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cliff wright · July 14th 08, 05:36 AM posted to rec.radio.amateur.homebrew

wrote:

wrote:

I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie

Ok, I went through the amp and it is a cheap made piece of junk
probably made to power a CB amp. No over current/vpoltage protection
except fuses on both the input an output. It does have a few things
going for it. The power transformer is truly massive. I dont doubt it
would do 50 amps intermittently. The computer grade capacitors are
120,000uf total. The rectifiers were a couple of 25 amp bridges
paralelled. I got rid of that and replaced thm with some leadless
rectifiers that came out of some old telco equipment. I think they
were part of a 200 amp bridge.

The main problem I have with it is the output transistors get hot,
they are well heatsinked. The voltage on the collector of the pass
transistors are 22volts. I think this is a little high. Is there a
good way to bring this down. The transformer doesnt have any taps. I
can put a big VARIAC on the front of of it to get it down but this is
not my first choice. I think I remember that Sorenson used to make a
power supply that used an SCR or TRIAC circuit on the primary side of
the transformer as the control element of their regulator.

Any ideas on how to make this a useful device would be appreciated

Jimmie
First off Jimmie I would check out the device numbers on the power
transistors. If a search tells you that they are Darlington devices get
rid of them right off!!!
Darlingtons have a minimum collector/emitter voltage of ~1.5 volts or
more so they will get very hot indeed at 30 amps or so.
The ideal would be to use a switching type regulator and power FET's but
even high current (30 amps or more) bipolar devices would be a big
improvement. Check out the figure for Vce(sat) at high currents, tha
lower the better.
BTW if they are not Darlington devices you might have to use external
transistors to drive them unless you have a very "grunty" zener and
waste a bit of power to give them a reference base voltage.
Cliff Wright.ZL1BDA

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