Odd voltages on home built PSU

[relativespeed]

I'm working on building a power supply (120VAC to ~12VDC). The Transformer puts out 13.6VAC without any modifications. But once I put on a diode on each side (make into DC) then put on a 1000 microfarad capacitor to eliminate the cosine curve it bumps up to 18VDC. Without said capacitor it registers 6VDC (probably due to the cos curve still existing). I'm at a loss here, I tried to use a larger (4700) capacitor and the voltage was the same, so I don't believe it was that at fault. It's a typical 3 output transformer with one black and two yellow wires with the yellow ones being used. Thnx ahead of time!

 

[Bob S]

To get full voltage with a full wave rectification, you need a bridge (full wave) rectifier (four diodes). Full-voltage half-wave rectification requires only one diode. Put a dc load across the capacitor to drain charge off between every (half) cycle to get a better voltage reading.

To get better voltage regulation, put a large NPN transistor (e.g., 2N3055) in series. Use a resistor and zener to control base voltage. Connect collector to capacitor, and emitter to load. Or you could use a 2N7912 or other regulator.

The black wire is a center tap on the transformer secondary, useful for full wave rectification with two diodes. The black wire is ground. But you get only half the voltage out.

 

[alxm]

I'd use a regulator with caps on both sides, to minimize ripple. It's pretty much the standard thing.
For a 12V regulator like a 7812 you'll need at least 15V though, although you can probably find a low-drop version. And remember to attach it to some cooling flange.

 

[relativespeed]

I'd use a regulator with caps on both sides, to minimize ripple. It's pretty much the standard thing.
For a 12V regulator like a 7812 you'll need at least 15V though, although you can probably find a low-drop version. And remember to attach it to some cooling flange.

Well, I have 18V now, so that wouldn't a problem. Nor would a couple heat sinks and a small fan from an old CPU.

To get full voltage with a full wave rectification, you need a bridge (full wave) rectifier (four diodes). Full-voltage half-wave rectification requires only one diode. Put a dc load across the capacitor to drain charge off between every (half) cycle to get a better voltage reading.

To get better voltage regulation, put a large NPN transistor (e.g., 2N3055) in series. Use a resistor and zener to control base voltage. Connect collector to capacitor, and emitter to load. Or you could use a 2N7912 or other regulator.

The black wire is a center tap on the transformer secondary, useful for full wave rectification with two diodes. The black wire is ground. But you get only half the voltage out.

I'd be a little nervous about running a couple amps thru a transistor though, wouldn't it burn out? I'm looking to build a 3AMP transformer box. And could you come up with a diagram or something? I'm not quite getting what you said on how to wire it all.

 

[relativespeed]

Well, asked an electrical engineer, and got a dur answer I should have known: it's not regulated, so the voltage will vary upon load, probably from 18-9 volts pending on ampheres going thru it.