How to mesure negative voltage with a positive device

hello all, i've a bunch of those chinese voltmeter 3.00-99.9volt (positive) they have only two wires one positive and one negative, they use it for self powering and measurement. i have a -12v line i would monitor, how can i convert it to positive? could someone help me please? thanks

Reply to
Juri Fossaroli
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It's not clear what your need is... perhaps just flip the leads on the voltmeter?

More detail please. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

I thought it was as clear as it needed to be. The device under test is ALSO what powers the meter.

So simply reversing the leads will not likely work.

Sounds like a strange device design to start with.

Reply to
Long Hair

neg wire to -12 and pos wire to com

Reply to
bloggs.fredbloggs.fred

Flipping the leads won't do as that would reverse the polarity of the power supply. Connecting them normally - positive wire to positive input - would probably work without displaying a negative sign. I have a similar module somewhere but I don't have a clear answer for the OP until I've had time to check it out more thoroughly.

These things are usually digital voltmeters with a built-in 5V linear regulator. Some have separate connectors for power and for signal while others have the two joined together by a jumper on the pcb and share the same external connector. In the latter case, the power and signal inputs can usually be separated by removing the jumper.

However, the OP says that his VM goes up to 99.9V whereas mine is rated for 30V, apparently because it uses a linear regulator.

*If* the OP's meters can really take 100V (I've seen the ads at AliExpress), they must use a different regulator. Dropping almost 100V with a linear regulator at enough current to power an LED display would generate a lot of heat for a small SMD.
Reply to
Pimpom

of course it does.

It makes some sense.

NT

Reply to
tabbypurr

connect the neg input to your neg wire connect the pos input to your pos wire

pretend there is a negative sign in front of the display.

m
Reply to
makolber

Agreed, this is the correct answer. Think of it this way, a two wire meter measures the difference in potential between the two wires (and powers the unit) thus the more negative wire must go to the more negative source (-12) and the positive wire goes to the more positive side of the supply - in this case common.

It will work just fine.

John :-#)#

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Reply to
John Robertson

You're an idiot.

No... A meter that ran on a supercap charged by a crank would make "some sense". Powering a measurement device with energy from what is being measured makes no sense at all.

Reply to
Long Hair

I have not seen one of these devices, but how do you know there isn't a diode bridge in front of the regulator? Then it will be properly powered regardless of the applied voltage polarity.

This would make very good sense for use in measuring the voltage of a power supply.

Rick C.

Reply to
gnuarm.deletethisbit

would make it interesting trying to measure the voltage on the other side of the diode bridge

Reply to
Lasse Langwadt Christensen

connect the black wire to your -12V and the red wire to 0V volts, the voltmeter will then see a positive difference -12 to 0 and display the result.

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Reply to
Jasen Betts

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** Your Chinese meters cannot display a negative reading.

However, they will display the "-12V" as "12V" if you wire the leads correctly.

Perhaps you can write the word "negative" next to the display.

.... Phil

Reply to
Phil Allison

Cheap meters are like six bucks at Harbor Freight. Sometimes the are on sale for three. Sometimes they give them away, no purchase necessary.

Reply to
jurb6006

Yeah... might as well carry the farce-ity of it all the way to the nth degree.

Reply to
Long Hair

How very true. Even WalMart has them now.

Reply to
Long Hair

ritto:

e only two wires one positive and one negative, they use it for self poweri ng and measurement.

hello i would like to measure the -12v line of pc psu, i will try to connec t it with positive-> 0v and negative-> -12v thanks :)

Reply to
Juri Fossaroli

timewaster plonked

Reply to
tabbypurr

Sure it does. The voltage feed into a reference reference and is compared with the input voltage. The result is a useful measure of the input voltage - providing that the input is above the minimum range the device needs and less than the voltage that produces smoke. I'm not sure I would like to use one over about 30 volts without looking closely at it first.

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Reply to
David Eather

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** I wonder if this guy has ever seen an analogue multimeter ??

An AC clamp meter would send him bonkers - no power source and no visible connection to the circuit ??

Bet he is one of them software engineer types.

.... Phil

Reply to
Phil Allison

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