I have a power supply that I built as one of my first projects when I was in high school, in the mid 1960s, and it has always worked well enough for my needs. It's in a Bud "Portacab" 6" wide, 8" high, and 8" deep. It has 2" analog meters, 0-10 VDC, and 0-3 ADC, and a ten-turn pot for output voltage adjustment. It has a 2 wire non-polarized lamp cord for AC supply, with a fuse on one of the input lines and one on the output.
Recently I have been doing some work with switching converters and I have tried to use this old supply instead of a 12 V SLA battery, but the regulation seems to be really bad, and when I try to get more than one or two amps output, the voltage drops to about 7 or 8 volts which is really unusable. I seem to remember it regulating pretty well when I built it 40+ years ago, and I figured it must just be a filter capacitor that has finally gone bad.
I probably have not opened this thing up since I first built it, and I don't have the schematic, but I knew it was a very simple circuit, using transistors in emitter follower mode. On the back of the supply are two black heat sinks probably good for 35 watts each, and on each I found a
2N1540 TO-3 transistor, which is a PNP Germanium device rated at 45 volts and 3 amps. They were tied in parallel, with no emitter resistors for current sharing, so that was not an indication of a good design.Then I traced the internal connections. The power transformer is a Stancor RT-204, labeled as a Selenium Rectifier type. It has two main output windings of 16 VAC each, at 4 amps, so it is capable of 128 VA, much more than the 10 volts at 3 amps as indicated by the meters. There is a large full wave bridge in a plastic package (Motorola), and the output is filtered by a large 450 uF 50 VDC metal can capacitor. There is a second capacitor of the same type that appears to be just a filter across the zener diode, which is a 1N2976B, nominally 12 VDC 10 watts.
There are also two more TO-3 power transistors mounted on a small aluminum heat sink inside, and these are 2N554, which are also PNP Germanium devices of unknown rating, but they appear to be connected in a darlington configuration to drive the bases of the main output pair, from the pot which is across the zener diode. There is a second DC voltage supply that was derived from an extra winding on the transformer, and this seems to be used to provide a higher voltage to assure that the output transistors stay turned on under heavy load conditions. There is also a 500 uF 15 VDC capacitor directly across the output.
So, it's not a good design, and it uses ancient parts, and the output voltage and current are not what I really need. Of course, there are many new (or even old) power supplies I could buy to meet my needs, and I already have several of various types, but not exactly what I want. Now my question is what to do with this so that it might be of some use, without a really major redesign.
The transformer should be able to provide about 14 VDC at 8 amps, which would be perfect for 12 VDC battery circuits. Rather than replacing the meters, I could add a switch for x2 on voltage, for 0-20 VDC, and I could do the same on the ammeter (although not as easily) for 0-6 ADC, which is enough.
If I use the existing PNP Ge transistors in emitter follower configuration, I think it will be enough to use two base resistors to balance the current, but I will not be able to use a simple pot on a zener to provide regulation. It is also difficult to wrap my brain around a circuit that uses a positive ground and a variable negative output, but that's how this needs to be unless I turn the drive transistors around. That would also make it easier to add a current limiting function, which I would like to have.
I worked out a possible circuit in LTspice (see end of post) that should work adequately, and I could keep many of the original components (except I need to use at least 10,000-30,000 uF filter capacitors for 6 amps). I simulated one darlington connected PNP transistor with an NPN/PNP pair, which I can use if the original Germaniums go bad. I have a 3 amp current limit for the single pair, but I'll add another set to get 6 amps maximum. It's not a great circuit, but it's much better than what I have now. It'll be cool to have a power supply made with 1960s technology, but better than my rather poor original efforts.
Any thoughts before I heat up the iron?
Paul
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Version 4 SHEET 1 880 680 WIRE -256 -192 -512 -192 WIRE 240 -192 -256 -192 WIRE -656 -128 -688 -128 WIRE -512 -128 -512 -192 WIRE -512 -128 -592 -128 WIRE -256 -128 -256 -192 WIRE -192 -128 -256 -128 WIRE -32 -128 -112 -128 WIRE 272 -128 -32 -128 WIRE 464 -128 368 -128 WIRE -576 -16 -624 -16 WIRE -512 -16 -512 -128 WIRE -32 16 -32 -128 WIRE 32 16 -32 16 WIRE 320 16 320 -64 WIRE 320 16 128 16 WIRE 464 16 464 -128 WIRE 464 16 368 16 WIRE 528 16 464 16 WIRE 592 16 528 16 WIRE -688 80 -688 -128 WIRE -576 80 -688 80 WIRE -256 96 -256 -128 WIRE -208 96 -256 96 WIRE -96 96 -144 96 WIRE 80 96 80 80 WIRE 80 96 -32 96 WIRE 80 112 80 96 WIRE 240 112 240 -192 WIRE 368 112 368 16 WIRE -688 128 -688 80 WIRE 528 160 528 16 WIRE -256 176 -256 96 WIRE -128 240 -160 240 WIRE -80 240 -128 240 WIRE 16 240 0 240 WIRE 368 240 368 192 WIRE 368 240 304 240 WIRE -688 272 -688 208 WIRE -624 272 -624 -16 WIRE -624 272 -688 272 WIRE -160 272 -160 240 WIRE 160 288 80 288 WIRE 240 288 160 288 WIRE 368 288 368 240 WIRE -576 304 -576 80 WIRE -464 304 -512 304 WIRE -688 400 -688 272 WIRE -640 400 -688 400 WIRE -464 400 -464 304 WIRE -464 400 -576 400 WIRE -256 400 -256 240 WIRE -256 400 -464 400 WIRE -160 400 -160 352 WIRE -160 400 -256 400 WIRE 160 400 160 368 WIRE 160 400 -160 400 WIRE 368 400 368 368 WIRE 368 400 160 400 WIRE 528 400 528 240 WIRE 528 400 368 400 FLAG 592 16 Vout FLAG -128 240 Vin FLAG 368 400 0 SYMBOL npn 16 192 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL npn 304 192 M0 SYMATTR InstName Q2 SYMATTR Value 2N3904 SYMBOL pnp 128 80 M270 SYMATTR InstName Q3 SYMATTR Value 2N2907 SYMBOL res 64 96 R0 SYMATTR InstName R1 SYMATTR Value 30 SYMBOL res 224 96 R0 SYMATTR InstName R2 SYMATTR Value 50 SYMBOL res 144 272 R0 SYMATTR InstName R3 SYMATTR Value 50 SYMBOL res 352 96 R0 SYMATTR InstName R6 SYMATTR Value 500 SYMBOL res 352 272 R0 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL voltage -688 112 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR SpiceLine Rser=.1 SYMATTR InstName V1 SYMATTR Value SINE(0 22 60 0 0 0 100) SYMBOL voltage -160 256 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 -91 198 Left 0 SYMATTR InstName V2 SYMATTR Value PWL(0 .1 5m .1 10m 1 20m 1 30m 2 40m 2 50m 4 60m 4 70m 8 80m
8 90m 10 100m 10) SYMBOL res 512 144 R0 SYMATTR InstName R8 SYMATTR Value 4 SYMBOL npn 272 -64 R270 SYMATTR InstName Q4 SYMATTR Value 2N3055 SYMBOL res -96 -144 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 0.2 SYMBOL diode -208 112 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode -96 112 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode -656 -112 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D3 SYMATTR Value MURS320 SYMBOL diode -576 0 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D4 SYMATTR Value MURS320 SYMBOL diode -512 320 M270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D5 SYMATTR Value MURS320 SYMBOL diode -576 416 M270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D6 SYMATTR Value MURS320 SYMBOL polcap -272 176 R0 WINDOW 3 24 64 Left 0 SYMATTR InstName C1 SYMATTR Value 6800µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=25 Irms=2.07 Rser=0.03 MTBF=2000 Lser=0 mfg="Nichicon" pn="UPR1E682MRH" type="Al electrolytic" ppPkg=1 SYMBOL res 16 224 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 1k TEXT -256 424 Left 0 !.tran 0 .1 10u