Electrical and Electronic Circuits
Electrical and Electronic Circuits ENGR 240
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This 3 page Class Notes was uploaded by Mckenna Lubowitz on Saturday October 3, 2015. The Class Notes belongs to ENGR 240 at Boise State University taught by Staff in Fall. Since its upload, it has received 12 views. For similar materials see /class/218030/engr-240-boise-state-university in Engineering & Applied Science at Boise State University.
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Date Created: 10/03/15
Thevenin Equivalent Engr 240 Spring 2004 By Sin Ming Loo In PSpice you cannot calculate the Thevenin equivalent with respect to a pair of nodes directly In order to replace a circuit with its Thevenin equivalent at a pair of nodes one needs the open cir cuit voltage VOC which is the Thevenin voltage Vth and the short circuit current Isc Using Ohm s law and Vth calculated Rth can be determined One of the methods that we can use to nd the open circuit voltage and short circuit current is by adding a dummy resistor to the given termi nals Enter the following circuit Figure 1 into PSpice R1 R2 7 v 5 4 v1 25Vdc i R3 39 I1 I 20 3Adc Figure l b To determine the Thevenin equivalent of the above circuit we are going to need a dummy resistor to assist us Add the dummy resistor with unknown resistance to terminals ab as shown in Fig ure 2 A ground reference is added at terminal b R1 R2 5 4 13 25Vdc i R3 l1 dummyiresistor 20 3Adc 7 A 11 D Figure 2 In order to nd the open circuit voltage set the resistance of the dummy resistor to very a large value lelZ Q Using Bias Point analysis you should obtain a open circuit voltage of 32V This is shown in Figure 3 dummyiresistor 1e12 Figure 3 To determine the short circuit current set the resistance of the dummy resistor to a very small value le12 Q Using Bias Point analysis you should obtain a short circuit current of 4A This is shown in Figure 4 AA 4 000A 8000mA 4 000A R3 I1 dummyiresistor 20 3Adc 193912 3000A Figure 4 Now apply Ohm s law to nd Rth In this case Rth is 8Q Rth V 8 Vth 32V Figure 5 The Thevenin equivalent problem that was solved in the previous example consists of indepen dent sources In the following example dependent source are involved Enter the circuit shown in Figure 6 The similar circuit enter into PSpice is shown in Figure 7 Figure 6 I2 4Adc R6 dummyJesIstor 8 2mg Figure 7 Once again we need to nd the open circuit voltage set the resistance of the dummy resistor to very a large value 1e12 Q Using Bias Point analysis you should obtain a open circuit voltage of 8V To determine the short circuit current set the resistance of the dummy resistor to a very small value le12 Q Using Bias Point analysis you should obtain a short circuit current of 8A With Ohm s law Rth is IQ The Thevenin equivalent is shown in Figure 8 RH Nv 1 Figure 8
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