Inductor and Transformers

7/29/2019 Inductor and Transformers

1/17

How Inductors WorkAn inductor is about as simple as an electronic component can get -- it is simply a coil of

wire. It turns out, however, that a coil of wire can do some very interesting things

because of the magnetic properties of a coil.

Examples of Coil are :


2/17

The Basics : In a circuit diagram, an inductor is shown like this:

To understand how an inductor can work in a circuit, this figure is helpful:

What you see here is a battery, a light bulb, a coil of wire around a piece of iron (yellow)

and a switch. The coil of wire is an inductor. After you have read How ElectromagnetsWork topic written below here , you might recognize that the inductor is an

electromagnet.


3/17

If you were to take the inductor out of this circuit, what

you would have is a normal flashlight. You close theswitch and the bulb lights up. With the inductor in the

circuit as shown, the behavior is completely different.

The light bulb is a resistor (the resistance creates heat to

make the filament in the bulb glow -- see How LightBulbs Work for details). The wire in the coil has much

lower resistance (it's just wire), so what you would

expect when you turn on the switch is for the bulb toglow very dimly. Most of the current should follow the

low-resistance path through the loop. What happens

instead is that when you close the switch, the bulb burns

brightly and then gets dimmer. When you open the

switch, the bulb burns very brightly and then quicklygoes out.

The reason for this strange behavior is the inductor. When current first

starts flowing in the coil, the coil wants to build up a magnetic field.

While the field is building, the coil inhibits the flow of current. Once

the field is built, current can flow normally through the wire. When

the switch gets opened, the magnetic field around the coil keeps

current flowing in the coil until the field collapses. This current

keeps the bulb lit for a period of time even though the switch is open.

In other words, an inductor can store energy in its magnetic field,and an inductor tends to resist any change in the amount of current

flowing through it. What is inductance?

The property of inductance might be described as "when any piece of wire is wound into

a coil form it forms an inductance which is the property of opposing any change incurrent". Alternatively it could be said "inductance is the property of a circuit by which

energy is stored in the form of an electromagnetic field".

We said a piece of wire wound into a coil form has the ability to produce a counter emf

(opposing current flow) and therefore has a value of inductance. The standard value of

inductance is the Henry, a large value which like the Farad for capacitance is rarelyencountered in electronics today. Typical values of units encountered are milli-henries

mH, one thousandth of a henry or the micro-henry uH, one millionth of a henry.

Think about water...One way to visualize the actionof an inductor is to imagine anarrow channel with waterflowing through it, and a heavywater wheel that has its paddlesdipping into the channel. Imaginethat the water in the channel isnot flowing initially.

Now you try to start the waterflowing. The paddle wheel will

tend to prevent the water fromflowing until it has come up tospeed with the water. If you thentry to stop the flow of water in thechannel, the spinning waterwheel will try to keep the watermoving until its speed of rotationslows back down to the speed ofthe water. An inductor is doingthe same thing with the flow ofelectrons in a wire -- an inductorresists a change in the flow ofelectrons.


4/17

A small straight piece of wire exhibits inductance (probably a fraction of a uH) although

not of any major significance until we reach UHF frequencies.

The value of an inductance varies in proportion to the number of turns squared. If a coilwas of one turn its value might be one unit. Having two turns the value would be four

units while three turns would produce nine units although the length of the coil alsoenters into the equation.


5/17

Henries : The capacity of an inductor is controlled by four factors:

The number of coils - More coils means more inductance.

The material that the coils are wrapped around (the core)

The cross-sectional area of the coil - More area means more inductance. The length of the coil - A short coil means narrower (or overlapping) coils, which

means more inductance.

Putting iron in the core of an inductor gives it much more inductance than air or any non-magnetic core would.

The standard unit of inductance is the henry. The equation for calculating the number of

henries in an inductor is:

H = (4 * Pi * #Turns * #Turns * coil Area * mu) / (coil Length * 10,000,000)

The area and length of the coil are in meters. The term mu is the permeability of the

core. Air has a permeability of 1, while steel might have a permeability of 2,000.

Applications : Let's say you take a coil of wire perhaps 6 feet (2 meters) indiameter, containing five or six loops of wire. You cut some grooves in a road and placethe coil in the grooves. You attach an inductance meter to the coil and see what the

inductance of the coil is.

Now you park a car over the coil and check the inductance again. The inductance will be

much larger because of the large steel object positioned in the loop's magnetic field. Thecar parked over the coil is acting like the core of the inductor, and its presence changes

the inductance of the coil. Most traffic light sensors use the loop in this way. The sensor

constantly tests the inductance of the loop in the road, and when the inductance rises itknows there is a car waiting!

Usually you use a much smaller coil. One big use of inductors is to team them up with

capacitors to create oscillators.

Inductance formula

The standard inductance formula for close approximation - imperial and metric is:

imperial measurements

L = r2 X N2 / ( 9r + 10len )


6/17

where:

L = inductance in uH

r = coil radius in inchesN = number of turns

len = length of the coil in inches

metric measurements

L = 0.394r2 X N2 / ( 9r + 10len )

where:

L = inductance in uH

r = coil radius in centimetresN = number of turns

len = length of the coil in centimetres

[ADDED 22nd May, 2002] Someone asked about a formula which takes into account thespacing bewtween windings, the 10len above automatically takes that into account, if

you're confused think about it!.

High "Q" Inductance formula

It has been found that the optimum dimensions for a high "Q" air core inductor is where

the length of the coil is the same as the diameter of the coil. A simplified formula for

inductance has been derived to establish the required number of turns for a giveninductance value.

metric measurements

N = SQRT [( 29 * L ) / (0.394r)]

where:

L = inductance in uHr = coil radius in centimetres

N = number of turns

Solenoid Inductors

Coils wound on a former (with or without a core) may have multilayers of windingswhich are called solenoid windings.

Self Resonant Frequency of an Inductance

All coils also exhibit a degree of self-capacitance caused by minute capacitances building

up around and between adjacent windings.


7/17

Depending upon the application this may be of considerable concern. This self-

capacitance combined with the natural inductance will form a resonant circuit (self-

resonant frequency) limiting the useful upper frequency of the coil. There are specialwinding techniques to to use on occassion to minimise this self capacitance.

Iron Cores

If the coil is wound on an iron core the inductance is greatly increased and the magnetic

lines of force increase proportionally. This is the basis of electro-magnets used in

solenoid valves and relays.

Power Transformers

When the coil is wound on special iron laminations or cores and a second winding is

placed on the core a "transformer" results. This is the basis of all power transformersalthough only alternating current (a.c.) can be transformed. The voltage relationship in

transformers is proportional to the turns. For example a power transformer might have2,500 turns on the primary side and the secondary side might have 126 turns. Such arelationship is 250 : 12.6 and if the primary were connected to 250V a.c. the secondary

would produce a voltage of 12.6V a.c.

Interesting, if the core size and the wire diameter on the primary supported a primarycurrent of 100 mA, the the primary power available would be 250V X 100 mA or 250 X

0.1 = 25 watts. Ignoring core and copper losses we could say that 25 watts is nowavailable on the secondary side at 12.6V which is 25W / 12.6V = 1.98 amps. In practice

we don't get that kind of efficiency however it would pay to remember that most power

transformers are designed to function most efficient at or near full design load.

R.F. Transformers

In many radio applications the coil is wound on a ferrite or powdered iron core. Typical

examples are the ferrite rod receiving antenna used in cheap transistor radios or the i.f.

transformers enclosed in metal cans in those radios - red, yellow, black, green cores. Thecore is manufactured to be optimum for the frequency range of interest and greatly

enhances the inductance for a specific number of turns. If we wound a coil on a blank

former we might get an inductance of say 10 uH, adding a specific core might increasethe inductance to 47 uH. By using screw in / screw out cores (as in the metal cans) we

can vary the inductance over a fair range of interest.


8/17

How Electromagnets WorkThe basic idea behind an electromagnet is extremelysimple: By running electric current through a wire, you

can create a magnetic field.

By using this simple principle, you can create all sorts ofthings, including motors, solenoids, read/write heads forhard disks and tape drives, speakers, and so on. In this

article, you will learn exactly how electromagnets work.

You will also have the chance to try several experimentswith an electromagnet that you create yourself!

A Regular Magnet : Before talking about electromagnets, let's talk aboutnormal "permanent" magnets like the ones you have on your refrigerator and that you

probably played with as a kid.

You likely know that all magnets have two ends, usually marked "north" and "south," and

that magnets attract things made of steel or iron. And you probably know the

fundamental law of all magnets: Opposites attract and likes repel. So, if you have two

bar magnets with their ends marked "north" and "south," the north end of one magnet willattract the south end of the other. On the other hand, the north end of one magnet will

repel the north end of the other (and similarly, south will repel south).

An electromagnet is the same way, except it is "temporary" -- the magnetic field onlyexists when electric current is flowing.


9/17

An Electromagnet : An electromagnet starts with a battery (or some othersource of power) and a wire. What a battery produces is electrons.

If you look at a battery, say at a normal D-cell from a flashlight, you can see that there

are two ends, one marked plus (+) and the other marked minus (-). Electrons collect at thenegative end of the battery, and, if you let them, they will gladly flow to the positive end.

The way you "let them" flow is with a wire. If you attach a wire directly between the

positive and negative terminals of a D-cell, three things will happen:

1. Electrons will flow from the negative side of the battery to the positive side as

fast as they can.

2. The battery will drain fairly quickly (in a matter of several minutes). For thatreason, it is generally not a good idea to connect the two terminals of a battery to

one another directly. Normally, you connect some kind ofload in the middle of

the wire so the electrons can do useful work. The load might be a motor, a light

bulb, a radio or whatever.3. A small magnetic field is generated in the wire. It is this small magnetic field

that is the basis of an electromagnet.


10/17

Magnetic Field : The part about the magnetic field might be a surprise to you, yetthis definitely happens in all wires carrying electricity. You can prove it to yourself with

the following experiment. You will need:

One AA, C or D-cell battery A piece ofwire (If you have no wire around the house, go buy a spool of

insulated thin copper wire down at the local electronics or hardware store. Four-

strand telephone wire is perfect -- cut the outer plastic sheath and you will findfour perfect wires within.)

A compass

Put the compass on the table and, with the wire near thecompass, connect the wire between the positive and

negative ends of the battery for a few seconds. What you

will notice is that the compass needle swings. Initially,

the compass will be pointing toward the Earth's northpole (whatever direction that is for you), as shown in the

figure on the right. When you connect the wire to the

battery, the compass needle swings because the needle isitself a small magnet with a north and south end. Being

small, it is sensitive to small magnetic fields. Therefore,

the compass is affected by the magnetic field created in the wire by the flow of electrons.


11/17

The Coil : The figure below shows the shape of the magnetic field around the wire.In this figure, imagine that you have cut the wire and are looking at it end-on. The green

circle in the figure is the cross-section of the wire itself. A circular magnetic fielddevelops around the wire, as shown by the circular lines in the illustration below. The

field weakens as you move away from the wire (so the lines are farther apart as they getfarther from the wire). You can see that the field is perpendicular to the wire and that thefield's direction depends on which direction the current is flowing in the wire. The

compass needle aligns itself with this field (perpendicular to the wire). Using the

contraption you created in the previous section, if you flip the battery around and repeatthe experiment, you will see that the compass needle aligns itself in the opposite

direction.

Magnetic field of a wire

Because the magnetic field around a wire is circular and perpendicular to the wire, an

easy way to amplify the wire's magnetic field is to coil the wire, as shown below:

One loop's magnetic field

For example, if you wrap your wire around a nail 10 times, connect the wire to thebattery and bring one end of the nail near the compass, you will find that it has a much

larger effect on the compass. In fact, the nail behaves just like a bar magnet.


12/17

A simple electromagnet

However, the magnet exists only when the current is flowing from the battery. What you

have created is an electromagnet! You will find that this magnet is able to pick up small

steel things like paper clips, staples and thumb tacks.


13/17

Experiments to Try! :

What is the magnetic power of a single coil wrapped around a nail? Of 10

turns of wire? Of 100 turns? Experiment with different numbers of turns and see

what happens. One way to measure and compare a magnet's "strength" is to seehow many staples it can pick up.

What difference does voltage make in the strength of an electromagnet? If

you hook two batteries in series to get 3 volts, what does that do to the strength ofthe magnet? (Please do not try any more than 6 volts, and please do not use

anything other than flashlight batteries. Please do not try house current coming

from the wall in your house, as it can kill you. Please do not try a car battery, asits current can kill you as well.)

What is the difference between an iron and an aluminum core for the

magnet? For example, roll up some aluminum foil tightly and use it as the corefor your magnet in place of the nail. What happens? What if you use a plastic

core, like a pen? What about solenoids? A solenoid is another form of electromagnet. It is an

electromagnetic tube generally used to move a piece of metal linearly. Find adrinking straw or an old pen (remove the ink tube). Also find a small nail (or a

straightened paperclip) that will slide inside the tube easily. Wrap 100 turns of

wire around the tube. Place the nail or paperclip at one end of the coil and thenconnect the coil to the battery. Notice how the nail moves? Solenoids are used in

all sorts of places, especially locks. If your car has power locks, they may operate

using a solenoid. Another common thing to do with a solenoid is to replace thenail with a thin, cylindrical permanent magnet. Then you can move the magnet in

and out by changing the direction of the magnetic field in the solenoid. (Please be

careful if you try placing a magnet in your solenoid, as the magnet can shoot out.) How do I know there's really a magnetic field? You can look at a wire's

magnetic field using iron filings. Buy some iron filings, or find your own iron

filings by running a magnet through playground or beach sand. Put a light dusting

of filings on a sheet of paper and place the paper over a magnet. Tap the paperlightly and the filings will align with the magnetic field, letting you see its shape!


14/17

Adapters or Battery Eliminators-cum-chargers :

The adapter can either be AC or AC/DC type. The AC adapter basically

only converts the ac voltage available at your power outlet to therequired voltage but that too only AC. Typically, we need to use an ACadapter to convert our 220VAC supply to 110VAC for using an itemimported from USA, because the device is made to work on the supplyof 110VAC which is supplied to every home and office in USA whereaswhat we are supplied is 220VAC. So, if we do not step down our ACsupply voltage from 220VAC to 110 VAC, we are most likely to damagethe device to a great extent.

The other type of Adapter is the one which not only converts the ACvoltage to the required voltage level but also converts these lower AC

voltage into a DC voltage for using it in lieu of a Battery, thats why theword Battery Eliminator !

Also this converted DC voltage can be further purified and regulated byadding a few simple parts like some capacitors and regulator ICs andcan very competently used to charge the rechargeable batteries likethe one being used in Mobile Phones very extensively.

Here's the transformer we will be exploring today:

here is what you find inside:


15/17

What you can see here are two windings. The purpose of a transformer is to convert one

AC voltage to another AC voltage. In this case the transformer converts the normal 120

or 220 volt AC current in your house down to three volts. The 120 or 220 volts comes inon the primary winding on the left. Running down the middle of that winding (as well

as around the outside) is an iron core. The AC current in the primary winding creates an

alternating magnetic field in the iron just as it would in an electromagnet. The otherwinding, known as the secondary winding wraps around the same iron core. In the

secondary winding the magnetic field in the core creates current. The voltage in the

secondary is controlled by the ratio of the number of turns in the two windings. So if the

primary and secondary windings have the same number of turns, the primary andsecondary voltage will be the same. If the secondary winding has half as many turns as

the primary then the voltage in the secondary will be half that of the voltage in the

primary. You can see in the following figure that the primary in this particulartransformer uses very fine wire while the secondary uses much thicker wire. To drop

down to 3 volts, there needs to be 40 times more turns in the primary than in the

secondary.


16/17

Turning the AC current into DC current

On the other side of the transformer you find two diodes wrapped in rubber insulation.

The diodes act as a rectifier, turning the AC current into DC current.


17/17

Most transformer cubes that you find around the house produce a low-voltage DC current(3 to 12 volts, and less than an amp of current). DC current is necessary because

rechargeable batteries store DC current, because most electronics require low-voltage DC

current and because small DC motors run directly from batteries and are the leastexpensive motors available.

Documents

Inductor and Transformers