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24VAC to 12VDC Power Supply

Discussion in 'Electronic Basics' started by Dean, Nov 13, 2003.

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  1. Dean

    Dean Guest

    Hi,
    I am trying to build a small power supply to take 24 volts AC down to
    12 volts DC. The load is about 200mA at 12 volts. I have tried a
    traditional bridge rectifier --> capacitor --> linear regulator. This
    works but generates alot of heat as the input to the regulator is
    about 33 volts.

    Is there a more efficient way of doing this? Ideally, I would like to
    fit this in a very small ( lipstick sized?) package and for under $10.
    I have seen complete units that do this for sale on the web but would
    like to learn how they work so I can build my own to suit the load.

    Thanks for any help or suggestions.

    Dean
     
  2. When size and heat matter, you go to a switching regulator. These
    chop the input voltage (turn it off and on very quickly) and average
    the pulses with an inductive input filter to produce the output
    voltage. See:
    An introduction to power supplies:
    http://www.national.com/an/AN/AN-556.pdf
    An example of a buck regulator control chip and how to design a
    regulator around it:
    http://www.national.com/an/AN/AN-711.pdf
     
  3. Dan Dunphy

    Dan Dunphy Guest

    If your transformer has a center tap, ground it, and use 2 diodes, or
    half the bridge rectifier.
    Tie the cathodes together and the anodes to the transformer, for
    +16.5V, or reverse the diodes for -16.5. Then your linear regulator
    won;t dissipate near as much heat.
    Dan

    Colorado Springs, CO
    My advice may be worth what you paid for it.
     
  4. The two transistor Black Regulator at www.romanblack.com may be able
    to handle that voltage and current with the right transistors.
    Bcause of the higher voltages, you would probably have to use a fast
    recovery rectifier in place of the Schottky diode.


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  5. Brian

    Brian Guest

    Hi Dean,

    I drew out a switching power supply for you (24V. to 12V.). You can see it
    at http://www.fncwired.com/12VoltExample/
    Brian
     
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