Re: Power supply
- From: Paul Keinanen <keinanen@xxxxxx>
- Date: Mon, 30 Jun 2008 08:04:50 +0300
On Sun, 29 Jun 2008 09:32:47 -0700 (PDT), jimmie68@xxxxxxxxx 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
.
- References:
- Power supply
- From: jimmie68
- Power supply
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