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Enforcing polarity with an AC adapter? Answered

Several months ago, I posted an I'ble for adding an AC adapter to something. The adapter I chose, from Radio Shack, is actually a two-piece "kit" -- the wall wart cord ends in a generic two-pin female jack, and they sell a bunch of different end plugs. The jack is symmetric, so that the end plug can be put on with either ground or hot on the outside.

I've just realized that this could be a problem. If the electronic device expects red to be positive and black negative, then putting the jack on backwards would be equivalent to installing all the batteries the wrong-way about. Not a great feature.

To "protect" the device, I'm considering adding a diode bridge to ensure that the internal "red wire" always carries the positive, and the "black wire" is always ground. Do more experienced folks consider this a good idea? Is it necessary? Would it be better, perhaps, to just epoxy the end plug so it can't be reversed?

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Kiteman
Kiteman

11 years ago

Is the jack really symmetrical?

As you may know, UK plugs are 3-pin, but some devices do have 2 pin jacks at the other end of the flex.

All the leads that are symmetrical are also before the transformer. All the two-lead jacks that are after the transformer and rectifier are actually slightly asymmetric, so that they only fit easily one way round.

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kelseymh
kelseymh

Reply 11 years ago

It's actually designed to be symmetric! the end of the wall-wart's cord has an indicator on it. The plug can be inserted with either "+" or "-" aligned with the indicator, specifying the polarity of the outer shell.

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Kiteman
Kiteman

Reply 11 years ago

!

In that case, I'd add the rectifier.

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NachoMahma
NachoMahma

11 years ago

. That will work for "idiot-proof" operation, but, as Binary Boy points out, for simple device protection, only one diode is required.
. Don't forget to take forward voltage drop into consideration.

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11010010110
11010010110

11 years ago

this option ensures right polarity with any connection or if you use ac to ac adapter for simple protection against accidental wrong connection 1 diode is enough the way you suggest is great and better than anythning with epoxy etc