TRANSISTOR AS SWITCH

Transcription

1 Exp. No #3 TRANSISTOR AS SWITCH Date: 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. PRELAB 1. Design and create a SPICE model of a transistor operating as switch.. 2. Analyze the circuit for different values of base current, collector current and collector to emitter voltage. Identify when the transistor operates in saturation mode. 3. From the 2N2222 datasheet, write down the parameters required to design the circuit. 4. From the design values, suggest a suitable transistor for switching action other than 2N2222. 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 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, James Cox, Fundamentals of Linear Electronics: Integrated and Discrete, Delmar Thomson Learning, 2nd edition,

2 DESIGN Design a driver circuit to turn on a 3mm LED with maximum forward current of 30mA and forward drop of 2.5V. Consider IL = 20 ma When BJT is saturated, IC = ICsat = IL = 2N2222 switching transistor is selected for this load current. From data sheet, the minimum value of hfe for 2N2222 is 75. With smallest hfe( ) IB = ICsat / = This is just on the Edge of Saturation (EOS). Consider an Overdrive factor of 10, then IB = ODF x IBEOS = Therefore, IB = VBB VBE / RB RB = VBB VBE / IB Wattage rating of RB = IB x (VBB VBE) = VCC = ICRC +VLED +VCE ICsatRC = VCC -VLED -VCE RC = (VCC -VLED -VCE) /ICsat = Wattage rating of transistor = IL x VCE = CIRCUIT DIAGRAM Figure 1 32

3 PRACTICE PROCEDURE 1. Transistor as Switch 1. Connect the circuit as shown in Figure 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 Inference 2. Transistor in Saturation 1. Connect the circuit as shown in Figure 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. 33

4 Table VBB IB VBE IC VCE Comments 0V +5V Inference 34

5 UNDERSTANDING & LEARNING 35

6 RESULTS AND CONCLUSION POST LAB INFERENCE 1. Calculate the value of forced. 2. Compare the theoretical and practical vaues of parameters in cut off and saturation mode. Prepared by: Name: Reg. No.: Experiment Date: ASSESSMENT Report Submission Date: Submission Delay:... Student Task Max. Marks Graded Marks Pre-lab Preparation 15 Performance 10 Signature Observation & Inference 10 Post-lab / Viva-voce 15 Total 50 36