It's been awhile, could someone clarify the difference between 12V and -12V? I have a wire in my car coming out of a switch/device that, in certain switch position, shows -12V on my DMM. I wish to tap into this wire, and run the -12V through the coil of a relay. Say I tap into this wire and connect it to pin 85 of my relay. For pin 86 (the "exit" of the coil), should I ground it? or what? I assume -12V means some sort of negative polarity, but I don't know what to do with it. FYI the car is a normal + polarity situation (i.e. - terminal of battery is grounded to chassis). Thanks in advance,
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destination moon
I would imagine its a ground wire. Plain and simple.
I don't actual know though. So wait for heamph or someone to tell you. I'm guessing.
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"I'm finished!" - Daniel Plainview
Do you have your leads crossed. Thats what mine shows if I use my positive as ground source and negative for the wire I'm testing.
kassdog hit on it. I can't think of anywhere a car would get a -12VDC, anyway.
Check your leads on your meter, and if you really DO have a -12VDC, just use it the same as a +12VDC in a relay application. If using a quenching diode, turn it around, ala
this page.
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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
Sounds like it is a reverse polarity switch/device.
Try using a ground reference that isn't a part of the switch (like the chassis of the vehicle). When referenced to the chassis you should not read -12vdc. The only time I've seen actual - voltgages in a car is when looking at certain wheel speed sensors and certain crank angle sensors (sensors that measure rotational speed by creating a sine wave). If this is your case (which it doesn't sound like it because you wouldn't actually read -12vdc) you wouldn't want to use either signals to drive a relay.
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Kevin Pierson
And to clearify your question:
Your volt meter reads voltage potentional between the ground leand and the positive lead. By displaying a -12 it is telling you that the voltage potential on the black lead is 12vdc greater then the potential on the red lead. This is why it is important to use a 'true' ground, so your voltage reference is correct. If you choose a voltage reference that is higher then 'true' ground you'll get all sorts of weird readings. With more and more 5vdc circuits creeping in to cars this is even more important, as you can get some really strange readings if you don't have a true ground and you're metering wires.
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Kevin Pierson