The Transistor 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

 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

 Given the supply voltage and the lamp resistance, we need to find the base resistance R B that will turn the transistor ON In order to determine R B, perform the following steps 1.Determine the required collector current 2.Determine the value of β 3.Calculate the required value of I B 4.Calculate the value of R B NB : In calculations, the voltage drop across the base emitter of the transistor V BE (which is 0.7V is only taken into account if the source voltage is > 10 V

 Assume V S = 28 V, lamp requires 50 mA and β = 75 I B = I C / β = 50 mA / 75 =2/3 mA R B = 28 / (2/3) = 42 kΩ

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

1. A 10 V lamp that draws 10 mA. β = A 5 V lamp that draws 100 mA. β = 50

1. A 10 V lamp that draws 10 mA. β = 100 I B = 10 mA / 100 = 0.1 mA R B = 10 / 0.1 mA = 100 k Ω 1. A 5 V lamp that draws 100 mA. β = 50 I B = 100 mA / 50 = 2 mA R B = (5 – 0.7) / 2mA = 4.3 / 2mA = 2.15 kΩ

 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

 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

 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

 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

 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

 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

 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

1.What effect does I B1 have on transistor Q 1 ? 2.What effect does turning Q 1 ON have on a)Collector current I C1 ? b)Collector Voltage V C1 ? 3.Where does the current through R 3 go? 4.In this circuit is the lamp on or off?

 1.I B1 along with a portion of Vs (0.7V for a silicon transistor) turns Q 1 ON  2. a)I C1 flowsb) V C1 drops to 0 V  3. I C1 flows through Q 1 to ground  4.Off

1.How much base current I B1 flows into Q 1 ? 2.Is Q 1 ON or OFF? 3.What current flows through R 3 ? 4.Is Q 2 ON or OFF? 5.Is the lamp ON or OFF?

 1.None  2.OFF  3. IB2  4.ON  5. ON

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

1.Determine load current IC2 2.Determine β for Q2. Call this β2 3.Calculate IB2 for Q2. Use IB2 = IC2/ β 2 4.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 Q1 7.Calculate the base current for Q1. IB1 = IC1 / β1 8.Find R1. R1 = Vs / IB1 9.Choose R2. For convenience R2 = R1

1. Find IB2 2. Find R3 3. Calculate the load current for Q1 when it is ON 4. Find the base current for Q1 5. Find R1 6. 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

1. Given IC2 = 1A and β2=20, IB2 = 1 /20 = 50 mA 2. R3 = 10 / 50 mA = 200 Ω 3. Ic1 (load current) = IB2 = 50 mA 4. β 1 = 100, IB1 = 5o mA /100 = 0.5 mA 5. R1 = 10 / 0.5 mA = 20 k Ω 6. For convenience R2 is the same as R1 (20 k Ω)

 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

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?

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?

Steps 1. Find the load current 2. Calculate I B3 given the value of β 3. I B3 = I C2 3. Calculate R 4 = V / I B3 4. Calculate I B2 given β 2. I C1 = I B2 5. Calculate R3 = Vs / I B2 6. Calculate I B1 given the value of β 1 7. Calculate R 1 = Vs / I B1 8. Choose R 2 = R 1 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.

Steps 1. Load Current I C3 = 10 A 2. I B3 = I C3 / β 3 =10/20 = 0.5A = I C2 3. R 4 = V / I B3 = 10 /0.5 = 20 Ω 4. I B2 = I C2 / β 2 =0.5/50 =10 mA= I C1 5. Calculate R3 = Vs / I B2 = 10/10mA = 1 k Ω 6. Calculate I B1 = IC1 /β 1 = 1omA / 100 = 0.1 mA 7. R 1 = Vs / I B1 = 10 /0.1 mA = 100 k Ω 8. Choose R 2 = R 1 = 100 k Ω 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.

Calculate the values of R1, R2, R3 and R4 given that R5 is a 75V lamp that draws 6 A. Assume that β1= 120, β2=100 and β3=30. Steps 1. Find the load current 2. Calculate I B3 given the value of β 3. I B3 = I C2 3. Calculate R 4 = V / I B3 4. Calculate I B2 given β 2. I C1 = I B2 5. Calculate R3 = Vs / I B2 6. Calculate I B1 given the value of β 1 7. Calculate R 1 = Vs / I B1 8. Choose R 2 = R 1

 R4 = 375 Ω  R3 = 37.5 k Ω  R1 = 4.5 M Ω  R2 = 1 M Ω