voltage measurementwire
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?
The cable colours will vary from cable to cable. Use a continuity checker or ohmmeter to determine which wire is connected to which pin on the plug.
Obligatory link to the XKCD comic about standards:
So yeah, there are standards. There are so many of them that it's effectively the same as having no standard, as every possible wire arrangement likely has a standard that describes it.
Best Answer
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.