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Odd voltages on home built PSU

  1. Mar 12, 2009 #1
    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!
  2. jcsd
  3. Mar 12, 2009 #2
    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.
  4. Mar 12, 2009 #3


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    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.
  5. Mar 12, 2009 #4
    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.

    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.
  6. Mar 13, 2009 #5
    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.
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