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Esc 102 Introduction to Electronics, 2004-2005/I Experiment 3 Verification of Network Theorems Aim of the Experiment The aim of the experiment is to verify the Superposition and Thevenin’s theorems in the laboratory. You will use two circuits containing DC sources only and another one containing both DC and AC sources. These circuits do not have any dependent sources and are hence simple ones. In this experiment you are given the freedom to do the experiment yourselves. You will be given the circuit elements, and a Digital Multimeter (DMM). DC voltage sources are available on the board. For the DC networks (Networks 1 and 2), you should use the DMM for your measurements. For Network-3 containing an AC source, you should use the CRO for voltage measurements. General Instructions and Precautions 1. DMM is an extremely useful general purpose Digital instrument, used for measuring voltages (AC and DC), currents (AC and DC), and resistances. Most DMMs have some extra functions, such as Diode check, Transistor check, etc. 2. Familiarize yourself with the DMM given to you. You will use it to measure the resistor values and node voltages only. 3. You need to be extremely careful when making measurements. Before measuring voltage or resistance, make it doubly sure that you have set up DMM correctly and the input leads are connected to the correct sockets of the DMM. 4. Before doing a measurement with the DMM you need to do TWO things. (i) Choose the correct FUNCTION using the rotary switch of the DMM. (ii) Connect the measuring leads of the DMM to the appropriate sockets of the DMM. One of the test leads (Black colour lead) should always be connected on the COMMON terminal. Note that the Voltage readings on the DMM are always with respect to the COMMON terminal. 5. The given DMM has an input resistance of at least 20 Mohms. Hence, it can be safely assumed that DMM voltage measurements will not alter the node voltages of your networks in this experiment. 6. When you are verifying the Superposition theorem, do not short circuit the DC power supplies. Instead, remove the DC power supply connections, and replace the power supply with a wire, thus giving 0 Volts to the network. 7. Before you connect a resistor in the circuit/network, measure its value using the DMM. 8. You should not use the DMM to measure currents. Instead, calculate the current in a branch from the measured values of the Node voltages and the known value of the resistors. 9. We expect you to come up with your own procedure to verify the theorems, as specified in the following networks. Before entering the Lab for this experiment, you will need to write clear steps and prepare tables, etc in your Lab Record and explain how you would proceed. Please note that you will not be allowed to enter the lab without the above. 10. All the theoretical calculations asked in the networks below must be done before coming to the Lab. 11. Please switch OFF the DMM after finishing the experiments on Networks 1 and 2. 1 Part A: Network 1 (Verification of Superposition and Thevenin’s Theorems) 1. A simple resistive network is shown in Fig.1. It is required to find VAB and IL for two values of RL, viz. (i) RL= 220 ohms, and (ii) RL= 680 ohms. 2. Measure VAB and estimate IL for the two values of RL. 3. Calculate VAB and IL for the two cases. Experimentally verify Superposition theorem for the above cases. Compare your theoretical and experimental results. 4. Calculate VTH, RTH, VAB and IL. Experimentally verify Thevenin’s theorem using the values obtained for VAB and IL. Compare your theoretical and experimental results. RL IL Figure 1 Part B: Network 2 (Bridge Circuit) 1. A bridge resistive network is shown in Fig.2. It is required to find V AB and IL for two values of RL, viz. (i) RL= 560 ohms, and (ii) RL= 180 ohms. 2. Measure VAB and estimate IL for the two values of RL. 3. Calculate VTH and RTH as well as VAB and IL for the two cases. (To solve the problem, it is easier to use Thevenin’s theorem separately at Nodes A and B, w.r.t. the ground, instead of finding Thevenin’s equivalent just across terminals A and B). Experimentally verify Thevenin’s theorem using the values obtained for VAB and IL. Compare your theoretical calculations with the experimental results. RL A IL B Figure 2 Part C: Network 3 (containing DC and AC sources) NOTE: Use only CRO to measure voltages. 1. A resistive network is shown in Fig.3. It is required to find VAB for RL= 1.2K. 2. Measure VAB on the CRO. 3. Calculate VAB. Experimentally verify Superposition theorem. Compare your theoretical calculations with the experimental results. A RL B Figure 3 2 + VAB -