enter image description here

Above figure shows the transformation of a circuit to its Thevenin equivalent. Imagine this is a power supply and A is the + terminal.

In reality the real circuit model is the one on the left side and just before the A terminal I circled the resistance with red color.

On the right side is its Thevenin equivalent and the Thevenin resistance is circled with green color.

I have the following questions:

1-) When one talks about the "output impedance" of this power supply, would you understand the red or green circled resistor?

2-) Lets say you don't know what lies inside the black box. You can measure the Thevenin voltage across A and B terminals by a voltmeter. But is it also possible to quantify the Thevenin resistance Rth(for example by shorting A and B)?


1 Answer 1


The green circled one represents the output impedance of the whole circuit. the red one is a component of it, but not all.

In theory you could measure the Thevenin resistance by shorting the supply, but if this were a real supply, it would likely protect itself (short circuit protection), and you wouldn't get the correct answer. Even if it didn't protect itself, if it has any nonlinear or active components (opamps, diodes, transistors etc.), the behavior of these devices when shorted might be significantly different (in a non-linear way) from the lightly loaded behavior and so you wouldn't get a good result.

Best approach is to put a light load (say full scale load) on the supply and measure the change in output voltage -- now Rth is deltaVOUT/ILOAD

  • \$\begingroup\$ how many ohms is light load if the supply is around V volts? \$\endgroup\$
    – user16307
    Dec 6, 2015 at 21:55
  • 1
    \$\begingroup\$ Use R = (V/I_max)/2, where I_max is the maximum current rating of the supply. If this is just a theoretical question, with all ideal components, any non-infinite value will work. \$\endgroup\$
    – jp314
    Dec 6, 2015 at 22:17

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