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Paul Keinanen · June 30th 08, 06:04 AM posted to rec.radio.amateur.homebrew

On Sun, 29 Jun 2008 09:32:47 -0700 (PDT), 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.

Some of the Astron power supplies had a separate (higher voltage)
power supply for the control and driver stages and the input-output
voltage difference could be as low as Vce(sat) i.e. below 1 V, thus
the power dissipation could be minimized.

However with a darlington, the minimum voltage drop would be Vce(sat)
for the driver transistor and Vbe for the big transistor, thus
requiring a higher transformer secondary voltage and hence suffer a
large dissipation in the series pass transistors.

Note also that even if the darlington could handle 50 A, the power
dissipation for TO-3 packages are typically in the 100-150 W range so
a single transistor could tolerate only 3-5 V voltage drop at 30 A.
However, those power dissipation figures apply for 25 C case
temperature, so in practice, this could be achieved only if the
transistor and the heat sing is submerged into running cold tap
water:-).

So in practice, you would have to use several darlingtons in parallel
with large heat sinks and also use larger emitter resistors to balance
out the gain differences between darlingtons. This may require a
slightly higher transformer secondary voltage.

Paul OH3LWR

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