# Determining polarity of charging jack

#### BobTPH

Joined Jun 5, 2013
2,499
I have a Lithium battery drill driver where the wires to the charger broke off at the plug. I cannot determine the correct polarity by looking at the plug since none of the wire is left.

I know I can open up the drill and find the circuit board and determine the correct polarity of the connector, but my experience with these things is that once opened they can be a bitch to stuff back together, so I would like to avoid that if possible.

So, the question is: Can I determine the correct polarity by applying a current limited voltage to the jack in both directions without blowing it up? I.e. is it likely that the correct polarity and the reverse polarity will behave in a distinguishable manner when voltage is slowly applied? I have a lab supply with current limiting so, I can do this while monitoring what happens. The wall wart that plugs into the drill puts out a constant 10V, so the charging circuit is inside the drill.

Any ideas?

Edit: It has a non standard power connector with no legend to show the polarity.

Bob

#### Dodgydave

Joined Jun 22, 2012
9,313
If you have a dvm, you can check continuity from the socket to the battery pins, chances are centre pin is Positive.

#### BobTPH

Joined Jun 5, 2013
2,499
I want ro avoid opening up the drill. So I do not have access to the battery terminals.

And the plug is not the usual barrel plug, it is a shaped two pin connector like you often see on electric shavers.

Bob

#### crutschow

Joined Mar 14, 2008
25,259
the question is: Can I determine the correct polarity by applying a current limited voltage to the jack in both directions without blowing it up?
That should work if you are careful.
Monitor the current as you increase the voltage.

Likely the correct connection won't draw much current until you approach the charging voltage.

The reverse connection may, or may not, draw any current, but if it does, it likely will be at just a couple volts or less.
Don't let the current get above a few mA if that happens.

#### BobTPH

Joined Jun 5, 2013
2,499
Thanks, I will try that.

Bob