Thevenin equivalent circuits

Transcription

1 Thevenin equivalent circuits We have seen the idea of equivalency used in several instances already same as 1 2 same as 1 2 R 3 same as = 0 V same as 0 A same as same as = EE 201 Thevenin 1

2 The behavior of any circuit, with respect to a pair of terminals (port) can be represented with a Thevenin equivalent, which consists of a voltage source in series with a resistor. load some circuit two terminals (two nodes) = port some circuit R L v RL load Thevenin equivalent R L v RL Need to determine and so that the model behaves just like the original. EE 201 Thevenin 2

3 Norton equivalent load load some circuit v RL I N R N v RL Ideas developed independently (Thevenin in 1880 s and Norton in 1920 s). But we recognize the two forms as identical because they are source transformations of each other. In EE 201, we won t make a distinction between the methods for finding Thevenin and Norton. Find one and we have the other. = I N R N = EE 201 Thevenin 3

4 Example 1.5 k! 1 k! 9V R L V 3 k! 6 ma Th 12 V R L Attach various load resistors to the original circuit. Do the same for the equivalent circuit. For each load resistance, calculate the load voltage (and current and power) for each of the circuits. The results are identical. In terms of the load that is attached at the port, the two circuits are indistinguishable. Check it yourself. R v i P 1! mv ma mw 10! mv ma 1.41 mw 100! V ma mw 1 k! 6.0 V 6.0 ma 36 mw 10 k! V 1.09 ma 11.9 mw 100 k! V ma 1.41 mw EE 201 Thevenin 4

5 Determining the Thevenin (or Norton) components How to find and? Need two components, so two measurements or calculations should suffice. Use two different load resistors. load load unknown circuit v 1 v 1 = load load unknown circuit v 2 v 2 = 2 equations, 2 unknowns: = ( ) = ( ) EE 201 Thevenin 5

6 More directly: open-circuit voltage, short-circuit current 1. Leave port open-circuited. (R L, i L = 0) Measure open-circuit voltage. unknown circuit v oc v oc v oc = open-circuit voltage is a direct measure of. 2. Short the output port. (R L = 0, v L = 0) Measure short-circuit current. unknown circuit i sc i sc = = = Note, that i sc can also be interpreted as a direct measurement of I N : i sc = I N. EE 201 Thevenin 6

7 Calculating Thevenin equivalent The open-circuit voltage / short-circuit current approach can be used to calculate the Thevenin equivalent for a known circuit. 1.5 k! Consider the circuit from slide 4: 9V 3 k! 6 ma Open-circuit voltage Use whatever method you prefer. We ll use node voltage in this case. v a = v oc = = = (.)().. = 12 V = v oc = 12 V. EE 201 Thevenin 7

8 Short-circuit current Use whatever method you prefer. We ll use node voltage in this case. But proceed carefully the short circuit introduces some unusual wrinkles into the circuit analysis. v a a: Because of the short circuit, v a = 0! I i S sc b: Because of the short, v R2 = 0 and i R2 = 0. So plays no role and can be removed. v a i sc = i R1 I i S sc = = = = = =. EE 201 Thevenin 8 =

9 Thevenin Norton 1 k! I N R N 12 V 12 ma 1 k! Alternate method for If the circuit consists of independent sources and resistors only, then the Thevenin resistance can also be found by de-activating the independent sources and finding the equivalent resistance as seen from the port. De-activate the sources = =(.) () = EE 201 Thevenin 9

10 Summary To measure and 1. Use a voltmeter to measure the open-circuit voltage at the port of the circuit: v oc =. 2. Connect a short circuit across the output and use an ammeter to measure the short-circuit current: i sc = I N. 3. Calculate = / I N. Note that shorting the output may not always be practical. For example, some devices may have over-current protection circuitry that prevents large short-circuit currents from flowing. Or the device might not be able to handle the large current that might flow when the output is shorted without being damaged. In those cases: 1. Use a voltmeter to measure the open-circuit at the port of the circuit: v oc =. 2. Attach a load resistance, R L that is small enough so that an appreciable current is flowing. Measure the resulting load voltage, v L. 3. Calculate = EE 201 Thevenin 10

11 Summary To calculate and 1. Using whatever techniques are appropriate, calculate the opencircuit voltage at the port of the circuit: v oc =. 2. Connect a short circuit across the output. Using whatever techniques are appropriate, calculate the short-circuit current: i sc = I N. 3. Calculate = / I N. Alternate method (for circuits that consist only of independent sources and resistors). 1. Using whatever techniques are appropriate, calculate the opencircuit voltage at the port of the circuit: v oc =. 2. De-activate all independent sources. Calculate the equivalent resistance as seen from the port. (If dependent sources are present in the circuit, the test generator method can be used to find equivalent resistance. See the equivalent resistance notes to review the test generator technique.) EE 201 Thevenin 11

12 Example 1 1 k! Find the Thevenin and Norton equivalents of the circuit at right, with the port as shown. 24 ma 6 k! R 3 3 k! v oc Find v oc. Start with a current divider. = = () =. = =(.)() =. = v oc = 43.2 V Find i sc. Note that R 3 is shorted out. Use a current divider again. = = () =. EE 201 Thevenin 12 i sc I N = i sc = ma

13 = =.. =. 2.1 k! 43.2 V I N ma R N 2.1 k! Alternatively, we could use the short-cut method to find. De-activating the current source: R 3 R eq = ( ) =() ( ) =. EE 201 Thevenin 13

14 Example k! R k! 20 V R k! 4.7 k! Find the Thevenin and Norton equivalents of the circuit at left, with the port as shown. R k! R k! R 4 i a i b R 5 v oc Find v oc. Use mesh current method. = = ( ) = ( ) = R 3 R 6 Insert values and solve: i b = ma. v oc = i b R 5 = 4.37 V. ( ) = ( ) = EE 201 Thevenin 14

15 R 4 Find i sc. Note that R 5 is shorted out by the short circuit. i S Use equivalent resistance to find i S. i sc = ( )=. R 3 R 6 = =. Use current divider to find i sc k! = i sc = 1.45 ma 4.37 V = =.. =. I N 1.45 ma R N 3.27 k! EE 201 Thevenin 15

16 Alternatively, we can use the short-cut method to find. R 4 R 5 R 3 R 6 = [ ( )] = 3.02 k! EE 201 Thevenin 16

17 Example 3 R 5 15! 4 A Find the Thevenin and Norton equivalents of the circuit at right, with the port as shown. 30! R 3 30! 30 V 20! R 4 15! Find v oc. Use node voltage. R 5 = = = = a R 3 R 4 b v oc = = solve: v a = 20 V, v b = 40 V. " v oc = v b = 40 V EE 201 Thevenin 17

18 Find i sc. Use node voltage, again. Note that R 4 is shorted out (so ignore it) and node b is shorted to ground, v b = 0. R 5 = = a R 3 b = i sc =. =. Need v a. =. =. = 6.36! = = = 8.57 V 40 V EE 201 Thevenin 18 I N 6.28 A R N 6.36!

19 Maximum power transfer Now that we have the ability to model any circuit using a simple Thevenin (or Norton) equivalent, we can answer another important question: How much power can a given circuit supply to an attached load? Start with the Thevenin equivalent and determine the load resistance that would lead to the maximum amount of power being dissipated in the load. load R L = v RL = ( ) In the usual way, find the max by setting the derivative to zero and solving. = ( ) = ( ) = = For maximum power to the load See slide 4 for an example look at power column in the table. EE 201 Thevenin 19