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Help with measuring AC current

Discussion in 'General Electronics Discussion' started by tcldarr50, Apr 6, 2021.

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  1. tcldarr50

    tcldarr50

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    Apr 6, 2021
    Recently bought a 15MHz 20vpp function generator. I can confirm that the frequency and voltage (rms) measure correctly using my DMM. If I set the generator for 1KHZ and 10vpp, then the DMM shows the correct freq and rms voltage.
    However, when I measure for AC current, the DMM does NOT measure what my calculation for rms current should be.
    I'm basically trying to convince myself that my new function generator is working properly or does it need to be returned and I only have a DMM, no OSCOPE yet. Freq and voltage seem to be ok, but current is throwing me a curveball. Any help would be appreciated.
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009

    I suspect you are not measuring correctly
    are you using a load ?
    if so what ?
    if not, then you will never measure current
     
  3. tcldarr50

    tcldarr50

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    Apr 6, 2021
    Source is 1KHZ Sine wave at 5vpp
    with a 1Kohm resister in series...that's it
    I'm measuring 1000Hz with my meter, and 1.76 volts as well (which should be the rms source voltage, right?) right off the source.
    Current should be 1.76 v divided by 1000 ohms, or 1.76 mA but I'm measuring 0.9 mA
     
  4. Bluejets

    Bluejets

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    Oct 5, 2014
    What is your meter...?
     
  5. tcldarr50

    tcldarr50

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    Apr 6, 2021
    I have an old radio shack true RMS digital multimeter. CAT# 22-174A
     
  6. Bluejets

    Bluejets

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    Oct 5, 2014
    Wow, that is OLD......taken from the manual.....may be important.....The AC current modes do not use an AC coupling capacitor to block DC. Both DC and the AC current flows through the shunt resistance. Too much DC current overpowers the AC component. lf you need to measure the AC component of a DC signal, we suggest you use a highquality capacitor to block the DC component. For normal AC line frequencies, we suggest you use a .47 pF or larger capacitor rated lor the vottage involved (500 volts or greater).

    https://www.antiqueradios.com/forums/viewtopic.php?f=8&t=351023
     
  7. tcldarr50

    tcldarr50

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    Apr 6, 2021
    anyone else have any other ideas? is the freq gen bad?
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Is the supposedly 1 kΩ resistor truly 1 kΩ?
    Why and how should the observed measurement be caused by a bad generator? 1.76 V / 1 kΩ would be 1.76 mA regardless of the generator. Even if the waveform should be non-sinusoidal and should exceed the max. crest factor allowed by your DMM, a comparable error should occur both when measuring current and voltage as the waveform and thus the crest factor would be the same across a pure resistor.
    The manual pages linked to by @Bluejets do not show the impedance of the meter's inputs when in current measuring mode. Do you have another meter to verify the Radio Shack meter doesn't have a too high resistance when measuring current? A high input resistance would increase the total resistance of the generator's load and thus decrease current.
    If you do have another meter, repeat the same measurements as with the Radio Shack meter and compare the results. Do they match?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Try measuring the current at 60Hz

    It may be that your multimeter can't handle higher frequencies well.
     
    davenn and hevans1944 like this.
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Good check.
    Acc. to the datasheet the meter is good (accurate) for ACV up to 5 kHz and ACA up to 1 kHz. But it may so happen that the low pass in Amp range sets in earlier such that at 1 kHz the damping is noticeable. The datasheet is not very specific here:(
     
  11. tcldarr50

    tcldarr50

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    Apr 6, 2021
    thanks everyone for your input.
     
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