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If I have a power supply (not a computer power supply, just a battery-replacing wall wart) and it doesn't have the usual polarity marking, how to I know if the tip is positive or negative?

Update: I couldn't find my multimeter. I did find a model number on the power supply and typed it into Google. Some guy selling a bunch of power supplies on eBay had checked it and found that it was tip positive, which is what I needed!

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Most of them I've found are tip positive, so that doesn't surprise me. – Brad Gilbert Jul 16 '09 at 1:13
up vote 2 down vote accepted

I have a simple voltage probe that has a bi-color LED, instead of a light bulb. It's wired so that if the point is positive relative to the ground clip, it's green. So I normally clip the ground clip to the outside of the connector, and put the point inside. That tells me at glance the polarity.

I also have used it for debugging RS232 signals, it's surprisingly fun to watch the LED flash red and green.

I would also, after determining the polarity, use the point to etch the polarity symbol into the case, or label. I mean it has a hardened tip already, why not use it?

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I know that was a long time ago, but do you happen to remember where you got that voltage probe? I was reading your answer out loud to my friend (I liked the part about watching the LED flash red and green) and he said he wished he had a voltage probe like you described. Thanks, if you happen to see this! – Ellie Kesselman May 17 '12 at 9:54
@FeralOink I might have bought it at the local Wal-Mart. I do remember swapping the pins of the LED because the colors were the other way around. Unfortunately I broke it some time ago. – Brad Gilbert May 18 '12 at 4:21

Multimeter, multimeter, multimeter.

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either that, or an LED – derobert Jul 15 '09 at 18:52
Ooo. An LED? That's interesting. Because it's a diode it only works one way? – Nosredna Jul 15 '09 at 19:55
OK. I like the LED answer, but I do have a multimeter around here somewhere. I think. – Nosredna Jul 15 '09 at 19:56
If you use a LED, make sure you use a resistor as well, unless you want to watch a LED die. Which isn't as much fun as it sounds. – Brad Gilbert Jul 16 '09 at 1:03
Any particular resistor? – Nosredna Jul 16 '09 at 1:52

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