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Really old homebrew power supply

P

Paul E. Schoen

Jan 1, 1970
0
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.
http://www.stancor.com/wrdstc/pdfs/Catalog_2006/Pg_008_10.pdf

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

==========================================================================

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
 
P

Paul E. Schoen

Jan 1, 1970
0
John Larkin said:
Good chance the electrolytic caps have dried up.

I will definitely replace those. They are only 450uF 50V, and only one is
used for raw filtering, so I don't know how it ever worked at 3 amps
output. The ripple was probably horrendous. I don't think I had a scope at
that time, and I usually did not use the supply for high current. I will
probably use 30,000 uF at 25 VDC or something similar that I have in my
surplus stuff.

A few years later I built a much better supply that had decent regulation
and current limiting. But it was 0-25 VDC at 1 amp, and it is now in my
junk box with other old projects.

Any idea on how long electrolytics last? What about ways to test them? I
would think injecting a constant current and reading the voltage to check
the capacitance value, and then monitor the self-discharge to determine
leakage?

Paul
 
J

Joerg

Jan 1, 1970
0
Paul said:
I will definitely replace those. They are only 450uF 50V, and only one is
used for raw filtering, so I don't know how it ever worked at 3 amps
output. The ripple was probably horrendous. I don't think I had a scope at
that time, and I usually did not use the supply for high current. I will
probably use 30,000 uF at 25 VDC or something similar that I have in my
surplus stuff.

A few years later I built a much better supply that had decent regulation
and current limiting. But it was 0-25 VDC at 1 amp, and it is now in my
junk box with other old projects.

Any idea on how long electrolytics last? What about ways to test them? I
would think injecting a constant current and reading the voltage to check
the capacitance value, and then monitor the self-discharge to determine
leakage?

Good ones last forever. The ones in the Sachsenwerk radio in my lab are
from the 40's and still fine. So is the main filter cap in the old
Hammond organ from 1961 even though they placed the rectifier tube right
next to it (arrgh!) so it's close to boiling all the time. They also
placed that tube way in back and I wonder how many curtains might have
been cinged, or worse ...

Bud Portacab? Man, you must have had a fat allowance back then. This is
some of my stuff I built when I was young, all enclosures either from
some scrap yard or made from scratch:

http://www.analogconsultants.com/ng/oldprojects.html

The camera doesn't count because I was over 20 by then.
 
P

Paul E. Schoen

Jan 1, 1970
0
Joerg said:
Good ones last forever. The ones in the Sachsenwerk radio in my lab are
from the 40's and still fine. So is the main filter cap in the old
Hammond organ from 1961 even though they placed the rectifier tube right
next to it (arrgh!) so it's close to boiling all the time. They also
placed that tube way in back and I wonder how many curtains might have
been cinged, or worse ...

Bud Portacab? Man, you must have had a fat allowance back then. This is
some of my stuff I built when I was young, all enclosures either from
some scrap yard or made from scratch:

http://www.analogconsultants.com/ng/oldprojects.html

The camera doesn't count because I was over 20 by then.

My father worked in an electronics lab and the PortaCab was, er, surplus,
as were some of the other parts. I'll have to take a photo of it, and some
of the other old projects I have on shelves and buried in boxes. Your
enclosures look really nice.

I have an old power supply that my father bought from a *blind* radio
repair man, probably in the late 30s, and I'll have to see if it still
works (but I cut the cord off when the rubber and cloth insulation totally
disintegrated). And I also have some WWII vintage meters that my father got
when he was in Germany during the occupation. The wooden cases on them were
really well made, and the wiring was beautifully done, with varnished
cambric insulation.

Paul
 
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