How to use voltmeter to test......

[Keith Bolog]

......polarity f a wall wart charger? SPECIFICALLY, I am snipping off the end plug and replacing it. I know the polarity of the new end plug but the wires are unmarked.

Thanks for help Im not good with electronic stuff like this. Merry Christmas

[Bill A. Moore]

The wall wart should tell you the polarity of the plug, wire the new connector the same, test with meter to be sure!

[Mike Auman]

If you have already snipped the wires on the DC side of the wall wart:

1. Set meter on DC Volts.
2. Clip meter leads to the bare ends of the wall wart output wire (either way, doesn't matter which to which). If your meter test leads have probe ends instead of clips, tape the two wall ward DC leads down on a nonconductive table and separate them.
3. Plug in wall wart and probe the DC leads.
4. If you read positive voltage (+9V, +12V, etc) then the RED/Positive lead of the meter is on the POSITIVE DC lead. If you read negative voltage (-9V, -12V, etc) then the BLACK/Negative lead of the meter is on the POSITIVE DC lead.
5. Mark the Positive DC lead with a bit of tape. It may also have a factory marking already (white stripe, etc).

[Steve Pavey]

IMG_0762.jpeg

If it’s a barrel connector like this one the inner is nearly always positive and the outer is zero volts or negative. Using your multimeter stick the red probe up the centre and touch the black probe on the outside of the barrel - it will display the voltage (assuming you’ve set the multimeter to display voltage of course!). If you swop the leads over it will still display the voltage but it will have a minus sign in front of the displayed voltage value. Knowing this will allow you to connect the new plug correctly.
You still need to find out which of the two conductors is which once you cut the old plug off. Pare back the insulation and separate the two conductors ( you can tape them down to the desk to make sure they stay separated). Use the multimeter again to check, just as above, and dab the positive one with a sharpie or white paint pen. You can then solder the wires correctly onto the new plug.
Apologies if this comes across as condescending - hope it helps!

[Jerry Overstreet]

I was just about to post the info that Mike left. The only thing I have to add is a charger can be either AC output or DC output.

Also use caution around bare wire ends.

Polarity on a charger with AC output, is not critical as in DC output voltage and may not show polarity on a meter. Still a good idea to maintain polarity even here if you can, but it really doesn't matter..

If your device isn't marked AC or DC, it should have one of these 2 symbols to determine the type of output voltage.

OIP-647857650.jpg

[Dennis Detweiler]

One of the wire leads may not be marked with a white line. One lead will have a small, raised rib on it. The ribbed lead should be positive. However, I would check it with the meter to be sure.

[Keith Bolog]

CHEERS - no marking on the wires, so only 50-50 chance if I guessed. The new plug different style and hot is well known.
THANKS

[Jerry Overstreet]

Good on you. 50-50 guess would kill a DC unit if polarity is reversed. Not worth a guess. Once you let the smoke out, they're no good.