DMM busted?

Discussion in 'General Electronics Chat' started by munuainen, Aug 1, 2014.

  1. munuainen

    Thread Starter New Member

    Jun 13, 2014
    11
    1
    Yesterday I noticed I had managed to burn the DMM fuse. Must've probably had the meter set to measure current while probing for voltage reading at some point.

    After replacing the fuse, I'm getting current readings again but the magnitude is way off, like plugging the meter in series with the circuit of a 1k resistor with a 10V DC supply shows ≈ 120 mA. If I split the supply voltage in half, I'm getting then half the 'wrong' current, ≈ 60 mA.

    This loss of magnitude only happens with the current measurements that pass the fuse. High current (10 A) setting still gives original correct values. I've triple checked that there's a good contact between the fuse and its holder.

    What might be causing this?
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Did you measure the 1 resistor or just assume it is correct to zero percent?

    Is the battery in the meter fresh?

    Possibly you burned out the internal low scale current shunt. You may be able to open the meter and find it, then worry about replacing it.

    Or just add a sticky note to the meter "scale X is Y% too high".
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,025
    I had trouble with a goofy fuse once - it had a DC resistance of over 5Ω and that was screwing up my DMM.
     
  4. munuainen

    Thread Starter New Member

    Jun 13, 2014
    11
    1
    This! Thanks man, your comment made me take another look inside the meter (took one yesterday but today with more focus). Upon closer inspection and simple schematic-drawing, I discovered the .99Ω shunt that measured almost 50Ω off-circuit. I used a spare 1Ω resistor as a replacement and I'm back online again, getting nice readings that are close enough for a $20 meter.
     
    Last edited: Aug 1, 2014
    ErnieM likes this.
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