the transistor switch. computers work with boolean algebra – two logic states – true or false ...

TEC 284The Transistor Switch

The Transistor as a Switch?

Computers work with Boolean algebra – two logic states – TRUE or FALSE

These states can be easily represented electronically by a transistor that is ON or OFF

Logic portions of microprocessors consist entirely of transistor switches

Turning a Transistor ON

In the circuit, a lamp can be substituted for a collector resistor

Rc, the resistance of the lamp is referred to as the load

Ic, the current through the lamp is referred to as the load current

When the transistor is turned on, the collector voltage is 0 ( there is actually a small voltage drop between the collector and the emitter – saturation voltage)

We will consider this negligible and assume the collector voltage to be 0 for a transistor that is ON

Calculating RB needed to turn a transistor ON

Given the supply voltage and the lamp resistance, we need to find the base resistance RB that will turn the transistor ON

In order to determine RB, perform the following steps1. Determine the required collector

current2. Determine the value of β3. Calculate the required value of IB

4. Calculate the value of RB

NB : In calculations, the voltage drop across the base emitter of the transistor VBE (which is 0.7V is only taken into account if the source voltage is > 10 V

Calculations

Assume VS = 28 V, lamp requires 50 mA and β = 75

• IB = IC / β = 50 mA / 75 =2/3 mA

• RB = 28 / (2/3) = 42 kΩ

Question

Assume that Vs = 9 V and that the lamp requires 50 mA and β = 75, find the base resistance RB needed to turn on the transistor. In this case since VS < 10, the VBE drop of 0.7 V has to be taken into account

Calculate RB for the following problems

1. A 10 V lamp that draws 10 mA. β = 100

2. A 5 V lamp that draws 100 mA. β = 50

Answers

1. A 10 V lamp that draws 10 mA. β = 100IB = 10 mA / 100 = 0.1 mARB = 10 / 0.1 mA = 100 k Ω

2. A 5 V lamp that draws 100 mA. β = 50IB = 100 mA / 50 = 2 mARB = (5 – 0.7) / 2mA = 4.3 / 2mA = 2.15 kΩ

Turning a transistor OFF

When a transistor is turned OFF, it acts like an OPEN mechanical switch

When it is turned ON, it acts like a CLOSED mechanical switch

A transistor is turned off when no base current flows

Turning a transistor OFF

You can be sure there is no base current in the circuit to the left by opening the mechanical switch

To ensure that a transistor remains off when it is not connected to the supply voltage, add a resistor to the circuit (R2)

The base of the resistor is connected to ground or 0 V through the resistor

No base current can possibly flow This resistor should be between 1

kΩ and 1MΩ

Why Transistors are used as switches Operating equipment in a

dangerous environment Turning a lamp on in a dangerous

environment e.g. radioactive chamber

A switch can be used outside the chamber

In mobile devices (e.g. radio controlled airplane) using switches minimizes power, weight and bulk required

If a switch controls equipment that requires a large amount of current, a transistor switch can be turned on an off using small, low voltage wires to control the larger current flow If the switch is located some distance from

the equipment that requires large current, this can save time and money

Why Transistors are used as switches

Because switching a transistor on and off can be controlled by an electrical signal, it can be controlled very accurately Mechanical devices are not as accurate This is important in photography where

an object is illuminated for a precise period of time

A transistor can be switched on and off millions of times a second and will last for many years Transistors are one of the longest lasting

and most reliable components known

Why Transistors are used as switches

Signals generated by most control devices are digital (high or low voltage) and are ideally suited for turning transistors on and off

Manufacturing techniques allow miniaturization of transistors Millions of them can fit on a single chip Electronic devices continue to get

smaller and lighter

Multiple Transistor Switch

Switch is in position A IB1 flows through the

base of Q1 and transistor Q1 is turned ON

Collector current IC1 flows causing the base of the transistor Q2 to be 0V

Q2 is thus OFF and no current flows through the lamp

Multiple Transistor Switch

Switch is in position B No base current flows

through Q1 IB2 flows through the

base of Q2 and transistor Q2 is turned ON

Collector current IC2 flows causing the lamp to turn ON

Multiple Transistor Switch Analysis

1. What effect does IB1 have on transistor Q1?

2. What effect does turning Q1 ON have on a) Collector current IC1?b) Collector Voltage

VC1?3. Where does the current

through R3 go?4. In this circuit is the lamp

on or off?

Multiple Transistor Answers

1. IB1 along with a portion of Vs (0.7V for a silicon transistor) turns Q1 ON

2. a)IC1 flows b) VC1 drops to 0 V

3. IC1 flows through Q1 to ground4. Off

Multiple Transistor Switch Analysis

1. How much base current IB1 flows into Q1?

2. Is Q1 ON or OFF?3. What current flows

through R3?4. Is Q2 ON or OFF?5. Is the lamp ON or OFF?

Multiple Transistor Answers

1. None2. OFF3. IB24. ON5. ON

Two transistor switch calculations

For the following circuit calculate the values of R1, R2 and R3 that are required to operate the lamp. How do we accomplish this?

Steps for the calculation

1. Determine load current IC22. Determine β for Q2. Call

this β23. Calculate IB2 for Q2. Use

IB2 = IC2/ β 24. Calculate R3 to provide the

base current (Vs / IB2)5. R3 will have the same

current as the base current for Q2

6. Calculate β1, the β for Q17. Calculate the base current

for Q1. IB1 = IC1 / β18. Find R1. R1 = Vs / IB19. Choose R2. For

convenience R2 = R1

Question

1. Find IB22. Find R33. Calculate the load current for Q1 when it

is ON4. Find the base current for Q15. Find R16. Choose a suitable value for R2

• Given that a 10 V lamp draws 1A and that • Vs = 10V• IC2 = 1 A• β1 = 100• β2 = 20

• Find R1, R2 and R3

Answers

1. Given IC2 = 1A and β2=20, IB2 = 1 /20 = 50 mA

2. R3 = 10 / 50 mA = 200 Ω3. Ic1 (load current) = IB2 = 50 mA4. β 1 = 100, IB1 = 5o mA /100 = 0.5

mA5. R1 = 10 / 0.5 mA = 20 k Ω 6. For convenience R2 is the same as

R1 (20 k Ω)

The Three-Transistor Switch

This circuit uses three transistors to switch a load on and off

Q1 is used to turn on Q2 ON and OFF and Q2 is used to turn Q3 ON and OFF

The calculations are similar to the two transistor switch but an extra step is introduced

Questions

With the switch in position A

1. Is Q1 ON or OFF?2. Is Q2 ON or OFF?3. Where is the

current through R4 flowing?

4. Is Q3 ON or OFF?

Questions

With the switch in position A

1. Is Q1 ON or OFF?2. Is Q2 ON or OFF?3. Where is the

current through R4 flowing?

4. Is Q3 ON or OFF?5. Which switch

position turns the lamp ON?

6. How do the ON/OFF positions for the switch in the three-transistor switch differ from the ON/OFF positions for the two-transistor switch?

Determining resistor values

Steps1. Find the load

current2. Calculate IB3 given

the value of β3. IB3 = IC2

3. Calculate R4 = V / IB3

4. Calculate IB2 given β2. IC1 = IB2

5. Calculate R3 = Vs / IB2

6. Calculate IB1 given the value of β1


8. Choose R2 = R1

Calculate the values of R1, R2, R3 and R4 given that R5 is a 10V lamp that draws 10 A. Assume that β1= 100, β2=50 and β3=20.

Answers

Steps1. Load Current IC3 = 10 A2. IB3 = IC3 / β3 =10/20 = 0.5A = IC2

3. R4 = V / IB3 = 10 /0.5 = 20 Ω4. IB2 = IC2 / β2 =0.5/50 =10 mA= IC1

5. Calculate R3 = Vs / IB2 = 10/10mA = 1 k Ω

6. Calculate IB1 = IC1 /β1 = 1omA / 100 = 0.1 mA

7. R1 = Vs / IB1 = 10 /0.1 mA = 100 k Ω

8. Choose R2 = R1 = 100 k Ω


Question


Steps1. Find the load

current2. Calculate IB3 given

the value of β3. IB3 = IC2

3. Calculate R4 = V / IB3

4. Calculate IB2 given β2. IC1 = IB2


6. Calculate IB1 given the value of β1


8. Choose R2 = R1

Answers

R4 = 375 ΩR3 = 37.5 k ΩR1 = 4.5 M ΩR2 = 1 M Ω

the transistor switch. computers work with boolean algebra – two logic states – true or false ...

Documents