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I found this voltmeter on eBay, and I know what the red and black wires are for (live and ground respectively, or alternative names), but I have no idea what the white wire is for. As its top range is 99 volts it can't be high voltage.

The listing does not say what it is for, and it has a really complicated circuit diagram which I cannot understand at all.

Please could somebody tell me what this is for?

enter image description here enter image description here

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  • \$\begingroup\$ Just for the reference, I am not going to buy it, I have my own voltmeter, but I just saw it on a sellers other items \$\endgroup\$ – George Mar 23 '14 at 19:31
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    \$\begingroup\$ The white wire is there only to confuse the bomb squad. \$\endgroup\$ – Olin Lathrop Mar 23 '14 at 19:38
  • \$\begingroup\$ Oh, you can just try it anyway. If that's the correct schematic (!) it's pretty much idiot-proof if you put less than 30V on any combination of wires. Find the two that make it light up (try 12V) and the third is the voltage input wrt the (-) power. \$\endgroup\$ – Spehro Pefhany Mar 23 '14 at 20:09
  • \$\begingroup\$ Those current-limiting resistor packs look an awful lot like dip-switches. \$\endgroup\$ – Tut Apr 21 '14 at 20:01
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My guess would be that black is ground, red is power supply (5 - 28 volts) and white is voltage to be measured (0 - 99 volts) - but that is only a guess, and I would examine the unit carefully, comparing it to the schematic provided, before applying any voltage.

Personally, I would not buy anything that did not come with the necessary documentation.

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  • \$\begingroup\$ Thanks for the answer, and I wouldn't buy it either - I would buy a proper voltmeter, or at least one from a UK seller that has more info on it. \$\endgroup\$ – George Mar 23 '14 at 19:31
  • \$\begingroup\$ @GeorgeH There is nothing wrong with the voltmeter. It's typical for this type to have a separate power wire and input wire. \$\endgroup\$ – alexan_e Mar 23 '14 at 19:37
  • \$\begingroup\$ @alexan_e plenty wrong. They tend to be inaccurate the farther they are from ~12v, can't accurately measure a voltage less than vcc, subjected to the accuracy of the microcontroller and whatever code is being used, etc. \$\endgroup\$ – Passerby Mar 23 '14 at 19:39
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    \$\begingroup\$ @Passerby The recommendation to avoid the device was given based on the missing documentation, and my objection was based on that. The accuracy is something completely different and I don't think a generalization can be representative of how such devices will perform, and buying a similar device locally will not guarantee the performance of the device either. \$\endgroup\$ – alexan_e Mar 23 '14 at 20:09
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    \$\begingroup\$ @Passerby What do you compare it to, and why do you insist on the accuracy issue? Have I missed any given accuracy requirement by the OP? The device is intended as a cheap and compact indicator, and has a xx.xV format which will give a resolution of 100mV anyway, so if you want to measure a few mV then obviously that meter is not the type of device you should choose. \$\endgroup\$ – alexan_e Mar 23 '14 at 20:38
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Two of the wires (2 and 1 in the middle of the schematic) provide power to the device. The third (3) is for sensing voltage along with the ground at 1.

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  • \$\begingroup\$ Thanks for the answer. Please can I just clarify your answer - 1 is the black wire, which is the ground. 2 is the live wire, which is the red and 3 is the power wire, which is white? \$\endgroup\$ – George Mar 23 '14 at 18:39
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    \$\begingroup\$ I can't say which is which; I can't see the back of the device to see where they connect (and no, trying to turn it around in Photoshop won't help). \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 23 '14 at 18:40
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The answer is: black is - (negative) and red is display + (positive), and white is the wire that the voltage is measured off of. How I came to this conclusion, was I asked my wife, who has absolutely no electrical background and is deathly afraid of electricity. lol. So I decided to test this for myself. I put the black on ground and the hot on red - the meter lights came on and displayed 0.0 volts. So then I added the white wire to the red one that was on hot - it displayed the voltage. so the last test just to be sure? Add a ground from another DC source and a hot from that other source to just the white wire and leave the red on the hot from the other battery? Can you guess what happened? Yes! Exactly, it showed the voltage of the new source on the white wire and not use the power from white to light the device. Instead lighting its self from the red and measuring from the white. :)

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Connector P1 is a three wire connector for the red, black and white wires. Nobody is able to categorically tell you which colours relate to the three pins but the three pins are: -

  1. Pin 1 on P1 is ground
  2. Pin 2 on P1 is battery or dc supply to power the LED display and chips - it needs to be 4.5V to 30V.
  3. Pin 3 on P1 is the pin that you use to measure a voltage so that its value can be displayed on the LED display.
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  • \$\begingroup\$ -can the ground for Pin1 be the negative of the battery used to supply the power (4.5-30v)? Sorry to bring up an old question, need help. \$\endgroup\$ – Filek Sep 12 '14 at 7:02
  • \$\begingroup\$ I believe it can. \$\endgroup\$ – Andy aka Sep 12 '14 at 8:57

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