Tim Wescott 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

Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.

--

Tim I made a few repairs that were obvious and discovered the power
supply design was minimal at best. It does have what seems to be a
very heavy duty transformer probably 30 amps CCS. There is no
overcurrent/overvoltage protection. The heat sinks are small in
comparison to my Pyramid brand 20 amp power supply. The transformer is
center tapped but a paralelled pair of bridge rectifers are being used
with the (-) leads disconnected. The regulator doesnt have its own
power supply or even operate from higher voltage taps on the
transformer. Oh well, I bought it for the transformer and it looks
like I got a good deal on that and the box.

Jimmie

On Jul 1, 6:20 pm, wrote:
Tim Wescott 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

Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.

--

Tim I made a few repairs that were obvious and discovered the power
supply design was minimal at best. It does have what seems to be a
very heavy duty transformer probably 30 amps CCS. There is no
overcurrent/overvoltage protection. The heat sinks are small in
comparison to my Pyramid brand 20 amp power supply. The transformer is
center tapped but a paralelled pair of bridge rectifers are being used
with the (-) leads disconnected. The regulator doesnt have its own
power supply or even operate from higher voltage taps on the
transformer. Oh well, I bought it for the transformer and it looks
like I got a good deal on that and the box.

Jimmie



Personally I use a cheap FET,100 amps is like $3.00. With 0.004 ohms
on resistance. Dual 30 amp schottky diodes for a 60 amp bridge. A
TL431 as under voltage detector to clamp the gate to ground for short
circuit protection. I gain a few extra volts because of the diode drop
is lower and low on resistance of the FET. Which translates into more
current from the same supply, that used bipolar transistors and
regular diode bridge.

73,

N8ZU