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I am considering buying a few small wall- plug DC transformers to power a few of my DC accessories.   I can buy these on Amazon of course, but they all come with a "socket" plug at the end, just like phone chargers.  I will have to cut this plug off and separate the two wires in order to wire things up.

But, the DC sensors I use will only work if a plus wire is connected to one place, and a negative wire to another. IF this is done backwards, it will burn the sensor out.

So, is there any way, using my voltmeter, to determine which wire is plus and which is minus, coming out of the plug in transformer?

(These sensors come in a three-pack, but I've already burned one up when trying to hook it up to my regular train transformer, because I didn't know the sensor was plus/minus sensitve. )

Thanks,  Mannyrock

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With a digital meter set to DC Volts, touch its leads to the DC wall wart output wires and observe the reading on the meter display.  If the reading is positive, then the red positive meter lead is connected to the power supply positive wire.  If the meter shows a minus in front of the voltage reading, then the positive meter lead is connected to the power supply negative wire.

Note that an analog meter is different and the meter needle will try to move backwards if polaity is reversed.

Last edited by SteveH

Thanks for all of the great info.

But Steve, my meter is a $10.00 meter with a needle from Harbor Freight.  It doesn't have a digital read out.

So, I a little bit confused by your explanation of what the needle will do.

If I hook it up correctly, with the positive output connected to the red meter lead, and the negative output connected to the black meter lead,  what will the needle do please?



With an analog meter correctly configured to measure DC Voltage and its red positive lead connected to the DC power supply positive and negative test lead to supply negative, the needle will point to the Voltage the power supply is producing.

This voltage may be higher than specified on the power supply's label when no load is connected other than the meter.  When a greater load is connected, the measured voltage will drop a little.  This is normal.

If you have additional questions about setting up your specific meter to measure DC Voltage or its other functions, either posting a picture of its front panel or its make and model would be helpful.  There are some variations for the test lead connections to it and for the settings among different analog meters.

I hope this helps.

Last edited by SteveH

On an analog meter if the polarity is wrong the needle will attempt to go backwards (what usually happens IME is you see it jerk slightly, then nothing, as opposed to no voltage, where it won't show any sign of movement.

Easiest way to tell is put the leads to the wire, if it doesn't move, reverse it and see if it moves. If it doesn't move at all, wall wart isn't working.

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