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Battery Charger for power supply.

Discussion in 'Electronic Basics' started by Jack P. Flash, Feb 4, 2005.

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  1. Hi,
    I am attempting to build a power supply that will supply 6 to 14v DC at
    around 20amps. I'm going to parallel some LM338's for the regulator side
    and now am looking for a transformer.
    Is there any reason I couldn't use a transformer out of a battery
    charger? I've got some large caps, 22,000uf 50v electorlytics and a
    large bridge rectifier, 40amps at 50v I believe.

    Any help?

    Thanks

    James
     
  2. John Fields

    John Fields Guest

    Just paralleling the regulators doesn't sound like a good idea. You
    might want to look at the adjustable 15A regulator on page 14 of
    National's LM138 data sheet at:

    http://cache.national.com/ds/LM/LM138.pdf
     
  3. Bill Bowden

    Bill Bowden Guest

    It may be hard to get 14 volts at 20 amps from
    a battery charger transformer.

    12 volt battery charger transformers are
    typically 17 volts peak.

    You can work out the filter cap size
    using about 8,000uF per amp per volt.
    So, if the supply is 17 and the load is
    20 amps, and the voltage can only fall
    a couple volts, the cap will be
    (8000*20)/2 = 80,000uF

    The other problem is heat from the
    regulators. If you adjust the output
    to 6 and the input averages around
    15, and the load is 20 amps, the
    heat dissipation will be
    (15-6)*20 = 180 watts.

    This will require a fairly large heat
    sink, and possibly a fan to cool it off.

    If the transformer has a center tap, you
    can switch between using the entire winding
    with a bridge rectifier or the two halves
    with 2 diodes and the center tap at ground.
    This will reduce the heat losses when you
    need a lower voltage.

    A multi-tapped transformer would be even better
    since you can keep the input very close to the
    output and minimize the heat loss.

    -Bill
     
  4. James

    James Guest

    John,
    I had the same thought.. Parelleling the regulators is a bad idea.
    However.. I ran across this application note on National's site
    outlining the way to parallel the regulators and get them to load share
    within 60mv.
    http://www.national.com/an/LB/LB-51.pdf

    Thoughts on that??

    Thanks
    James
     
  5. jsmith

    jsmith Guest

    So what do you need the capacitor for anyway???
     
  6. Uhh The 138 is a 5amp jobbie isnt it ??
     
  7. John Fields

    John Fields Guest

     
  8. John Fields

    John Fields Guest

     
  9. John Fields

    John Fields Guest

     
  10. jsmith

    jsmith Guest

    Since when is "charge reservoir" necessary when charging a lead acid
    battery??
     
  11. John Fields

    John Fields Guest

    ---
    If you don't use a charge reservoir, then reverse leakage current when
    the battery conduction angle is less than 43° will cause osmotic
    heating of the separators and the rectifiers will lose their
    passivation.

    Duh...
     
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