Thanks Jim for posting the flaws in that design !

Here is a better schematic http://www.wa8dbw.ifip.com/rs-70a.gif

for an Astron 70a linear ps.

clarke


Jim wrote:

On Mon, 20 Oct 2003 13:57:46 GMT, in
, clarke wrote:

Hi, check this link and see what you need to do,
one or two pass trans. will not do the job for you,
easier to use 6-10 pass trans to share the load.

http://www.mitedu.freeserve.co.uk/Ci...er/1230psu.htm

Is your transformer able to supply necessary power to this supply ?

I'm not sure anyone has ever built the PSU shown at the above
URL, and if so, I'd guess they haven't run it at the full 30-amp
output for more than a few seconds.

The basic design demonstrates a generally acceptable approach to
designing a power suppy, but some of the components chosen just
don't seem up to meeting the design criteria of 24 volts in, 12
volts out at 30 amps.

The ballast resistors are rated at 1 watt, but will be expected
to dissipate roughly 2.5 watts at the full 30-amp output.

The text analyzing the regulator performance has me baffled. It
correctly describes a current gain of about 35 for the pass
transistors, which at a glance suggests the regulator has to
handle a bit under 1 amp. The web page author says 871 ma is the
current into the regulator input and 866 ma is the current out,
for a net current of 5 ma and a cool regulator. Looks to me like
that poor thing is going to be asked to dissipate about 10 watts
and will run about 110 degrees C even if mounted on an *infinite*
heat sink. I'd redesign to include a transistor driver.

I'd say the designer of this supply never ran it at full output
for more than a few seconds.

--
Jim Higgins, KB3PU

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