I got the PSU from a hamfest recently as I was interested in getting a PSU that could drive tens of amperes that my homemade series dissipative PSU simply can't handle. I didn't expect what I got.

I thought that these old PSUs were mostly linear series dissipative regulators, and a characteristic of these supplies is a large heatsink in the back, quite possibly with fans - mostly because switching power supplies were available once high speed high current transistors became available in the 70s. Indeed this PSU does have a large heatsink, but curiously no fans. Also the heatsink isn't immense oddly enough, and I couldn't find any series dissipative transistors in the heatsink either. As I did more research into this curious specimen I learned that this was using Silicon Controlled Rectifiers to control power. There are two mounted on the heatsink. As SCRs are either on or off, this counts as an ancient, low frequency switching power supply. So I was wrong, this is a type of switch mode PSU and not series dissipative!

The HL 6427A has a voltmeter and an ammeter. It has fine and coarse controls for voltage limits and current limits, and goes from 0V to 18V and 0A to 15A. This means it can regulate up to 270W. At 12V it will supply 180W. I measured this device to have fairly large overshoot on powerup (seems to be around 33% of setpoint!) but regulation is fair when powered on. It has three 70,000µF capacitors and a large, heavy transformer. The control circuitry is on its own power, which has a small transformer powering it - and this is the rest of the "wasted" power.

I found out that this was how it worked when I tried to power a 12V 4A lamp. The power output is on the back on a large screw terminal distribution strip, where the sense lines are available as well. In any case, if this supply had been series dissipative mode assuming 18V was its high voltage (though it needs to be even higher than 18V), then it would need to drop 24W ((18V-12V) * 4A) of power which I should have been able to feel - but the heatsink didn't really get warm - it remained mostly cold. As this supply could not have been a MOSFET high frequency switching system, a SCR system would be the only explaination of the lack of heat. A SCR that has been triggered and latched on has a drop of just 1V, and thus would only dissipate only around 4W when passing 4A - at a theoretical efficiency of Pout=12V*4A Ploss~=4A*1V Pin=Pout+Ploss = 48W+4W, and finally Efficiency=Pout/Pin=48W/52W=92% (theoretical, does not include transformer and control inefficiencies!) which is fairly good. At low voltages it's not as good because the drop across the SCR is a higher percentage of the output (note: new switching PSUs make this issue better by using MOSFETs instead of diodes to reduce the drop further), but MUCH better than straight series dissipative. Driving a 3V 15A load would mean over 200W wasted in a linear series dissapative, but only 15W or so in this switching SCR system (though the system is now only 75% efficient, again not counting control and other losses.)

My homemade supply driving a 2A 3V load would be Pout=3V*2A=6W, Pin=2A*18V=36W, Ploss=Pin-Pout=36W-6W=30W, and Efficiency=Pout/Pin=6W/36W=17%... and the series pass transistor would be roasting trying to deal with the 36W. I have killed that TIP42 transistor before doing exactly that. The SCR system will have no problem with this kind of load.

Note that the power factor of this PSU must be fairly bad, though it changes depending on the load. At full load it would be better, and at low load the power factor wouldn't matter too much.