Exp. No #4 TRANSISTOR AS SWITCH OBJECTIVE The purpose of the experiment is to design and analyze the operation of transistor as switch. Also, to design a suitable driver circuit for a given load using transistor. EQUIPMENT AND COMPONENTS USED 30 MHz Dual Channel Cathode Ray Oscilloscope 3 MHz Function Generator 0-30 V dc dual regulated power supply 4 ½ digit Digital Multimeter Transistor 2N2222, BC 3mm LED Resistors ¼W Breadboard and Connecting wires BNC Cables and Probes THEORY A transistor can function as a single-pole single-throw switch controlled by an electronic signal driving the base terminal. When the control signal on the base terminal turns the transistor off, it acts like an open switch. When the control signal on the base terminal turns the transistor on, it acts like a closed switch. When transistor is used for switching, it is in one of two states: on or off. In the off state, the base bias current is zero and the transistor is cut off. In the on state, the base bias current is set large enough to drive the transistor into saturation. Adding a capacitor in parallel with the base resistor improves the transistor switching speed. FURTHER READING 1. Paul Horowitz and Winfeld Hill, The Art of Electronics, Cambridge University Press, New York, 2nd edition, 1989. 2. James Cox, Fundamentals of Linear Electronics: Integrated and Discrete, Delmar Thomson Learning, 2nd edition, 2001. 3. Robert Boylstad, Louis Nashelsky, Electronic Devices and Circuit Theory, PHI, 2008 39
DESIGN Design a driver circuit to turn on a 3mm LED with maximum forward current of 20mA and forward drop of 2.5V. Consider I L = 20 ma When BJT is saturated, I C = I Csat = I L = 2N2222 switching transistor is selected for this load current. From data sheet, the minimum value of h fe for 2N2222 is 75. With smallest h fe ( ) I B = I Csat / = This is just on the Edge of Saturation (EOS). Consider an Overdrive factor of 10, then I B = ODF x I BEOS = Therefore, I B = V BB V BE / R B R B = V BB V BE / I B Wattage rating of R B = I B x (V BB V BE ) = V CC = I C R C +V LED +V CE I Csat R C = V CC -V LED -V CE R C = (V CC -V LED -V CE ) /I Csat = Wattage rating of transistor = I L x V CE = CIRCUIT DIAGRAM Figure 1 40
PRELAB 1. Design and create a SPICE model of a transistor operating as switch. Analyze the circuit for different values of base current, collector current and collector to emitter voltage. Identify when the transistor operates in saturation mode. 2. From the 2N2222 datasheet, write down the parameters required to design the circuit. 3. From the design values, suggest a suitable transistor for switching action other than 2N2222. State reasons. 4. Simulate the circuit shown in figure2 and observe the transistor switching action. 41
Exp. No. : OBJECTIVE TRANSISTOR AS SWITCH Date: OBSERVATION Circuit Diagram PRACTICE PROCEDURE 1. Transistor as Switch 1. Connect the circuit as shown in Figure 1. 2. Apply 5V, 1 khz square wave to the base. 3. Apply +5V dc voltage to collector. 4. Observe the input and output waveforms simultaneously using oscilloscope. Volt/div = Time/div = Volt/div = Time/div = Graph 1: Input square wave Graph 2: Output Waveform 42
Inference 2. Transistor in Saturation 1. Connect the circuit as shown in Figure 1. 2. Connect the base terminal to ground (0V). 3. Record the base current, collector current and collector-emitter voltage. 4. Apply 5V dc to the base terminal. 5. Record the base current, collector current and collector-emitter voltage. Table V BB I B V BE I C V CE Comments 0V +5V Inference 43
3. Transistor switching circuit 1. Connect the circuit as shown in Figure 2. 2. Observe the switching action of transistor. Figure 1 Inference UNDERSTANDING & LEARNING 44 6
RESULTS AND CONCLUSION Prepared by: Name: Reg. No.: Date of Experiment : ASSESSMENT Date of Report Submission: Submission Delay (if any):... Student Task Max. Marks Graded Marks Pre-lab Preparation 20 Inference 10 Signature Results & Conclusion 10 Post-lab / Viva-voce 10 Total 50 45