# Battery Charger for power supply.

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

1. ### Jack P. FlashGuest

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 FieldsGuest

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 BowdenGuest

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. ### JamesGuest

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. ### jsmithGuest

So what do you need the capacitor for anyway???

6. ### Rheilly PhoullGuest

Uhh The 138 is a 5amp jobbie isnt it ??

10. ### jsmithGuest

Since when is "charge reservoir" necessary when charging a lead acid
battery??

11. ### John FieldsGuest

---
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...