Rth and vth of powersupply

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

  1. ajcircuit

    Thread Starter New Member

    Mar 25, 2009
    Hi there;

    how can we measure thevenines Rth and vth of any power supply ?
    please help me....
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    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?