Helper Instruments PS 250 Power Supply.
I finally got around to testing this Helper Instruments PS 250 Power Supply which was among some items given to me by a client. With the power supply set for 5 Volts DC output, the ripple was outrageous. Helper Instruments sold out to Zetron Corporation many moons ago, and this power supply hasn’t been in production in decades. To make matters more interesting, Zetron discarded all the files on the Helper Instruments equipment many years ago as well. Schematics and documentation on this power supply are impossible to find. Time to do a little detective work and experimenting.
With the oscilloscope set to DC coupling, this was the output of the power supply. Swinging between positive and minus 1.8V or so was a sign of definite problems.
Although cosmetically the power supply is in excellent condition, given its age I was sure the electrolytic capacitors were well beyond their rated life. After replacing all of them, things looked better, but there was still way more ripple on the output than I wanted to see. Finally I resorted to simply installing a 47 uF electrolytic capacitor internally across the output leads. The power supply now works fantastic.
When all else fails, just go back to the basics. In this case, it meant adding a 47uF ripple filtering capacitor between the positive and ground leads at the output terminals. Problem solved.
The cool thing about this power supply is that you can preset a current limit or trip value between 0 and 3 Amps, then dial the output voltage up or down from 0 to 19.9 VDC. If your preset current value is exceeded, the power supply will beep and either limit the current or trip, depending on which mode you have it switched to. Quite handy for breadboarding and testing at the bench when you can’t keep your eyes on everything at once but don’t want to damage a device, circuit, or your power supply.
Fluke multimeter says: 5.01 VDC with the PS 250 set for 5.00 VDC on its internal metering. Quite acceptable.
DC output as seen on the oscilloscope with the PS 250 set for 5 VDC after replacement of all electrolytic capacitors and the addition of a filter capacitor at the output terminals.