Old school PSU - Page 1
Forum section: electronics / beginners

author: cs.dk
author_rank: Unknown
timestamp: December 05, 2013, 07:33:46 pm
content: Just get a very old home made Power Supply - Cool to see some homemade equipment from entusiasts.It has 10-turn pots for Volts and Ampere.When voltage is dialed up (unloaded), it is reading the output voltage. (because there is no load?)The max. output current seems easy to set, short the output, and dial up to what you want ie. 0.5AWhen the output is shorted, the voltage drops to nothing, and a OC-diode (yellow) turns on. - Tested with a cars blinker bulb - 12,6V was dialed in, and it draws 1,75A per the display, this seems reasonable. My multimeter comfirms the display readout within allmost nothing.Some strange thing, when i turn the amp down, the volts is dropping with it? - ie. Same bulb as above, turned down to 1,25A the voltage dropped to 6,3VDoes anyone have an idea, whats inside that sucker? - I think it is a 80's design, but it seems to work flawless. I'll take it apart, if i can open the glued wooden holster, without to much damage. - Or it could be opened and come in a new aluminium case.. Thinking, thinking.EDIT: Forgot the pics!First picture - Leads open.Second picture - Leads shorted.Third picture - Pretty large heatsink.

author: Unknown
author_rank: Unknown
timestamp: December 05, 2013, 07:50:14 pm
content: I think some basic understanding of voltage sources, current sources and Ohm's law would help. I'm sure basic electronics textbooks and sites like allaboutcircuits.com) have done a much better job describing these than I'd do, so I'm not going to repeat what you can easily find in these resources. The power supply can either act like a voltage source, in which case the output voltage is controlled by the voltage pot, or as a current source with the current determined by the current knob. It sounds like the OC diode will light when it's acting like a current source, and will be off when it's acting like a voltage source.Assume the light bulb acted like a resistor (a not very accurate assumption, but close enough) with a resistance of about 7 ohm. Putting 12.6 V across it would force about 1.75 A of current through the resistor according to Ohm's law. If you then lower the current by turning the current knob, the voltage also needs to go down to keep obeying Ohm's law.Power supplies from that era would often either use an LM723 or a regulator built out of two op-amps. Schematics of each type should be easy to find. I know Elektor published several designs during the eighties, for example.

author: sdg
author_rank: Regular Contributor
timestamp: December 05, 2013, 10:56:21 pm
content: Quote from: cs.dk on December 05, 2013, 07:33:46 pmThat darned U=RI acting again ?

author: cs.dk
author_rank: Unknown
timestamp: December 06, 2013, 04:45:18 am
content: Quote from: alm on December 05, 2013, 07:50:14 pmSure ohms law will stay in place.I just had in my mind, that the display allways should show the dialed in voltage. - Your explanation makes sense.It could be a LM723, except for the lowest value. "Output Voltage Adjustable from 2V to 37V". Max. voltage seems to be ~36V unloaded, haven't testet with load yet. I can though get lower then 2V. Gotta open this thing.

author: Jonny
author_rank: Regular Contributor
timestamp: December 06, 2013, 09:21:00 am
content: Quote from: cs.dk on December 05, 2013, 07:33:46 pmDon't turn it on.....TAKE IT APART!