Yet another exclusive KL7AJ story problem!

Discussion in 'General Electronics Chat' started by KL7AJ, Feb 20, 2009.

  1. KL7AJ

    Thread Starter Senior Member

    Nov 4, 2008
    How well do you REALLY understand AC circuits? The correct answer for this one will earn you a GENUINE PLASTIC STAR!

    Tommy Technician has a problem. He has a mystery inductor he needs to identify. A cursory measurement with an ohmmeter shows the DC resistance to be essentially a dead short. This tells Tommy that it's going to be a REAL small value of inductance....assuming it's a good component.
    Tommy knows he probably can't measure the thing with his trusty DMM, even though it has an inductance function. He opts for a more sophisticated determine the inductance indirectly by measuring the REACTANCE, and then working backwards. Clever lad, that Tommy.
    Tommy breaks out his trusty wideband oscilloscope and R.F. signal generator, as well as a calibrated 1000 ohm, non-reactive resistor. His plan is to put the resistor and inductor in series, and measure the voltage drop across both the resistor and the mystery inductor. He knows that when both values are equal, the reactance of the inductor will be 1000 ohms. It's a simple step backwards from there, knowing the frequency, to determine the inductance value.
    Tommy sets his signal generator to precisely 10 volts peak to peak output, as measured with his oscilloscope. He decides it's safest to use his dual trace scope in the differential mode, using two probes, so as to eliminate any possibility of ground conflicts and such. He puts the probes across the resistor, and gradually increases the frequency of the signal generator until he sees 5 volts peak to peak on his scope. He reads the frequency, it's 4.53 MHz. He re-measures the signal generator output, just to be sure the level hasn't changed during the frequency sweep. He is pleased to see it still reads precisely 10 volts.
    He then decides to measure across the inductor itself. If he did everything right, it should also read 5 volts. He attaches the probes and measures a little over 7 volts. He scratches his head. He re-measures everything again. 10 volts from the signal generator. 5 volts across the resistor. A little over 7 volts across the inductor. Just to be sure his test equipment isn't lying, he reverses the polarity of his probes. Same readings everywhere. Something just doesn't add up.

    What is wrong with this picture? IS anything wrong with thiis picture?

    Plastic Star awaiting.

  2. hgmjr


    Jan 28, 2005
    My suspicion it that the scope probes are introducing a small amount of capacitance and the resulting RLC is producing the unexpected output.

  3. davebee

    Well-Known Member

    Oct 22, 2008
    He's adding voltages that aren't in phase?

    10 volts = 5 volts + (7 volts * cos(angle))

    angle = acos(5/7) = 45 degrees

    tan(angle) = XL/R

    XL = R tan(45) = 1000 ohms inductive reactance

    L = XL/(2 pi F) (microHenries, megaHertz) = 1000/(2 pi 4.53) = 35 microHenries
  4. KL7AJ

    Thread Starter Senior Member

    Nov 4, 2008

    DaveBee gets the gold star! A twenty-one kazoo salute in your direction


  5. davebee

    Well-Known Member

    Oct 22, 2008
    thank you!
  6. mik3

    Senior Member

    Feb 4, 2008
    Good question KL7AJ.