Hello David, (and Alan), and thanks to John for the link showing the
added parts.
David, your PS sounds very similar to the one I built, also in the
70's. Mine has four 3055's on very large heat sinks that are overkill,
but I didn't know what to expect. It was at a time when we were going
from high voltage PS for tubes to Low voltage PS for solid state. I had
not seen a low voltage supply to power a 100 watt rig yet. So I went
heavy duty. My diodes for the full wave bridge I made were stud mounted
on 1/4 inch aluminum pieces for heat sinking, and mounted on wood to
insulate them from each other. There were no full wave bridges in a
package yet.
I'm curious how your supply trips out, and if you used a relay? It
seems to me that I did that, and when the 723 clamps down, the relay
drops to open the primary. I have not had the time to review the
schematic diagram, but as I recall, if a pass transistor shorts out, the
crowbar may try to shunt the unregulated voltage, but the situation is
not going to be good. That is why I like the device ad5x is showing on
his website. It will protect the radio from over-voltage by blowing a
fuse in the DC line.
If Alan is reading this, I'm wondering if I missed where that
'strategically placed diode' is located?
Dick, n0ce
On 7/22/2016 10:16 AM, David Anderson via Elecraft wrote:
I have a couple of homebuilt DC regulated supplies that I built a great many
years ago, the first is a simple 3A 13.5V one that was the very first supply I
ever built and it used a 723 with a single 2N3055 series pass transistor. I
remember I bought a lovely oil filled mains transformer for it, but
accidentally wired up the 120V windings in parallel instead of series for our
240V mains and watched it start to bulge when I switched it on. (after I put a
bigger fuse in it). I ended up using another transformer and the power supply
is still in daily use, built in the early 70's.
I later built a 25 Amp version with 4 3055s and a 723 with overvoltage trip,
and also short circuit protection. It can be shorted out and with barely a
spark it trips out and has to be reset before it comes on again. No fuses to
blow and replace, no destroyed series pass transistors. I built it in a chassis
that was lying around and 35 years later it is still in daily use and still not
got a proper cover made for the chassis. I checked the output the other day on
my scope to see if there was any hum or ripple and on full load I had 10mV p-p
of noise. Regulation still excellent.
Nothing much wrong with the humble 723.
I have seen some truly awful "commercial" supplies with no short circuit
protection, that blow the series pass transistors like fuses, or worse make them into
short circuits that apply the full unregulated supply on to the load.
73 from David GM4JJJ
On 22 Jul 2016, at 15:55, Alan Bloom<n...@sonic.net> wrote:
The 723 regulator has some known reliability issues, but with proper design
they can be mitigated. In particular the differential voltage on the error
amplifier inputs is only rated for 5v. If one input is connected to the 7.15V
reference, then if the power supply output is shorted the voltage rating is
exceeded. The solution is a strategically-placed diode.
Alan N1AL
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