AC-DC converter

Discussion in 'The Projects Forum' started by marksan, Mar 18, 2009.

  1. marksan

    Thread Starter New Member

    Mar 18, 2009
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    I need a circuit that converts a 2-phase, 26 vAC, 400 Hz input to +/- 15 vDC (may get away with +/- 12 vDC). Only need about 125 mA for power.
    I have very little board real estate to work within. Can I do this with a 4-diode full wave rectifier and some voltage regulation, thereby skipping the need for a transformer?
    Is the 2-phase AC input versus a single phase going to complicate things?

    Thanks.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    By 2 phase you mean a neutral wire and two live wires?
     
  3. marksan

    Thread Starter New Member

    Mar 18, 2009
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    Yes, that's correct. Two live and one neutral.
    Thanks.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You'll lose 1v across the rectifier bridge, and more across the regulators.

    What is the board space that you have to work with?
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Use one phase and neutral to produce the positive supply and the other phase and neutral to produce the negative supply.
    You can use a bridge rectifier for each of the voltages you want. However, a half wave rectifier might work because you need only 125mA if you use a large filter capacitor and if the AC source can provide enough current to charge the capacitor quickly.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sorry I closed this thread - I saw 400 Hz and assumed 120 VAC.

    As the lady on SNL used to say, "Never mind".
     
    Last edited: Mar 18, 2009
  7. Papabravo

    Expert

    Feb 24, 2006
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    Emily Litella, played by Gilda Radner

    http://en.wikipedia.org/wiki/Emily_Litella
     
  8. marksan

    Thread Starter New Member

    Mar 18, 2009
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    I have approx. 1.0 x 1.25 inches. Not much room.

    Thanks for the response.

    Semper Fi, brother.
     
  9. marksan

    Thread Starter New Member

    Mar 18, 2009
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    Thanks mik3. I'll see if I have the board space for a bridge rectifier, otherwies I'll give the half wave a shot.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Have a look at the attached. I've used a simple configuration that won't protect against shorts or overheating, but has a much lower dropout than an LM317M or 78Mxx/79Mxx series regulators, and as the parts are available in TO92 cases, are reasonably small. You could opt for SMD/SMT equivalents as well. Regulation won't be as good as the aforementioned regulators, but it should be reasonably stable within a few mV's, depending upon the nature of your load.

    Your big problem (literally) is finding suitable capacitors that have a small enough footprint.

    The diodes are no big deal (sic); you can position them all vertically.

    I'm afraid you're not going to make it with a halfwave rectifier scheme; you'd need to use much larger capacitors. Their increased footprint will send you back to the drawing board.
     
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