# Rth and vth of powersupply

Discussion in 'Homework Help' started by ajcircuit, Mar 25, 2009.

1. ### ajcircuit Thread Starter New Member

Mar 25, 2009
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0
Hi there;

how can we measure thevenines Rth and vth of any power supply ?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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782
Are you thinking of a regulated DC supply?

If so, this could be difficult.

It's possible you could do a test on a small battery - 'though I'd wouldn't try evaluating the theoretical definition given below

Rth = Vopen_circuit/Ishort_circuit ...!

I remember dropping a spanner across the terminals of a 12V car battery once - the spanner went cherry red pretty quickly. Fortunately the guy next to me had the presence of mind to knock it away with a screwdriver before we ended up covered in acid.

Short circuiting even a small harmless looking battery could be hazardous.

The safer way ...

You'd need to apply a few different loads to obtain a graph of V vs I from which you could deduce the Rth value. So you'd need a voltmeter and an ammeter for the taking of the two measurements at the various load conditions. The load(s) would have to be of low enough resistance to cause a discernible drop in the output voltage without overheating the battery. The mean slope of the V-I droop will give a reasonable estimate of Rth. Vth is just the open circuit value. Thing is - the battery probably won't behave like it has a nice theoretical Thevenin equivalent. It's discharge behaviour will have / introduce some non-linearity, but I'm not sure to what extent that would effect the result.

Why do you want to know this?