work with AC, and that's called a 'shading' ring (or shadingcoil). If you look closely at the photo of the larger octal relay in Figure 2, you can see it (well, ok, youcan't really see it clearly, so look at Figure 7 instead). There's a thick piece of plated copper pressedinto the top of the polepiece, and that acts as a shorted turn, but only on half the diameter of thecentre pole. The shorted turn causes a current that's out-of-phase in its part of the polepiece, andthat continues to provide a small magnetic field when the main field passes through zero. Howeverunlikely this might seem, it works so well that the AC relay pictured above is almost completelysilent, with no chatter at all.


15 of 35 5/21/2015 9:09 PM

Figure 7 - AC Relay Shading Ring

This is the very same principle as used in shaded-pole AC motors (look it up if you've never heardthe term). The small magnetic field created by the shading ring is enough to hold the relay'sarmature closed as the main field passes through zero, eliminating chatter and/or high speedcontact movements that would eventually wear out the contacts just by the mechanical movement.Chattering contacts will also create small arcs with high current loads that will damage the contactsand possibly the load as well.

AC relays can be used with DC, but a few problems may be encountered. You will need to reducethe DC voltage by enough to ensure that the coil can pull in the relay reliably but withoutoverheating. You might also experience possible armature sticking - see below for more info on thatphenomenon. In my case, the 32V AC relay works perfectly with 24V DC, but it draws almostdouble the current that it does with AC. The coil has a resistance of 184 ohms and draws 62mA at32V AC - an impedance of 516 ohms. For roughly the same current, it should be operated at nomore than 12V DC, but it will not pull in at that voltage. At 24V DC the coil will draw 129mA anddissipate over 3W, and it will overheat. The pull-in current with 32V AC is 104mA, because theinductance is low when the armature is open and more current is drawn. That means that theimpedance is only 307 ohms when the armature is open.

Never use a DC relay with AC on the coil, as it will chatter badly and may do itself an injury due tothe rapid vibration of the armature. Contacts will almost certainly close and open at twice the mainsfrequency rate (100 or 120Hz). If you must operate a DC relay from an AC supply, use a bridgerectifier and a filter capacitor. Release time will depend on the value of the filter cap, coil resistance,etc. If there is a capacitor across the relay coil of more than a few microfarads (depending on relaysize of course), you don't need a diode because the capacitor will absorb and damp the smallback-EMF. You can include the diode if you like - it won't hurt anything, but it won't do much goodeither.

The yoke and armature of most relays is just mild steel, not the 'soft iron' that you'll see claimed inmany articles. Mild steel is magnetically 'soft' in that it doesn't retain magnetism very well (holding amagnetic field is known as remanence), but it does have some remanence so may become slightlymagnetised. This can lead to the armature sticking to the polepiece, and that can be a real issue. Ifthe armature sticks, the contacts will not release back to the 'normal' state when coil current isremoved. This can be overcome by a stronger spring, but then the coil needs more current to pull inthe armature against the tension provided by the spring.


16 of 35 5/21/2015 9:09 PM

In many DC relays, the centre polepiece may have either a very thin layer of non-magnetic materialon the top (where the armature makes contact) or a tiny copper pin, placed so that the armaturecan't make a completely closed magnetic circuit. This small gap is designed to be enough to ensurethat the relay can always release without resorting to a stronger spring. You will almost certainly seethis technique applied in 'sensitive' relays - those that are designed to operate with the lowestpossible current.

With AC relay coils, if you need back-EMF suppression then you have to use a bidirectional(non-polarised) circuit. This can be a TVS with suitable voltage rating to handle the peak ACvoltage, two back-to-back zener diodes, again with a voltage rating that's higher than the peak ACvoltage, or a resistor/capacitor 'snubber' network. It may be necessary to allow a higher back-EMFthan you might prefer to ensure that the armature returns to the 'rest' position without being sloweddown by the suppression circuit.

Relay Drive Circuits

This article will not cover drive circuits in any detail. This is simply because there are so manypossibilities that it would only ever be possible to cover a small selection. Common circuits areshown throughout this article, but there are many others that will work too.

I've shown the most basic NPN transistor drive, where the relay coil connects to the supply rail andthe drive circuit connects the other end to earth/ ground. A PNP transistor can be used instead, butused to switch the supply to the relay coil (the other end is earthed). Relays can be driven byemitter followers, but that's not very useful as a stand-alone switching circuit, but can be handy insome cases. Some relays with particularly low coil current can be driven directly from the output ofan opamp, and using 555 timers as relay drivers is also common.

You can also use low-power MOSFETs (such as the 2N7000 for example), and once upon a timeeven valves were used to drive relay coils in some early test equipment and industrial controllers.There are dedicated ICs that can be used, and of course any relay can be activated using a switch(of almost any kind) or another relay. You might want to do that if a low power circuit has to control ahigh power load, and relays are used as a form of amplification. For example, your circuit mighthave a reed relay switching power to a heavy duty relay that applies mains power to a contactor'scoil (if you recall from the intro, a contactor is just a really big relay).

Where switch-off time is particularly critical, controlled avalanche MOSFETs might be appropriate.These are specifically designed to allow any transient over-voltage to be dissipated harmlessly inthe parasitic reverse-biased diode that's a standard feature of all MOSFETs. Don't push anyMOSFET that is not specifically rated for avalanche operation (such devices may be classified as'ruggedised' or avalanche rated) into forward voltage breakdown. For most relay applications Iwouldn't even consider this approach, as it's simply not necessary for most 'normal' drive circuits. Ifyou want to play with using avalanche rated MOSFETs, the IRF540N is a low cost MOSFET thatshould survive with no diode in parallel with the coil.

Driving AC relay coils is most commonly done using either a switch or another relay. It's certainlypossible to make an electronic circuit that can drive an AC coil, but in general it would be a pointlessexercise. The vast majority of all control systems will use DC coils, and it's an uncommon instancewhere AC coils are the only relay you can get that will handle the power of the controlled system(whatever it might be). If that is the case with a microcontroller or other IC based controller, then it'sfar easier to use a relay with a DC coil to switch power to the AC relay coil.


17 of 35 5/21/2015 9:09 PM

You need to be aware that switching the coil of a relay on or off can induce transients into low-levelcircuitry. PCB layouts generally need to be carefully optimised to ensure that the relay power -including the return/ earth/ ground circuit - is isolated from the supply used for the low-level circuitry.If this isn't done in audio circuits, clicks and pops may be audible when relays operate. For controlor measurement systems, the relay coil transients may be interpreted as valid data, causing errorsin the output. If you opt for a circuit using a diode and zener for example, the turn-off transient isvery fast, which makes it more likely to induce transients into surrounding circuitry.

Relay Logic

Taking relays to the extreme, you can even have relay logic! This used to be quite common forprocess controllers and other industrial systems, where control switches and relay contacts arearranged to create the basic logic gates - AND, NAND, OR, NOR and NOT (inverter). One of themost common (and complex) forms of relay logic was used in telephone exchange ('central office')switches. These interpreted the number dialled and routed the call to the requested destination -often through several exchanges. The exchange switches used a combination of conventionalrelays and rotary 'stepper' relays. A uniselector worked on one (rotary) axis, and the step-by-steptwo axis stepper (one rotary and one vertical) was commonly known as a Strowger switch after itsinventor. Later exchange switches used a crossbar matrix switch, with the last of them beingelectronically controlled.

The diagrams used to describe relay logic are generally referred to as 'ladder' diagrams, and you'llalso see the term 'ladder logic' used. This used to be (and perhaps still is in some cases) a requiredarea of study for anyone involved in industrial electronics. It is so entrenched that manymicroprocessor based control systems are still programmed using a ladder diagram, even thoughmost of the functions are in software. One manual I saw for a 'logic relay' extended for nearly 300pages!

This is a very specialised area, and while it's certain that there are still some early relay based logicsystems still in use, in most cases they will have been replaced many years ago. Unlike amicrocontroller, re-programming a true relay logic system is generally done with hard wiring. All therequired inputs are brought to the main 'logic' unit, and the outputs control the machinery.

Inputs can include push-buttons, pressure sensors, limit switches, thermal sensors, magneticdetectors and/or the output signals from another relay logic unit. Outputs are typically motors,heaters, valves for water, hydraulic fluid, gas, etc. Generally not thermionic valves (aka 'tubes'),although that's possible too - older high power RF amplifiers for high frequency welding systems forexample.

Another related use for relays is a switching matrix. Crossbar telephone exchange switches are oneexample, but matrix switches are used to divert all manner of signals to a required destination, andto direct outputs of other equipment to the right place. Process control, automated test equipment,audio, video and RF switching matrixes are just a few of the possibilities. Reed relays areparticularly well suited to matrix switching systems for low power signals.

Relay logic and matrix switching are vast topics, and I have no intention to go into any more detail.There is so much information and the applications so diverse that even scratching the surfacewould occupy several books. If you are at all interested, it's worth doing a search for 'relay logic' or'relay matrix' - you'll be surprised at the number of web pages that are devoted to the topics.


18 of 35 5/21/2015 9:09 PM

Pull-In And Release Voltages

Most detailed specifications for relays will provide the pull-in (or pick-up) and release (drop out)voltages. These vary widely depending on the relay's construction, but you might see figures thatindicate that a particular relay should pull-in at 75% of the rated voltage, and should release whenthe voltage falls to 25% of rated voltage. Based on this, a typical 12V relay should pull-in at about9V, and should release when the voltage has fallen to 3V. This is a test you might be able to runyourself, but in the majority of cases it doesn't make a lot of difference. The pull-in and releasevoltages may also be referred to as the 'must operate' and 'must release' voltages, and they varywith different relays.

Most circuits are designed to switch the power to relays quickly, commonly using a circuit such asthose shown in Figure 4. The full voltage appears almost instantly, and when the transistor switchturns off the supply current is interrupted immediately. The relay current continues to flow via thediode, but that doesn't affect the actual voltage at which the relay releases. What these numbers dotell us is that once a relay has pulled in, a significantly lower voltage and current will keep it in theenergised state. This means that it's possible to reduce the current and keep the relay energised.This leads us to ...

Relay 'Efficiency' or 'Economy' CircuitsThere is one application where the release or drop-out voltage needs to be known. In somesystems (especially battery operated), it may be important to get the maximum possible efficiencyfrom a relay. This means that the coil is supplied with a low holding current after the relay has beenactivated. This is the minimum safe current that will keep the relay energised, and battery drain isreduced accordingly. Early systems used a resistor, but there are now ICs available that use PWMto modify the current profile after the relay has settled [ 3 ].

When first activated, the relay coil receives the full voltage and current for a preset period, afterwhich the circuit reduces the current to a known value that will keep the relay energised. If you planto use this type of device, you will need to know the coil inductance because that's needed so theproper PWM switching frequency can be set. A simple system such as that shown below may be allyou need though. It doesn't have the high efficiency of a switchmode solution, but it's simple, cheapand effective. I've assumed a relay coil resistance of 270 ohms.

Figure 8 - Simple & PWM Relay Efficiency Circuits

Looking at the simple R/C circuit, when Q1 is switched on, C1 is discharged and can only chargevia the relay coil. The coil therefore gets the full voltage and current when Q1 is turned on, but as


19 of 35 5/21/2015 9:09 PM

C1 charges, they are both reduced. It will eventually be reduced to exactly half the normal current,in this case about 22mA instead of 44mA. The same trick can be used with higher than normalsupply voltages, allowing the resistor to limit the current to a safe holding value, but providing a'boosted' current as the relay is energised. Putting up to 24V or so across a 12V coil momentarilyusually won't damage it, provided the long term operating current is not more than the rated value.In most cases the coil current can be halved and the relay will not release. This must be tested andverified of course. The capacitor should be selected to give a time constant of at least 100ms, whichis more than enough time for the relay to pull in properly. The time constant is determined by ...

t = R * C where R is the series resistance in ohms (R2), and C is in Farads (C1)t = 270 * 2,200uF = 126ms

The PWM driver is a little harder to understand unless you have some knowledge of PWM circuitsfeeding inductive loads. The PWM driver is 'symbolic' only, and does not represent any particulardevice. 'Ct' is a timing cap, used to set the operating frequency. When the circuit is triggered, therelay gets a steady current for a preset time (perhaps 1/2 second or so - the waveform is not toscale). Then the internal transistor turns on and off rapidly, usually at 20kHz or more. D1 is noweither a very fast or preferably Schottky diode, and every time the switch turns off, back-EMFmaintains current through the coil. If the final duty cycle is 50%, then the average current throughthe coil and diode will be 50% of the maximum (44mA reduced to 22mA for the demonstrationrelay). The advantage is that there is no power lost in an external resistor, and because of theswitchmode circuit the current drawn from the supply will only be 11mA ... in a perfect world. Inreality there will be some losses, so supply current may be a little higher than the ideal case.

The driver IC is a switching regulator, so the overall efficiency is much higher than the resistor-capacitor version. The cost is relative complexity, and the ICs are more expensive than a transistor,but if battery life is paramount then you don't have a choice, other than to use a latching relay. Thecurrent reduction can be well worth the effort if you need to conserve power. In many cases amicrocontroller can be programmed to do the same thing, driving a switching transistor instead ofthe dedicated IC. Ideally, if you plan to use a PWM efficiency circuit, if possible get relays intendedfor that purpose. General purpose (solid yoke and armature) relays may overheat due toeddy-current losses if the ripple current through the coil is too high.

I ran a test of the PWM efficiency circuit on a general purpose 12V relay with a nominal 240 ohmcoil and an inductance measured at 300mH. Even with a 1kHz drive waveform, there was only veryminor heating detected in the yoke. For the 'main' test, I used a 1N4148 diode and a BC550transistor (neither is ideal, but both ran almost cold) and drove the base with a 5kHz squarewave.The input current measured 48mA with a steady-state input, and it fell to 11.7mA when driven by a50/50 squarewave. Although the voltage across the coil varies across the full 12.8V range (thediode forward voltage is added to the supply voltage), the current through the coil is fairly steady at23.4mA with about 5mA of ripple, so eddy current losses are lower than you might expect. The fastswitching waveform will cause interference in low level signals that are nearby, and that willprobably rule out PWM control in audio or test and measurement applications.

Note that the measured inductance is wrong according to a low frequency test as described earlier,but we still don't care. Most inductance meters test at a fairly high frequency, and PWM isperformed at a high frequency too. The measured inductance is a good indicator of the minimumPWM frequency that can be used, and if it turns out that it's higher than measured, that simplymeans there's less ripple current with PWM operation.


20 of 35 5/21/2015 9:09 PM

Regardless of the type of circuit, the optimum hold current may be more or less than the 50% usedas an example. This means that the resistor value may not be the same as the coil's resistance, butis adjusted to suit the relay. Likewise, the duty-cycle of a PWM circuit may also need to be changedto suit the relay. The 50% figure works with most relays, but some will be happy with less, othersmay need more.

An unexpected advantage of using an 'efficiency' scheme (whether active or passive) is that therelay's release time is reduced because there's a much lower magnetic field and less back-EMF.However, this is something that you'd have to test thoroughly for your particular application,because every relay type will be somewhat different from others, even if superficially the same.

Keep in mind that the relay coil is temperature sensitive because of the thermal coefficient ofresistance of the copper wire (about 0.004/C). This can be approximated to 4% resistance changefor each 10C. When the relay coil is hot the pick-up voltage will be increased in proportion to thetemperature. This may be because the coil has been operated for some time and become warm (orhot), or due to high ambient temperature. The drop-out voltage will also be increased, so the relaymay release at a higher voltage than expected. In most circuits this is not a problem, but it issomething you may need to consider in some applications.

There is at least one version of a very flawed efficiency circuit on the Net. The circuit uses normallyclosed contacts to short out the series resistor, so when the relay operates the short is removed andthe resistor is in circuit. There's only one problem - the relay is placed in series with the coil beforethe relay armature has contacted the polepiece. This means that the relay will probably never reallyclose properly because its full current isn't available for long enough. If contact pressure is too low(as it almost certainly will be), resistance may be much higher than it should be and contact failurewill follow, or it may not make contact at all. The idea might work with some relays, won't work at allwith others. It would be a clever idea if it could be trusted, but it's far too risky in a high currentapplication. I strongly recommend that you avoid copying the mistake. I tested it, and the relayactivated just far enough to open the NC contacts, but not enough to close the NO contacts. Thearmature was in limbo, at about half travel. Epic fail.

Reed Relays

Reed relays are often used when switching low-level ('signal') voltages. Because the contacts arehermetically sealed in a glass tube there is no risk of contamination, and the only limit to their life ismechanical wear of the contact surfaces. Because the contacts close and open with no slidingforces, mechanical wear is minimal. The reed switch is yet another product that came out of thetelephone system - it was invented by an engineer at Bell Labs in 1936. Reed switches are usedwith a separate magnet for door and window switches for intruder alarms and for safety interlockson machinery. When the magnet (attached to the moveable part of the door/ window) moves a fewmillimetres away from the switch, the contacts open signalling that the safety cover/ door/ windowhas been opened. There are countless other applications as well.

The reed switch itself uses two magnetic contact arms/ blades, one of which is flexible. There is nomechanical hinge or pivot, so reed switches can be considered to have no moving parts as such.The flexing of the moveable contact arm is designed to be well within the normal elastic range of themetal, so metal fatigue is not a limiting factor. A semi-precious metal is used for the contact faces.When the two contact arms are surrounded by a solenoid, one becomes magnetised with a Northpole, and the other is South. Since opposites attract, the two contacts are drawn together, closingthe circuit. In some cases a bias magnet is used to provide a normally closed contact, and the


21 of 35 5/21/2015 9:09 PM

solenoid opposes the magnet to open the contacts. A bias magnet can also be used to increasesensitivity, but at the expense of being potentially unreliable in the presence of other magneticmaterials. A bias magnet can also be used to create a latching relay, and the coil's polarity isreversed to open the contacts again.

Most reed switches have a single pair of normally open contacts, but there are versions withnormally closed and changeover contacts [ 4 ]. A reed relay consists of the magnetically operatedreed switch inside a solenoid. The two parts may be completely separate, or sealed into a smallenclosure as seen in the photo above (top right, Figure 2). They are also installed in small PCBmount cases, looking somewhat like an elongated IC. Reed relays are mostly designed for lowvoltage, low current applications. The contact opening is very small and usually cannot withstandhigh voltage, although high voltage reed switches do exist! 200V AC at up to 1A is not uncommon.Reed switches and relays can be rated for billions of operations, depending on the load. If thevoltage or current is towards the maximum rated for the switch it may last for less than 1 millionoperations due to contact erosion.

Reed relays are very fast. I tested the one shown in Figure 2 up to 1kHz, and it was switching atthat speed. The output was more contact bounce than anything else, but at 500Hz there was analmost passably clean switching waveform (still with about 150s of contact bounce though).Contact bounce notwithstanding, that is very fast for a relay of any kind. Operating it at that kind ofspeed isn't recommended because of contact bounce, and even at a rather leisurely 100Hz you geta billion (1E9) operations in a little over 115 days.

Reed switches were used for commutation of some high-reliability brushless DC fan motors beforesemiconductor Hall effect sensors became available. Even in this role the switches would mostlikely outlast the bearings ... somewhere in the order of 9 years for a billion operations. No,nothing to do with relays as such, but interesting anyway.

If you ever need to know, reed relays typically need around 20-30 ampere turns to activate, so if youhave to make your own coil for a reed switch you'll need to use about 1,000 turns at 30mA fortypical examples. They vary, so you will need to run tests for yourself. It's obviously far easier to buyone than to mess around winding your own coil, but it can be done if you like to experiment. I testedone with 30 turns, and it required 1A (close enough) to operate, so that's 30A/T. Remember that youneed to add a safety margin, so you'd probably aim for around 45A/T for a reed switch that operatesat 30A/T to ensure that it will always pull in with the rated voltage - even if the resistance hasincreased due to self-heating of the winding.

Latching Relays

There are many different types of latching relay, sometimes also known as bistable relays (twostable states). A conventional relay is a monostable, having only one stable state. Some latchingrelays use an 'over-centre' spring mechanism similar to that used in toggle switches to maintain theselected state, and others use a small permanent magnet. There are single coil and dual coil typesas well. A single coil is a bit of a nuisance because the driving electronics become more complex,but dual coil types are usually somewhat more expensive. With a single coil, the driving circuitneeds to be able to provide pulses with opposite polarities, which typically requires four drivetransistors rather than two. Latching relays have the advantage that no power is consumed tomaintain the relay in the 'set' or 'reset' state.


22 of 35 5/21/2015 9:09 PM

Figure 9 - Dismantled Latching Relay [ 5 ]

The photo shows one kind of latching relay - it uses a magnet with two pole pieces on the armature,which pivots around its centre point. The coil is centre-tapped, so it can be latched one way or theother by energising the appropriate half of the winding. This type of relay only needs a momentarypulse on the appropriate coil to set or reset the contacts, and the pulse will be in the order ofperhaps 250ms. This means that the relay draws no power most of the time, only when it changesstate.

Unless the relay has an additional contact set that can be used to monitor which state it's in, there'sno way to know. Because it has two stable states, there is no real distinction between 'normallyopen' and 'normally closed' because both states of the relay are equally valid. For this reason,latching relays should never be used to turn on/off machines or power tools. For example, if there'sa power outage while the machine is running, when power comes back on the machine will startagain. This can easily create a risk of serious injury because the machine will start without warning.

If a microcontroller is used to drive latching relays, in theory it knows (thanks to the internalprogramming) which state the relay is in. However, if the equipment is portable and is dropped, therelay may change state due to the G-force created when it lands. Without separate contacts, themicro has no way to know that the relay's state has changed. This is a very real problem and it mustbe addressed in the software so that invalid states can be recognised and dealt with appropriately.

Figure 10 - Essential Parts Of Latching Relay (Contacts Not Shown)

The drawing shows the way the relay works. The magnet assembly has a central pivot, allowing theentire armature to rock back and forth. When there is no power to either coil, the armature can be ineither position and will be stable. If current is applied to the set (or reset) coil so the top of the yokebecomes a magnetic South pole, the bottom becomes North. In this state, the magnet and its polepieces will be repelled from both ends, and will snap clockwise so unlike poles are together. Again,the relay can remain in this state indefinitely, until the other coil is pulsed briefly and it will changestate again.

If the set coil is pulsed multiple times with no intervening pulses to the reset coil, nothing happens.Once the relay is in one state, multiple pulses or continuous current to that coil has no effect. It'sonly when the other coil section is pulsed that anything happens, and that will cause the relay tochange state.

Below are two simplified circuits of dual-coil (A) and single coil (B) latching relay drivers. As is


23 of 35 5/21/2015 9:09 PM

readily apparent, the dual coil version is far simpler, and just uses a transistor to connect one side ofthe coil or the other to ground to set or reset the relay. The two transistors should never be turnedon at the same time because the relay state will be indeterminate when power is removed.Otherwise, no harm is done. Note the way the diodes are connected - this only works if the coil anddrive transistors are connected as shown, and the peak voltage across the transistor that remainsoff is three times the supply voltage (3 x 12V or 36V in this case).

Figure 11 - Dual & Single Coil Latching Relay Drive Circuits

The single coil (B) is more complex, requiring another two transistors and resistors. Note thatdiodes can't be used to suppress the back-EMF because the polarity across the coil changes. Well,you can use diodes, but you have to add four of them. You need a diode from each end of the coilto earth/ ground, and another to the supply. The resistor shown (R5) is simpler and cheaper, andagain assumes the coil resistance to be 270 ohms and limits the flyback voltage to double thesupply (24V in this case). There should be no concern about the extra dissipation in the resistor,because it's on for such a brief period.

Some explanation is needed. If a signal is applied to 'Input 1 - Set', Q1 will turn on. This will turn onQ3 because the lower end of R3 is now at close to zero volts and Q3 gets base current. Q2 and Q4remain dormant. Current therefore flows through Q3, the relay coil, then Q1 to ground. If voltage isnext applied to 'Input 2 - Reset', Q2 and Q4 turn on, and current flow is now through Q4, the relaycoil (but in the opposite direction), then Q2 to ground.

With the Figure 10 (B) circuit, it is imperative that the software (or other control system) can neverapply a signal to both inputs at the same time. If that happens, all transistors turn on, and thetransistor bridge becomes close to a short circuit across the supply. This will almost certainly causetransistor failure and may damage or destroy the power supply.

While it's possible to include a 'lock-out' function to prevent this type of failure, that will simply addmore complexity. A crude (but probably effective) method would be to connect a Schottky diodebetween the base of Q1 to the collector of Q2, and another from the base of Q2 to the collector ofQ1. When either transistor is turned on the diode bypasses any base current intended for the othertransistor.

There are other ways a single coil can be driven, and if the relay coil voltage is significantly lessthan the supply voltage Q3 and Q4 can be replaced with appropriately sized resistors (270 ohms fora 24V supply and 270 ohm relay coil for example). If you use a resistor feed, the parallel resistorand/or diodes aren't needed. It's still far more effort than a dual coil relay though. Basically, the


24 of 35 5/21/2015 9:09 PM

whole process just gets messy, and the moral of this story is quite clear - if at all possible, use dualcoil latching relays.

There are also 'bistable' latching relays, where one impulse operates or 'sets' the contacts, and thenext (on the same coil) 'resets' them. If this type of relay is used, there should always be a spare setof contacts that can be used for an indicator or to tell a microcontroller the current state of the relay.Without that, there is no way to know which contacts are closed, and such an arrangement must beused with great care if it controls anything that could cause damage if the relay is in the wrong orunexpected state at power-up.

Figure 12 - Self-Latching Relay (Including Basic Relay Logic)

A fairly common control application is where you have two push-buttons to turn a machine on or off.These are sometimes mechanical, but momentary contact switches can be used as shown above.Provided the safety interlock switch is closed, when the 'On' button (normally open) is pressed therelay energises. The circuit is completed by the first set of relay contacts (A) which cause the relayto remain energised. It will remain on for as long as power is applied, or until the 'Off' button(normally closed) is pressed or the safety interlock switch opens. Power to the equipment isprovided by contact set B.

As shown the 'Off' button and safety interlock have absolute precedence, and as long as either isopen, the 'On' button cannot switch the circuit on. There might be several additional contacts inseries with the 'Off' button, perhaps used for sensing that a safety screen is in place or otherswitches that signal that the machine is safe to turn on. Should any safety switch open while themachine is in use, it will stop because the relay will de-energise. It cannot re-start until all interlockswitches are closed and the 'On' button is pressed.

This is a very basic form of relay logic, acting as a set/ reset circuit with an 'AND' function in the'Stop' circuit. The safety interlock and the 'Stop' button must be closed before the machine willoperate. Including other logic functions is just a matter of adding more contacts, relays, sensorswitches or external switching devices.

Semiconductor (Solid State) Relays

The common term is something of a misnomer, but anyone in the 'business' knows what a solidstate relay (SSR) is, and may even know how to control them and what loads are safe with a giventype. There is a huge variety of different types, not just for switching devices but for inputrequirements as well. Some SSRs are designed exclusively for use with AC, others are exclusivelyDC. A small number of commercial SSRs can be used with AC or DC. In this respect they are farmore restrictive than conventional (electro-mechanical) relays, but they also offer some uniqueadvantages. Needless to say, they also come with some unique disadvantages as well.

SSRs can use a wide variety of isolation and control techniques, including reed relays (which strictlyspeaking makes it a hybrid), DC/AC converters, mains frequency transformers, or (and mostcommonly) infra-red light within an IC package. This creates an optocoupler, and these outnumberthe other techniques by a wide margin. If significant power is being controlled, the control circuitrymay use various means to amplify the relatively low output current from the optocoupler [ 6 ].

Like conventional relays, SSRs provide galvanic isolation between input and output, commonly


25 of 35 5/21/2015 9:09 PM

rated for 2-3kV as a matter of course. Rather than using a coil to operate the relay, most SSRs usean optocoupler, so the activating medium is infra-red light rather than a magnetic field. Where anelectro-mechanical relay may require an input power of up to a couple of Watts, SSRs generallyfunction with as little as 50mW, with some needing even less.

However, where the contacts of a conventional relay may dissipate only a few milliwatts, an SSRwill usually dissipate a great deal more, with high power types needing a heatsink to keep theelectronic switching device(s) cool. This is because the switching element is a semiconductordevice, and therefore is subject to all the limitations of any semiconductor. This includes the naturalenemy of all semiconductors - heat! Common switching devices are SCRs, TRIACs and MOSFETs,and each has its own specific benefits and limitations.

Switching Used For Comments

SCR Wave AC Two are commonly used in reverse-parallel forhigh-power full-wave AC

TRIAC Full Wave AC Generally only used for low power versions (10A or lessfor example)

MOSFET AC or DC AC and DC versions are available, but are generally notinterchangeable

To make things more interesting, many SCR and TRIAC based SSRs are available with internalzero-voltage switching circuitry. This means that when switching AC loads, the electronic switchingwill only allow the SSR to start conducting when the applied AC voltage is close to zero. This is asimple way to reduce electrical interference, but you must be aware that they are only suitable forresistive loads.

Never use a zero-voltage switching SSR with transformers or other inductive loads.Doing so ensures maximum possible inrush current, which can result in trippedcircuit breakers and possible damage to the SSR itself. To see a complete articledescribing this phenomenon and more, read Inrush Current Mitigation. Inductiveloads behave very differently from what you might expect when switched on!

To see come of the techniques used for MOSFET relays, see the article MOSFET Relays whichdescribes the various techniques that can be used. DC MOSFET based SSRs simply use aMOSFET and an opto-coupler. There is generally little or no advantage to using the pre-packagedversion over a discrete component equivalent, except in cases where the certification of the SSR isneeded for safety critical applications.


26 of 35 5/21/2015 9:09 PM

Figure 13 - Internal Wiring & Solid State Relay

The general arrangement shown in the schematic of Figure 13 is common to most SCR and TRIACbased SSRs. The optocoupler can be purchased as a discrete IC in either 'instantaneous/ random'or 'zero-crossing' versions. In this case, 'instantaneous' simply means that the opto-TRIAC willtrigger instantly when DC is supplied to the LED, regardless of the AC voltage or polarity at thatmoment in time. The zero-crossing versions will prevent triggering unless the AC voltage is within 5to 20V of zero. Examples are the MOC3051 (instantaneous/ random phase) or MOC3041 (zerocrossing).

As noted above, zero-crossing trigger ICs must never be used with transformer or other inductiveloads, and they are completely unsuited for use in phase controlled light dimmers. They should beused when switching resistive loads (including incandescent lighting). They are also commonlyused for switching heaters, especially when thermostatically controlled, as there is almost noelectrical noise when the AC is switched as the voltage is close to zero.

Most solid state relays are not suited for use with electronic loads, and thatincludes lighting such as compact fluorescent or LED. In some cases they mightseem to work, but if the mains current waveform is examined you may see currentspikes of several amps occurring every half-cycle - for a single lamp! This will (notmight - will) eventually lead to failure of the lamp, the SSL or both. Electronic loadsshould only ever be switched using electro-mechanical relays, unless testedthoroughly as a complete installation and verified to ensure that operation is safefor both relay and load.

You will no doubt have noticed that there are two prominent notes with regard to solid state relays.These are just two of the things that you have to be very aware of if you decide to include a SSL inyour project. The comments regarding electronic loads are particularly important, and an 'electronicload' is anything that has a bridge rectifier across the mains, then uses a capacitor or active PFCcircuit to create a DC voltage. Virtually all switchmode power supplies meet the definition of anelectronic load, and therefore most cannot be controlled by a SSR unless such usage is specificallypermitted in the datasheet. If it's not mentioned, then assume that it's not allowed. If you choose notto accept that this is true, you will almost certainly damage the load and probably the SSR as well.It's something that's not well documented, poorly understood, rarely tested properly and can causesignificant damage, including the risk of fire.


27 of 35 5/21/2015 9:09 PM

You also need to carefully read through the documentation to make sure that your supply and loadcan never exceed any of the limits described in the datasheets. A momentary over-voltagegenerally won't cause the contacts of a standard relay the slightest pain, and even short-termexcess current is usually not a problem. With a solid state relay, no limiting value can be exceeded... ever. You also have to ensure that the voltage and/or current don't change too fast, becauseSCRs and TRIACs have defined limits, known as DV/DT (critical change of voltage over time) andDI/DT (critical change of current over time). If either is exceeded, the device may turn onunexpectedly. You will also see these terms written as V/t and I/t.The maximum peak voltage can't be exceeded either, and woe betide you if the load draws morethan the rated peak current. You also have to use a heatsink if the load current would otherwisecause the temperature to rise above the rated maximum (typical junction temperature might bearound 100C). There are many disadvantages, but sometimes there is no choice. For example,you can't use a mechanical relay in a 'phase-cut' dimmer because it can't act quickly enough. Youalso can't ensure that a mechanical relay switches on at a particular phase angle of the ACwaveform - for example the ideal for an inductive load is to apply power at the peak of the ACwaveform. This is easily done with a SSR.

Figure 14 - MOSFET Relay

The MOSFET relay shown above is based on the one described in the article MOSFET Relays.There are several types, including those intended for DC operation, but the one shown is a fairlycommon arrangement. Exact details will differ, but the general principles are the same. Somephoto-voltaic couplers have the turn-off circuit (R2 + Q1) inbuilt, and it's needed because theMOSFET gates have extremely high impedance and significant capacitance. Without the turn-offcircuit the MOSFET could remain (partially) conducting for several seconds after LED current isremoved. Because the photo-voltaic cells have very limited output current, turn-on time may bemuch slower than expected.

The same principles are also used with a pair of IGBTs. These are useful for very high power orhigh voltage applications. IGBTs can also be used in DC solid state relays where a MOSFET maybe unable to give the required performance. There are countless possibilities with semiconductordevices, but all components have limitations, and it can be difficult to make the right decision whenthere are so many variables. IGBT based SSRs are even available as miniature low current devices(around 1A), and the PVX6012 is an example if you want to run a search for the datasheet. It'sworth reading, if only to see how they are made and see some specifications. They are non-linearand are unsuitable for switching signal voltages.


28 of 35 5/21/2015 9:09 PM

It's worth looking at the (generalised) advantages and disadvantages of semiconductor compared toelectro-mechanical relays.

Advantages ...

Some have a smaller case allowing more devices per unit volume, but if a heatsink is neededthis advantage goes awayNo contacts, so no arcing, and can be used in hazardous environmentsIncreased lifetime regardless of number of switching cycles. No moving parts to wear outSilent operation (no audible noise)Much faster than electromechanical relays, and their switching time is in the order ofmicrosecondsNo contact bounceDC versions can break high voltage and/or high current that would cause a melt-down withcontactsLess sensitive to mechanical shock, vibration, humidity and external magnetic fieldsSensitive input circuit means low drive power is needed for operation

Disadvantages ...

Most are limited to '1 Form-A' - one normally open contactVoltage & current characteristics of semiconductors rather than mechanical contactsHigher internal impedance when 'closed', generating heatRelatively high voltage dependent leakage current when 'open'Waveform distortion due to non-linear voltage and current characteristicsSome SSRs have polarity-sensitive switching devicesSCR and TRIAC relays generally cannot be used with DC (they can't be turned off)May switch randomly due to voltage transientsLike most semiconductors, SSRs are likely to fail short circuitSensitive input circuit means that noise transients may cause unexpected operation

The inability of most SSRs to provide changeover contacts or multiple sets of contacts can be aserious limitation, and can also increase costs significantly. It costs very little to add another set ofcontacts to an electro-mechanical relay, but with the SSR you need an extra high current switchingdevice, and an improved driver to suit. In most cases if you need a circuit to be normally closed withpower off then you're probably out of luck. Such things do exist, but I've never come across oneother than in datasheets.

Although solid state relays offer some worthwhile advantages, they have many limitations that willnegate their use in a great many applications. Especially if you need multiple contacts orchangeover (double throw), then you will have difficulty finding what you need and it will almostcertainly be far more expensive than a standard electro-mechanical relay. In some cases it will besimpler and cheaper to make your own SSR using a suitable opto-isolator and SCR, TRIAC orMOSFET.

One area where MOSFET and IGBT based SSRs excel is interrupting high voltage, high currentDC, which is fundamentally evil. At voltages over around 30

Documents

Relay and how to use term