Multimeter reading wrong voltage

Discussion in 'General Electronics Chat' started by Razor Concepts, Jun 12, 2011.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
    Recently my digital multimeter started acting up... I noticed this when a AA battery measured 1.7 volts :eek:

    It turns out that my multimeter (an old craftsman one) is giving out higher voltage readings than it should for any voltage.

    Any ideas on how to fix this?
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The first thing to check is that any batteries in the meter are still good. If a weak battery resulted in an internal reference voltage falling, this could cause the meter to read high.
     
  3. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    What other power supplies have you tested? Generally power supplies produce more voltage than their rating states, including new batteries. Just something to take into consideration ;)
     
  4. Jotto

    Member

    Apr 1, 2011
    159
    17
    Its a old meter, you didn't say if its analog or digital, analog will have an adjustment on the meter itself. Digital usually has a adjustment on the inside.
     
    Last edited: Jun 12, 2011
  5. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    You know the old saying: a man with one watch knows what time it is; a man with two watches is never sure.

    What you need is a voltage reference. You can make a nice 1.23 volt reference (that's stable to around a mV or better) from an LM285Z chip. I made one four years ago from a 100 kΩ resistor, three AAA batteries in series, and the chip. I've measured the temperature coefficient and it's about 50 ppm/°C. Really, for the money (the chip is around a buck), I don't know why everyone doesn't have one -- they're trivial to build. Mine draws around 30 μA. See the attached picture -- I made it in an old Maxim samples box. Doesn't need a power switch.

    Now, that doesn't fix your meter, but then you'll have a way to check it. I checked my voltage reference's output with both my HP 3456A voltmeter and my Fluke 893A differential voltmeter and they agree within one digit in 5 significant figures (not bad for 30+ year-old equipment).
     
  6. Jotto

    Member

    Apr 1, 2011
    159
    17
    This would be a good item to add to a power supply. Clamping the voltage to a certain level with a diode will give you a good reference voltage Could be done with a schottky diode also.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
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    For this purpose you need a reference diode. Many years ago the best option might have been a Zener of about 5.6V, but nowadays the "diode" would most likely be an IC like the LM285, based on the band-gap principle.

    The forward voltages of ordinary PN diodes and Schottkys are unsuitable as meter references: they vary too much with current and temperature.
     
  8. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
    Thanks for the help everyone! Turns out it was a simple low battery problem as Adjuster suggested. Using the meter i dug through my pile of 9Vs, found the one that read the highest at 11.9v :rolleyes:, popped it in and everythings okay now :D Thanks!
     
    leobert likes this.
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