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

    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
    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


    Apr 5, 2008

    I have changed the tiltle for you.

  4. Shagas

    Thread Starter Active Member

    May 13, 2013
    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 .