I bought a 12V wall transformer from radioshack yesterday. So I plugged it in and tested the outputs with a voltmeter and got 14.7V. This is fine, I knew that the voltage was not going to be exactly 12V. what concerns me is that I need a ground for my circuit that I'm working on. So I simply ran a wire from the ground prong in another wall outlet, thinking that I could use that. But when I measured the voltage from the ground to either of the wall transformer outputs, I get .65V. This doesn't make any sense to me. If the voltage difference between the transformer outputs is 14.7V shouldn't the difference between the ground and one of them be half of that? Why is it going down to .65V?

 

Could you post a schematic? That would certanily make it easier to understand.

 

Hi,

You have just discovered that wall transformers' outputs float - that is, they are not referenced to the outlet ground.

If your circuit needs a ground reference, use that wire to the outlet ground pin. If you need +12 volts, just tie the negative lead out of the wall transformer to the ground line, and its output will be referenced to ground.

 

Thanks beenthere. You were absolutely right. I did what you told me to do, and now it works perfectly. Thanks a lot.