ECE480-App_Note-Fields

8/7/2019 ECE480-App_Note-Fields

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VOLTAGE REGULATORS

A Voltage Regulator (also called a "regulator") has only three legs and appearsto be a comparatively simple device but it is actually a very complex integratedcircuit. A regulator converts varying input voltage and produces a constant

"regulated" output voltage. Voltage regulators are available in a variety ofoutputs, typically 5 volts, 9 volts and 12 volts. The last two digits in the nameindicate the output voltage.

Name Voltage

LM7805 + 5 volts

LM7809 + 9 volts

LM7812 + 12 volts

LM7905 - 5 volts

LM7909 - 9 volts

LM7912 - 12 volts

The "LM78XX" series of voltage regulators are designed for positive input. Forapplications requiring negative input the "LM79XX" series is used.

Symbol for a Voltage Regulator.

This device looks like a Transistor,but it is actually a complex

Integrated Circuit.

A LM7805 Regulator

The circuit diagram below represents a typical use of a voltage regulator.


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The LM7805 can reliably deliver 1 amp at a constant voltage.

How it Works:The transformer drops the 240 volt 'mains' voltage to 8.5 volts. The diode 'bridge'rectifies the 8.5 volts AC from the output side of the power transformer into DC.The 2500uF capacitor helps to maintain a constant input into the regulator.

As a general guide this capacitor should be rated at a minimum of 1000uF foreach amp of current drawn and at least TWICE the input voltage. The 0.1uFcapacitor eliminates any high frequency pulses that could otherwise interfere withthe operation of the regulator.

Voltage regulators are very robust. They can withstand over-current draw due toshort circuits and also over-heating. In both cases the regulator will shut downbefore damage occurs. The only way to destroy a regulator is to apply reversevoltage to its input.

Reverse polarity destroys the regulator almost instantly. To avoid this possibility

you should always use diode protection of the power supply. This is especiallyimportant when using nine volt battery supplies as it is common for people to'test' the battery by connecting it one way and then the other. Even this short'test' could destroy the regulator if a protection diode is not used.

All of the interfaces described on this site have protection diodes connected intothe power supply circuit to prevent damage due to incorrect polarity. Generally a1N4004, 1 amp power diode is connected in series with the power supply. If thesupply is connected the wrong way around, the regulator will be protected fromdamage.

Input Voltage:As a general rule the input voltage should be limited to 2 to 3 volts above theoutput voltage. The LM78XX series can handle up to 30 volts input, but thepower difference between the input voltage/current ratio and outputvoltage/current ratio appears as heat. If the input voltage is unnecessarily highthe regulator will get very hot. Unless sufficient heat-sinking is provided theregulator will shut down.


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Transistor Circuits

This page explains the operation of transistors in circuits. Practical matters such as testing,precautions when soldering and identifying leads are covered by the Transistorspage.

General: Types | Currents|Functional model | Darlington pairSwitching: Introduction|Use relay?|Chip output | for NPN|and PNP| Sensors|Inverter

Next Page: Analogue and Digital SystemsAlso See: Transistors(soldering, lead identification)

Types of transistor

There are two types of standard transistors, NPN andPNP, with different circuit symbols. The letters referto the layers of semiconductor material used to makethe transistor. Most transistors used today are NPNbecause this is the easiest type to make from silicon.This page is mostly about NPN transistors and if youare new to electronics it is best to start by learninghow to use these first.

The leads are labelled base (B), collector(C) and emitter(E).These terms refer to the internal operation of a transistor but they are not much help in

understanding how a transistor is used, so just treat

In addition to standard (bipolar junction)transistors, there are field-effect transistorswhich are usually referred to as FETs. Theyhave different circuit symbols and propertiesand they are not (yet) covered by this page.

Transistor circuit symbols
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Transistor currents

The diagram shows the two current paths through a transistor. You can build thiscircuit with two standard 5mm red LEDs and any general purpose low powerNPN transistor (BC108, BC182 or BC548 for example).

The small base current controls the largercollector current.

When the switch is closed a small current flows into the base (B) of thetransistor. It is just enough to make LED B glow dimly. The transistor amplifiesthis small current to allow a larger current to flow through from its collector (C) toits emitter (E). This collector current is large enough to make LED C light brightly.

When the switch is open no base current flows, so the transistor switches offthe collector current. Both LEDs are off.

A transistor amplifies current and can be used as a switch.

This arrangement where the emitter (E) is in the controlling circuit (base current) and in thecontrolled circuit (collector current) is called common emitter mode. It is the most widely usedarrangement for transistors so it is the one to learn first.

Functional model of an NPN transistor


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The operation of a transistor is difficult to explain and understand in terms of itsinternal structure. It is more helpful to use this functional model:

The base-emitter junction behaves like a diode. A base current IB flows only when the voltage VBE across the base-emitter

junction is 0.7V or more. The small base current IB controls the large collector current Ic. Ic = hFE IB (unless the transistor is full on and saturated)

hFE is the current gain (strictly the DC current gain), a typical value for hFEis 100 (it has no units because it is a ratio)

The collector-emitter resistance RCE is controlled by the base current IB:o IB = 0 RCE = infinity transistor off

o IB small RCE reduced transistor partly on

o IB increased RCE = 0 transistor full on ('saturated')

Additional notes:

A resistor is often needed in series with the base connection to limit thebase current IB and prevent the transistor being damaged.

Transistors have a maximum collector current Ic rating. The current gain hFE can vary widely, even for transistors of the same

type! A transistor that is full on (with RCE = 0) is said to be 'saturated'. When a transistor is saturated the collector-emitter voltage VCE is reduced

to almost 0V. When a transistor is saturated the collector current Ic is determined by the

supply voltage and the external resistance in the collector circuit, not by

the transistor's current gain. As a result the ratio Ic/IB for a saturatedtransistor is less than the current gain hFE. The emitter current IE = Ic + IB, but Ic is much larger than IB, so roughly IE =

Ic.

There is a table showing technical data for some popular transistors on thetransistors page.
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Darlington pair

This is two transistors connected together so thatthe current amplified by the first is amplifiedfurther by the second transistor. The overall

current gain is equal to the two individual gainsmultiplied together:

Darlington pair current gain, hFE = hFE1 hFE2(hFE1 and hFE2 are the gains of the individualtransistors)

This gives the Darlington pair a very high currentgain, such as 10000, so that only a tiny basecurrent is required to make the pair switch on.

A Darlington pair behaves like a singletransistor with a very high current gain. It hasthree leads (B, C and E) which are equivalent to the leads of a standardindividual transistor. To turn on there must be 0.7V across both the base-emitter

junctions which are connected in series inside the Darlington pair, therefore itrequires 1.4V to turn on.

Darlington pairs are available as complete packages but you can make up yourown from two transistors; TR1 can be a low power type, but normally TR2 willneed to be high power. The maximum collector current Ic(max) for the pair is thesame as Ic(max) for TR2.

A Darlington pair is sufficiently sensitive to respond to the small current passedby your skin and it can be used to make a touch-switch as shown in thediagram. For this circuit which just lights an LED the two transistors can be anygeneral purpose low power transistors. The 100k resistor protects thetransistors if the contacts are linked with a piece of wire.

Touch switch circuit


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Using a transistor as a switch

When a transistor is used as a switch it must be eitherOFF orfully ON. In thefully ON state the voltage VCE across the transistor is almost zero and thetransistor is said to be saturated because it cannot pass any more collector

current Ic. The output device switched by the transistor is usually called the'load'.

The power developed in a switching transistor is very small:

In the OFF state: power = Ic VCE, but Ic = 0, so the power is zero. In the full ON state: power = Ic VCE, but VCE = 0 (almost), so the power is

very small.

This means that the transistor shouldnot become hot in use and you do notneed to consider its maximum powerrating. The important ratings inswitching circuits are the maximumcollector current Ic(max) and theminimum current gain hFE(min). Thetransistor's voltage ratings may beignored unless you are using a supply

voltage of more than about 15V. Thereis a table showing technical data for some popular transistors on the transistorspage.

For information about the operation of a transistor please see the functionalmodel above.

Protection diode

If the load is a motor, relay orsolenoid (or any other device with a coil) a diode

must be connected across the load to protect the transistor (and chip) fromdamage when the load is switched off. The diagram shows how this is connected'backwards' so that it will normally NOT conduct. Conduction only occurs whenthe load is switched off, at this moment current tries to continue flowing throughthe coil and it is harmlessly diverted through the diode. Without the diode nocurrent could flow and the coil would produce a damaging high voltage 'spike' inits attempt to keep the current flowing.
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When to use a relay

Transistors cannot switch AC or high voltages(such as mains electricity) and they are not usuallya good choice for switching large currents (> 5A).

In these cases a relay will be needed, but note thata low power transistor may still be needed toswitch the current for the relay's coil!

Advantages of relays:

Relays can switch AC and DC, transistors can onlyswitch DC.

Relays can switch high voltages, transistors cannot.

Relays are a better choice for switching large currents (> 5A).

Relays can switch many contacts at once.

Disadvantages of relays:

Relays are bulkierthan transistors for switching small currents.

Relays cannot switch rapidly, transistors can switch many times per second.

Relays use more powerdue to the current flowing through their coil.

Relays require more current than many chips can provide, so a low power transistormay be needed to switch the current for the relay's coil.

Connecting a transistor to the output from a chip

Most chips cannot supply large output currents so it may be necessary to use atransistor to switch the larger current required for output devices such as lamps,motors and relays. The 555 timer chip is unusual because it can supply arelatively large current of up to 200mA which is sufficient for some output devicessuch as low current lamps, buzzers and many relay coils without needing to usea transistor.

A transistor can also be used to enable a chip connected to a low voltage supply(such as 5V) to switch the current for an output device with a separate higher

voltage supply (such as 12V). The two power supplies must be linked, normallythis is done by linking their 0V connections. In this case you should use an NPNtransistor.

A resistor RB is required to limit the current flowing into the base of the transistorand prevent it being damaged. However, RB must be sufficiently low to ensurethat the transistor is thoroughly saturated to prevent it overheating, this isparticularly important if the transistor is switching a large current (> 100mA). A

Relays

Photographs Rapid Electronics
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safe rule is to make the base current IB about five times larger than the valuewhich should just saturate the transistor.

Choosing a suitable NPN transistor

The circuit diagram shows how to connectan NPN transistor, this will switch on theload when the chip output is high. If youneed the opposite action, with the loadswitched on when the chip output is low(0V) please see the circuit for aPNP transistor below.

The procedure below explains how tochoose a suitable switching transistor.

1. The transistor's maximum collectorcurrent Ic(max) must be greater thanthe load current Ic.

load current Ic =supply voltage Vs

load resistance RL

2. The transistor's minimum current gain hFE(min) must be at least five timesthe load current Ic divided by the maximum output current from the chip.

hFE(min) > 5 load current Ic

max. chip current

3. Choose a transistorwhich meets these requirements and make a note ofits properties: Ic(max) and hFE(min).There is a table showing technical data for some popular transistors on the transistorspage.

4. Calculate an approximate value for the base resistor:

RB =Vc hFE

5 Ic

where Vc = chip supply voltage(in a simple circuit with one supply this is Vs)

5. For a simple circuit where the chip and the load share the same power supply (Vc = Vs)

you may prefer to use: RB = 0.2 RL hFE6. Then choose the nearest standard value for the base resistor.7. Finally, remember that if the load is a motor or relay coil a protection diode

is required.

ExampleThe output from a 4000 series CMOS chip is required to operate a relay with a 100 coil.

NPN transistor switch(load is on when chip output is high)

Using units in calculationsRemember to use V, A and orV, mA and k . For more detailsplease see the Ohm's Law page.
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The supply voltage is 6V for both the chip and load. The chip can supply a maximum current of5mA.

1. Load current = Vs/RL = 6/100 = 0.06A = 60mA, so transistor must have Ic(max) > 60mA.

2. The maximum current from the chip is 5mA, so transistor must have hFE(min) > 60(5 60mA/5mA).

3. Choose general purpose low power transistorBC182 with Ic(max) = 100mA andhFE(min) = 100.

4. RB = 0.2 RL hFE = 0.2 100 100 = 2000 . so choose RB = 1k8 or 2k2.

5. The relay coil requires a protection diode.

Diodes:


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