Amperemeter jumping around

Discussion in 'The Projects Forum' started by Spi D, Jul 4, 2011.

  1. Spi D

    Thread Starter New Member

    Jul 4, 2011
    2
    0
    Hi All

    New to this forum and wondering if anybody can explain:

    A have this old traditional car battery charger. Works fine but the build-in analog ammeter got stuck. 230V in and 12/24 V out.

    Got the idea of digital replacement and fitted a digital voltmeter at the same time (to monitor battery and check behavior when charger is shut off). Voltmeter works as expected.

    The ammeter is digital, connected to 12v pos & neg, to power it, and to each side of the shunt (200A, 75 mV) to measure amps. Terminals marked 5V, O, IN- and IN+.

    Whenever power is on, no matter if the measuring wires are connected, the digits jumps about. All sorts of readings, even negative.

    Conncting the measuring wires will change the jumping max/min values but it stil jumps around.

    Any ideas? Any ideas on how to solve?
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    You didn't exactly say you gave it 5 volts for its power supply. If you gave it battery voltage, you're lucky it didn't smoke.

    That out of the way, a big capacitor across the shunt might fix it.
    If you know the impedance of the meter (so you don't foul the accuracy), you could use resistors to lower the voltage applied to the meter and use the resistance network you just built to get a smaller capacitor to stop the jumping.
     
  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,913
    2,187
    Some of the old battery chargers don't have a ripple filter on the output so your DC waveform might look like this.
    [​IMG]

    It might be a good idea to install a small DC power supply (wall-wart) to power the digital circuits.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    And it's possible the digital meter you installed is sampling a waveform like nsaspook posted, as that's what my battery charger puts out. An average-responding AC ammeter that you calibrate with an RMS measurement tool (e.g., an RMS DMM) would be the way I would go. The ammeters I've seen on battery chargers are typically quite poor in terms of accuracy if they bother to put any numbers on the meter at all.
     
  5. Spi D

    Thread Starter New Member

    Jul 4, 2011
    2
    0
    Thanks a lot guys! Ofcourse you were right..

    The meter manufacturer stated that it would accept 5 to 15 volts so I was pretty sure it wouldn't blow up.

    I hooked up a 6 volt battery (camera-type) for testing and it solved the problem.

    I'm going to add a small supply (5v easily available these USB-days :))
     
  6. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Cool! Just leave cash or a check in the collection plate on the way out. :p
     
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