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#165753 07/04/07 10:40 PM
Joined: Dec 2004
Posts: 78
Cinner Offline OP
If you take the common lead (black) to a (-) dc supply source and the red lead to a + DC supply source, will the voltage be negative or positive?

Cinner #165754 07/04/07 10:42 PM
Joined: May 2005
Posts: 247

techie #165774 07/05/07 09:39 AM
Joined: Aug 2001
Posts: 7,520
It doesn't matter whether one or the other terminal you connect to is grounded or not, positive or negative with respect to ground.

The reading you get on the meter will depend entirely upon the potential difference between the two leads. If the "+" lead is more positive than the "-" you'll get a plus reading. If the "-" lead is more positive than the "+" you'll get a minus reading.

You could connect the positive test lead to a point which is at +6V to ground, the negative lead to a point which is at +9V to ground. Even though both test points are positive with respect to ground, the meter will give you a reading of -3V.

pauluk #165782 07/05/07 12:11 PM
Joined: Jun 2007
Posts: 301
If you are using a digital meter it does not matter. But look for the plus or minus symbol on the display. Then you can reverse the leads. But the voltage value will not change.
I think with an analog meter your pointer will try to go negative, thus providing no reading. This is when you need to put the right meter lead on the right conductor?????
Anyone else?

JValdes #165783 07/05/07 12:36 PM
Joined: Aug 2001
Posts: 7,520
I think with an analog meter your pointer will try to go negative

It will. Depending upon the range selected and the applied reverse voltage it will do anything from moving slightly to the left of the zero mark to slamming hard into the left-hand stop (and possibly causing damage).

You can get center-zero movements intended to allow either polarity to be applied (the ammeter in your car is a good example).

The VTVM (vacuum tube voltmeter) or its modern transistor equivalent often has a polarity reversing switch so that negative values can be read with the cable shield left connected to a grounding point.

pauluk #165784 07/05/07 12:46 PM
Joined: Jul 2002
Posts: 717
Man, I remember one sure fire way to cause the veins in the vocational school instructor's head to pop out was to reverse the leads on the schools analog multimeters while doing dc testing since he had repeatedly warned that it would damage the meters.

macmikeman #165785 07/05/07 01:00 PM
Joined: Jul 2004
Posts: 9,745
Likes: 13
There are lots of things that will blow up an analog meter and a digital meter will shrug off.
I let the smoke out of a meter once switching from DC to AC, transiting the milliamp range.

Greg Fretwell
gfretwell #165792 07/05/07 04:57 PM
Joined: Dec 2005
Posts: 152
Come on guys: Black (common) lead to -ve side of cell (DC source), red lead to +ve side of cell = POSITIVE reading on an analog meter (needle moves to the right)

Ann Brush #165794 07/05/07 05:53 PM
Joined: Nov 2005
Posts: 826
There is one, not so obvious, thing to watch out for on DVMs, VTVMs, and VOMs. Some are reversed on resistance or diode checks. All of my Flukes have been correct, or should I say intuitive, with - on the black lead for diode checks. My Sabtronics DVM at home has - on the red lead on all resistance scales. I need to keep that in mind because I often use it to check LEDs on the 100 ohm X10 scale. I try not to reverse bias LEDs because they typically have a very low reverse breakdown voltage.

Joined: Aug 2001
Posts: 7,520
Most of the older, conventional-style VOMs were wired in the way that Joe describes, so that polarity from the internal battery was the reverse of what you might expect on ohms ranges. It simplified the switching to do it that way.

Black (common) lead

The Avo 8 series meters (among others) actually use the positive terminal as the common for all ranges. The negative test lead is then connected to one of three terminals for switched ranges, 2500V A.C., or 2500V D.C. (it's not "negative" as such on A.C., of course).

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