[DMM] Different reading on reverse polarity - diyAudio
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Old 28th January 2011, 06:39 AM   #1
canvas is offline canvas  Taiwan
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Default [DMM] Different reading on reverse polarity

I tried to measure a 7915 regulator on my HP3478A 5.5 DMM. With correct polarity (Lo=-15V & Hi=GND), the reading is -15.82xx volt, but if I reverse the test lead (Lo=GND & Hi=-15V) the reading becomes +15.63xx volt! According to service manual, there is no calibration on negative DC, and all calibrations on DC is only based on positive reference. So, I don't think a full calibration will solve this problem. Is there anyone having this problem?
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Old 28th January 2011, 06:06 PM   #2
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Why not measure some other DC voltage sources like your automobile battery.
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Old 29th January 2011, 11:20 AM   #3
AndrewT is offline AndrewT  Scotland
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One of the tests for a correctly working voltmeter is to check it reads exactly -Value = +Value when reversed.
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Old 18th February 2011, 03:08 AM   #4
tomchr is offline tomchr  United States
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It sounds like your meter has developed an offset. What do you get when you short the inputs to the meter?
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Old 18th February 2011, 04:15 AM   #5
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I don't know if that's a bench type or a hand held. If the former, it could be leakage to the power line. Does it have a ground lead or are both leads floating?
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Old 18th February 2011, 04:35 AM   #6
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Perfectly normal behaviour for handheld DMM's in my experience.........don't worry about.

Eric.
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Old 18th February 2011, 08:34 AM   #7
canvas is offline canvas  Taiwan
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It's a bench type DMM. When both leads are shorted, it is about -0.0014mv ~ -0.0019mv. I will try a 12V car battery next time.

Last edited by canvas; 18th February 2011 at 08:38 AM.
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Old 18th February 2011, 10:25 AM   #8
Mooly is offline Mooly  United Kingdom
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Any battery will do... 1.5 or 9 volt etc.

It's possible the error measuring on the reg is down to some noise etc on the rails or AC "noise" on the chassis (ground), so reading a battery will prove beyond doubt. I would guess with a meter like yours it should be equal to within the last digit or so.
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