Signal current using a DMM

Discussion in 'General Electronics Chat' started by Shagas, Sep 5, 2013.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hello

    The title of this thread is misleading , but I kept getting server errors so I had to change it.


    Signal current using a DMM

    I was trying to measure the current going out of a constant current driver with my DMM the other day ( feeding an led).
    When I connected the TTL to +5volts (I'm powering the whole thing with 5V)
    then I got a current of say 50ma , then without doing any adjustments I connected the TTL to the micro which was outputting a few tens of Khz square wave and the DMM showed more than twice the amount of current , about 120ma.

    I understand that the DMM current mode is not designed to measure signal currents so this is an error right? When I give the TTL say.. a 50khz square wave then driver should output half the average current than if I connected the TTL to 5volts right?

    Just want to make sure there isn't some trickery going on ...

    Thanks in advance for the answers
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    This is probably to do with the way the DMM samples and converts the analog value to a digital number.

    The best way of measuring this current is to insert a low value series resistor (say, 1ohm) and measure the voltage drop across it. Best to use an oscilloscope if you've got one. Use a non-inductive resistor, not wire-wound.

    The oscilloscope will also show if anything else is going on, like ringing or slow rise/fall times.

    If you don't have an oscilloscope, make a simple low-pass filter between the series resistor and the DMM voltage input. Bear in mind that most DMMs have a 10M ohm imput impedance on the DC voltage range and your series filter may reduce the voltage reading slightly.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    I have changed the tiltle for you.

    Bertus
     
  4. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Thanks bertus .

    Thanks JDT ,
    I know how I can measure the current using a low value resistor and using the voltage drop across to calculate the current but I wasn't bothered to try it , probably should.

    I was mostly curious as to why the DMM shows such an error but yeah it's probably something to do with how the current is sampled .
     
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