Richard, After a big search through my old documentation I have found all my design notes on the power supplies I built, I still have the Fairchild voltage regulator application handbook that I consulted at the time. Like you, I needed to build this supply for my first solid state high power equipment and I still use it to this day. The smoothing electrolytics are 2 large 31,000 uF military standard capacitors which were used in battlefield computers.
The short circuit protection of the 25 Amp power supply appears to be something that I "designed" as it doesn't appear in the handbook in the form I used. Basically it uses a transistor that senses the output voltage dropping (as it would with a short circuit on the output) and this then triggers a small SCR which pulls the 723 compensation pin low which makes the 723 output voltage stop. This will remain like that until the short circuit is removed and the current through the SCR is broken by manually pushing a NC switch in series with the SCR. It works well, I have used it to zap NICADS that have gone short circuit! It just gives a brief pulse of high current and trips the supply, no fuse blowing etc. Overvoltage protection is even cruder, it is a standard Crowbar circuit with very big 40 amp SCR and 16 V zener to sense. This will blow a 20 Amp fuse in the raw supply. It has never been used in anger, though I did test it with a smaller fuse. It is there only to protect against either a shorted pass transistor or some fault with the regulator circuitry. Of course having a good current limit circuit with short circuit protection makes the likelihood of a series pass transistor failing short circuit more unlikely. My design has 4 2N3055's in parallel (with low value resistors in the emitters to help share the current equally) driven by another 3055, the current sense resistor is in the base drive of the output series pass transistors. I have over temperature protection of the transformer with a bimetallic switch but that has never tripped. I do have a soft start circuit on the primary using an AC relay and some high wattage resistors. I avoided using fans by using large heatsinks as fans are a source of failure over time. [I recall a commercial 5V 80 amp supply cooling fan failing at night, the supply shutting down for over temperature then coming back on again, rinse and repeat, until nearly every IC in a multimillion $ machine was damaged.] Like so many projects this was developed from earlier projects, starting with a simple single transistor 3A PSU, then an 8A using more transistors, and finally the 25A supply with much more in the way of protection from likely fault conditions. I am not sure I could source the mains transformer or smoothing capacitors so easily nowadays if I wanted to build another one, as nearly all new designs are high efficiency switched mode supplies. I think it best if anyone else wants more details that they take this off the list and contact me directly as we are well over the limit on postings on this subject. 73 from David GM4JJJ > On 23 Jul 2016, at 07:46, Richard Fjeld <rpfj...@outlook.com> wrote: > > 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. > > 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 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. >> >> >> 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 >>> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
