I have a few DC power jacks of various sizes and shapes with 3 pins, 2 of them are connected when not plugged in, 1 is isolated for positive. When a plug is plugged into the jack usually the 2 that were connected disconnect, one goes to negative, the other - which I will call the third pin - isn't connected to anything. This is true for all power jacks and none of them show voltage in the third pin except for one.

That's strange, where is this voltage coming from to the third pin if the pin is disconnected? Is this something to worry about and should I get a new adapter?

This is a picture of the adapter that shows voltage, I only have one piece: 5.5mm x 2.5mm DC Power Jack

 

You would need to show all the connections? if only 2 wire DC is used then what is connected to the other jack connection, if any?
Max.

 

Nothing. I just tested it bare with no circuit. Multimeter straight onto the jacks terminals.

 

If it is not connected to anything why worry?
It is probably not opening the third pin for some reason.
Max.

 

Oh I don't know, maybe I should just use it without worry. Third pin isn't going to be connected to any circuit or casing. I really don't know what the third pin is there for? What are some applications of it?

 

It is sometimes used on a power jack socket to disconnect the internal power when the external power jack is plugged in.
Max.

 

It's for battery power. You connect the positive battery terminal there. The jack automatically disconnects it when you plug in. If you see voltage there, it's broken, but if you don't have a battery or other power source, it doesn't matter.

 

It's for battery power. You connect the positive battery terminal there. The jack automatically disconnects it when you plug in. If you see voltage there, it's broken, but if you don't have a battery or other power source, it doesn't matter.

Thank you for the responses. I don't have any use for it, and no longer need to worry.
As for battery power, it would make more sense to connect the negative terminal of the battery there? Since the third pin is connected to the negative terminal when the power plug is disconnected.

 

Depends which way around you're wiring it! Most devices use a negative tip, if they use batteries.

 

The jack your talking about is more likely to be an audio jack instead of a power jack.
The pin disconnecting shuts off the devices internal speaker when an earphone is plugged in.

 

The jack your talking about is more likely to be an audio jack instead of a power jack.
The pin disconnecting shuts off the devices internal speaker when an earphone is plugged in.

Hmmm, I doubt it. The form factor might be slightly different, but I'd guess the OP is talking about something like this:

 

Many different configurations, the PDF for this shows the schematic of the connections.
http://www.digikey.ca/product-detail/en/PJ-006B-SMT-TR/CP-006BPJTR-ND/408457
Max.

 

Hmmm, I doubt it. The form factor might be slightly different, but I'd guess the OP is talking about something like this:

Didn't see the link to a picture, sorry.

 

Didn't see the link to a picture, sorry.

The pic displays in the body of the post?
Max.