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Highland Ham · July 2nd 08, 12:04 PM posted to rec.radio.amateur.homebrew

wrote:

Highland Ham 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.
===========================================
If the design of what you have is 'debatable' and having NPN Darlintons
which require relative low drive current you might consider a very
simple circuit with a LM317 in a wrap-around circuit.
Two references :
1) Experimental Methods in RF Design , by W7ZOI ,et al ,page 1.15

2) Radcom (RSGB) October 2001 ,page34 : A simple rugged power supply
,by OZ1XB

Frank GM0CSZ / KN6WH

For what it worth almost any 3 legged regulator will work in this
circuit. The minimum voltage of the power supply will depend on the
voltage of the regulator. Basically the LM317 is a 1.25 volt
regulator.
===========================
The LM317 ,being basically a 1.25 v regulator can have a variable output
by means of a potmeter or any fixed output by means of 2 fixed resistors
,that's why it is often used as a universal device.

Frank GM0CSZ / KN6WH

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