12V polarity question

pencilneck

Registered User
Hello, I have a power transformer sitting around with a 1.6 amp output of 12 volts dc. The device was used for some electrical gadget, can't remember what. What I would like to do is to use it as a 12V test device to test automotive components but the output leads are not labeled. So I don't know which is "positive" and which is "negative". Is there a simple electrical test with a multimeter I could do to see which lead should be "negative" so I don't fry out the diodes in the device? I do not have a multimeter with a sweep meter to see which way the needle deflects. I could even use it to charge small batteries manually with its 1.6 amp current. I already tested both sides of the output to the power plug and neither side is continuous with "ground". Thanks, Roy
 
Take your meter and touch the red lead to a terminal and the other to a ground and watch for 12V.
 
Harbor Freight has digital multimeters for $10 or less. $3.99 in the store. If one of the leads has a colored tracer on it, it's +.
 
you wont fry a digital multimeter if you get it backwards... itll just read negative. what colors are the wires?
 
Neither wire is colored, but one of the two does have the UL requirements printed on it. My guess would be that that one is the "positive" one. I do have a nearby harbor freight store, I could make a sweep by sticking a pin into a potato I guess, just hoping there was an "ole timer method" that someone might know of... I just didn't want to run the risk of screwing up my very good digital multimeter by hooking up power backwards, but it is only 12V and 1.6 amps!!! Thanks, Roy
 
If you've got a radio shack store get a couple of LED's and a resistor. The longer lead on the LED is +. Twist one lead of the LED to the resistor, 1k-2.2k should work fine and connect them across the wires. If the LED doesn't light or it pops, it's backwards. If it lights the long lead is connected to +. It doesn't matter which side of the LED the resistor is connected.
 
just grab a light bulb, center is pos, outter is neg. it will only work 1 way:)
 
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you cannot damage a multimeter from polarity. In fact, about the only thing you can do is blow the fuse if you connect it to power while it's set to measure resistance.

Not to be rude, but why do you have a 'nice' multimeter if you're not willing to use it??
 
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