Transformer overheating

[sts1138]

So I'm working on a control box that basically controls all the lighting around my desk(i have blue underglow strips), as well as a couple external 12v builds I have made previously. This time around I decided to try my hand at my own power supply, using a center tapped transformer from radio shack that can output 6.3v or 12.6v(I'm using the 12.6v configuration) at 1.2 amps. The setup uses a 4 diode full wave rectifier with a 1 uf capacitor(just is one I picked, I've got a couple that I've tried that worked). The transformer itself is supplied by a salvaged 2 prong cord.

Now while it supplies power, the problem is that the thing overheats, and at one point started smoking. This happens pretty quickly after getting plugged in. I am still pretty new at this stage of electronics so I can't really troubleshoot the problem too much. Anyone have any thoughts as to what it could be?

 

[Cyparagon]

over-current somewhere. What sort of load are you putting on it? Keep in mind that after rectification and filtering, that 12.6V will be closer to 16V.

1µF is nowhere near enough for this setup. You want several hundred or several thousand µF worth.

 

[Polonium210]

What is the VA rating of the transformer?

 

[Cyparagon]

^12.6V at 1.2A is 15VA

 

[Things]

You've either got too much load on it, you've shorted something somewhere or the transformer has an internal short (unlikely if it's brand new).

Also like Cyparagon said, you're going to end up with a fair bit more than 12V unless you use a regulator.

 

[sts1138]

Duly noted on the true voltage output. After looking at what I soldered together a bit closer the issue may have to do with how the rectifiers are arranged, so a short. Especially considering the fact that it was overheating on open circuit. My testing method has been an led that can take a good amount of abuse, but that's only used for a total of 3 seconds before the circuit would be open again. Which is probably why I thought the layout worked fine until my workbench nearly caught fire. I should probably brush up on my multimeter reading....

So now that I have a theory on what's wrong with it I might as well ask this question: How do I calculate the size of capacitor I would need? Much thanks everyone

 

[Polonium210]

^12.6V at 1.2A is 15VA

I guess I read over the part where he stated the current

 

[Cyparagon]

Calculating the size of the capacitor is dependent on how much ripple voltage you can tolerate in your circuit. When in doubt, get a huge one. It can never be too big.