raypsi wrote:
The voltage gain of darlingtons is less that unity as a emitter
follower. but not the current gain. The base current is like 20 ma.
and the collector is like 20 amps that's a gain of 100. A good
regulator like a LM723 can regulate that easy. If the gain ain't there
in the darlingtons you need a drive transistor.

Huh, I thought the 723 was a voltage regulator, not current. Im sorry
but what you are saying just doent make sense to me. It is my
understanding that the greater current gain of the Darlington
transistors is just going result in the 723 seeing a lighter load wich
would be godd for the 723. Perhaps that could be a problem if it were
light enough, maybe this would result in the 723 becoming unstable but
this does not appear to ba a problem with light loads, no loads or
heavy loads.
When I first started working on the power supply there was one tiny
problem using the Darlingtons that was more a result of the poor
design of the origonal circuit than anything else. The the origonal
circuit monitored the ouput voltage at the base of the pass
transistors. Because of the 1.6 base to emiter drop of the darlingtons
the output voltage was a little low. After moving the pick off point
to the emitter I no longer had this problem.

I use a 0.1 ohm 50watt resistor series with the output, that gives me
an extra 2 volt drop that isn't across the transistor and keeps 40
watts from heating up the output. You still lose 40 watts. It just
goes up in the resistor. The 0.1 ohm 50 watt is only good for like 22
amps.

Put a fan on the heatsink. 170 watts will get hot all the time.
I put a computer power supply fan on my supply, the output stays alot
cooler.
Once the current drops the fan cools it really fast.

You have a high transformer voltage because it's a 50 amp supply and
you are running it at only 20 amps.

On Jul 15, 8:23�am, wrote:
happen.

The output of the regulator will likely try to go to its full input voltage rail
(because of the high gain of the darlingtons). �The feedback loop in the
regulator will try to bring it back down to the set output voltage. �It probably
can't because the gain of the darlingtons is too high, and the output stays
banged against the input rail (22V).

I dont understand what you mean by this. The voltage gain of the
darlington transistors is less than unity in this case since they are
being used as emitter followers.

I am dropping 8.5 volts at 20 amps or 170 watts is being dissipated by
the transistors. This may not be a real problem. I know my commercial
power supply of similar size only provides about 18 volts to the
collector of the pass transistors and this �would result in about half
as much power being wasted. What voltage would you normally expect at
the the collector of the pass transistors. I have also thought that
maybe I dont have that much of a real problem since I doubt if the
power supply will ever be put under the same demands as it does
working into my load bank. Also I think I should add two more
transistors in paralell with the two I have. I think this would
distribute the heat better over my heat sink and have less localized
heating.

I dont think the size of my heatsink is a problem, it is probably
overkill. it is a 8in chimney type heat sink( i think that what you
call them) 8x8x10 inches finned inside and out. The transistor body is
mount on the outside �withe the wires going down the inside of the
chimney. The end of the chimney is designed to take a muffin fan.

Jimmie


I added two more pass transistors and this seems to have helped a lot.
I was once given a rule of thumb that a TO3 transistor should never
have to handle more than about 7 amps continously. I was pushing them
to 10amps. Even though they are rated at 50A they will not handle
anywhere near that without extreme measures to keep them cool.
Interesting note, at 40amps they dont get is hot with 4 transistors as
it did with 20amps with 2 transistors. I didnt check yet but I suspect
that is because the voltage at the ouput of the filter rectifier is
beginning to drop off a bit. If I continue to try to use this power
supply I would probably increase the number of pass transistors to 8.
This would be easy to do as the existing heatsink is already prepared
for it.

Thanks much for the advice
Jimmie