Pwr suppley Problem

[scott93727g]

Pwr suppley Problem
I made a power supply with 3 paralelled LM338 Regulators in paralell
(24V 12A trans), with 2 paralelled 20A rectifirs, worked great for 1
week, then all of a sudden, output dropped to 2 amps max (from 15A), ,
all componed seem to check out ok, getting full current out of filter
caaps past rectifiers, any ideas, tried substituting a good regulater,
and same thing, any ideas.

 

[Lostgallifreyan]

scott93727g <eb3354@aol.com> wrote in news:5bf45acf-8ab7-461f-b777-
cecb22c2ef59@z28g2000prd.googlegroups.com:

Pwr suppley Problem
I made a power supply with 3 paralelled LM338 Regulators in paralell
(24V 12A trans), with 2 paralelled 20A rectifirs, worked great for 1
week, then all of a sudden, output dropped to 2 amps max (from 15A), ,
all componed seem to check out ok, getting full current out of filter
caaps past rectifiers, any ideas, tried substituting a good regulater,
and same thing, any ideas.

Parallel.
Now that that's out of the way, you say you're pulling 15A from a 12A
transformer for a week. Are you sure everything checks out? Or do you just
think it does? How did you test?
Did you remember to include the resistors, one low ohm high power resistor
for each of the LM338's? If not, one will have taken the strain that should
have been shared between them, and thermal shutdown probably prevented
outright failure. That might explain the low current you saw.
Check your rectifiers too, paralleling those won't share equally between
them, you need resistors just as you do with those regulators. If you need
more current than the rectifier accepts gracefully, get a bigger rectifier.

Another thing to check out carefully is your mountings, test to see if you
have case shorts to the heatsink because the outputs are on the case (I'm
assuming you're using TO3 cans) and must not connect together or to ground.