Lecture Chapter 4C Thevenin Equivalent Circuits
Problem 1 Two measurements are made on the same "linear mystery circuit" as shown. What would be the measured current Im if the 50-ohm resistor is
replaced by a short circuit? ( 96 Amps)
Thevenin: simple model for complex circuit• To find Thevenin Equivalent at a terminal a-b:
1. Find V open circuit2. Find I short circuit3. Vth = Voc4. Rth = Voc/Isc
Vth = Voc = 30V; Rth = 120Ω
"Lookback" resistance• Another way to find Rth– Turn off all sources • Replace Voltage source with short• Replace Current source with open• Calculate Rab, equivalent resistance
Problem 3 Find the Thèvenin equivalent circuit at the terminals A-B. Rt = 25Ω;Vt = 160V
Thevenin and Norton Equivalents would be indistinguishable at a-b
• To find Norton Equivalent, find Thevenin, set In = Vth/Rth, and Rn = Rth
• this is the basis of the "source transform" method, which can help simplify circuits
Another Way to Look at Thevenin Equivalents
• All our circuit elements and laws are linear– Vs = 9V, Is = 10A, V=IR, sum of v= 0, sum of i=0
• We can model the V-I curve of any linear circuit– define equation of line: slope and a y-offset• Rth and Vth
More Thevenin Equivalents• Dependent Sources– Find Voc the same (use NV, MC, Source Xform, etc)– For Rth:• You can always find Isc, then Rth = Voc/Isc• OR Turn off Independent sources,
– hook up a “Test Voltage Source” Vt (not Vth!!)– Find current produced by test source It,
» Then Rth = Vt/It
Problem 2 Find the Thèvenin equivalent circuit at the terminals U-V.(Rt = 9.74)
Circuits with only dependent sources are resistive. MUST hook up external test source
2) Find Thevenin Equivalent to left of 8 ohm RThis can be simplified with Source Xform, OR Solved with NV [72V, 4 Ohm]
3) Problem 1 Find the Thèvenin equivalent circuit at the terminals G-H.(Vth=12, Rth=34)
4) Problem 1 Find the Thèvenin equivalent circuit at the terminals S-T. Rt = 50Ω
Problem 2 Find the Thèvenin equivalent circuit to the left of the terminals A-B. (1V, 0 Ohm)
Problem 2 Find the Thèvenin equivalent circuit at the terminals U-V.(Rt = 9.74)
Circuits with only dependent sources are resistive. MUST hook up external test source
2) Find Thevenin Equivalent to left of 8 ohm RThis can be simplified with Source Xform, OR Solved with NV [72V, 4 Ohm]
3) Problem 1 Find the Thèvenin equivalent circuit at the terminals G-H.(Vth=12, Rth=34)
4) Problem 1 Find the Thèvenin equivalent circuit at the terminals S-T. Rt = 50Ω
Problem 2 Find the Thèvenin equivalent circuit to the left of the terminals A-B. (1V, 0 Ohm)