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Berkeley ELENG 100 - Equivalent Circuits Guide

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UNIVERSITY OF CALIFORNIACollege of EngineeringDepartment of Electrical Engineeringand Computer SciencesProfessors Fearing & White Fall 2005EE43/EE100 — LAB #3Equivalent Circuits GuideImportant Notes• Please make sure the current limit is set higher than the current required by the circuit but lower than 2amps. This is to insure that you provide your circuit with enough power without damaging the equipment.• Always use measuring devices (DMM) to take your measurements. Do not depend on the power supplyto report accurate voltage and current values.• In this lab, you will use , and resistors. For this lab, you can use resistor val-ues that are within 10% of your theoretical value. If you require the use of other valued resistors, thenyour theoretical calculations are incorrect.• These circuits are complicated. Good breadboard practice will be key in completing this lab.Equivalent Resistor Networks1. Build the circuit shown in Figure 1. Please use the breadboard to build this circuit. Assuming a maximum of10 volts, what is the maximum amount of current supplied by the power supply?2. From your prelab, you calculated the theoretical resistance across A and B. Disconnect the circuit from thepower supply and use the DMM to measure the actual resistance across terminals A and B.3. Reconnect the power supply, and record and for 5 different supply voltages between 0 and 10 volts.Plot the IV curve of this circuit. What value of resistance do you infer from the IV curve?a. When recording the value of and , it is important that you use the digital multimeter (DMM)to take your measurements. The readings from the power supply are inaccurate.b. Please set the current limit of the power supply to a value higher than that calculated in Step 1, butlower than 2 amps.1.2kΩ2.2kΩ220Ω,,1kΩAB+−IDC2.2k2.2k2.2k2.2kFigure 1VABIVABI2.0k2.0k2.0k2.0k2.0k2002002 of 34. Build the circuit shown in Figure 2. Use the value of calculated in the prelab exercises and measured inStep 2.5. Using the power supply, record and for 5 different supply voltages between 0 and 10 volts. Plot theIV curve of this circuit. What value of resistance do you infer from the IV curve?Thévenin’s and Norton’s Equivalence6. Build the circuit shown in Figure 3 leaving out the resistor labeled RL for now. Measure the voltage acrossterminals C and D. This is your open circuit voltage ( ) and should be the same as you calculated in yourprelab.7. Now measure the current flowing through terminals C and D. Remember, when measuring current usingDMM, there is 0-resistance across the probes. So, you are essentially measuring the short circuit current( ) and should be the same as you calculated in your prelab.8. Disconnect the power supply and short terminals A and B. You killed the voltage source. Measure the resis-tance across terminals C and D. This is your Thévenin resistance ( ) and should be the same as what youcalculated in prelab.9. Now, “unshort” terminals A and B and reconnect the power supply (thus restoring the circuit in Figure 3).For three different values of , and , install the resistor and measure the voltageacross and the current through .+−IDCFigure 2ABReqReqVABI+−25VABRL1.2k2.2k2201.2kFigure 3CDVTHISCRTHRL220Ω1.2kΩ,=2.2kΩRL+−Figure 4RTHVTHRL2.02.0k2002002.0k2001.2k1.2k3 of 310. Build the circuit shown in Figure 4 with the appropriate values of and that you calculated in yourprelab and measured in Steps 6 and 8.11. For the three values of , measure the voltage across and current through .12. Build the circuit shown in Figure 5 with the appropriate values of and you calculated in yourprelab and measured in Steps 7 and 8.Building a Current Source:a) Disconnect everything from the DC power supply.b) Set the power supply to output +25 V with a current limit that is close to (short circuit current). should not exceed 100 mA.c) Connect the DMM set up to measure current in parallel with the power supply and adjust the powersupply until is displayed on the DMM.d) Disconnect DMM and use the power supply at this setting as a current source for the Norton’s equiv-alent circuit.13. For the three values of , measure the voltage across and current through .Wheatstone Bridge14. Build the circuit shown in Figure 6. Measure the current . (The resistors are only 5% tolerance, so therewill be an imbalance.) If you swap the two 2.2k resistors, what is the current ?15. Deliberately unbalance the bridge by touching your index finger and thumb from your same hand across oneof the 22k resistors. Demonstrate to your TA and explain orally what is going on. (Keep your other hand inyour pocket.)16. From measured with fingers across this resistor, estimate your thumb-to-index finger resistance. Whatresistance do you measure using the DMM?VTHRTHRLRLFigure 5RTHISCRL+−ISCRTHISCISCISCRLRLig10V22k+−2.2k2.2k22kMFigure


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Berkeley ELENG 100 - Equivalent Circuits Guide

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