[G0HZU_JMR]

I dug my old homebrew 13.8V PSU out today and cleaned the dust off it. See the image below. This PSU design is the same as the one linked to in the original post. It hasn't been used in a long time but it still works! It produced 13.69V on a DVM and the electronic load which isn't bad considering it hasn't been adjusted in nearly 35 years.

I did a few tests on it with a modern electronic load and looked at it with a thermal camera. I found a couple of hot connections in a terminal block that needed tightening and one of the pass transistors was running a bit hotter than the others. The TO-3 fasteners needed a quick nip up to cure this. I did a 1 hour soak test at 12A load current and then looked at the temperature of the two big heatsinks and the metal case of each 2N3055 pass transistor.

Each heatsink reached 68degC when viewed right next to each 2N3055 and the hottest 2N3055 case temperature was at 80degC. The coolest 2N3055 was at 78degC.

I also modified an old excel spreadsheet to include the option of multiple heatsinks and pass transistors and this predicted 65degC for the heatsinks and 75degC for the case of the 2N3055 pass transistors. This was quite close to reality. I measured 6.5V drop across the pass transistors and guessed the heatsinks at 1.1degC/W. I'm pretty sure I bought heatsinks with the same rating as the original design but they look quite small. Each has a volume of about 135mm x 135mm x 31mm but they have lots of fins so this helps with the performance.

See below for an image of my PSU. As is often the case, I never really finished this PSU project. I never got around to adding a meter at the front and I never made any decent labels for the front panel. It also has an aux 5.0V supply output and I can't remember why I added this. I think it is only rated to 250mA max. There's also a screenshot of the excel spreadsheet.

If I increase the load current to 20A on the spreadsheet then it indicates that the junction temperature of the 2N3055 transistors would be quite worryingly high if soak tested at full duty. However, I only really built it to power a typical 100W transmitter and this would never be run at full duty like this.