Sometime in the mid 1960s I bought an EICO 1064 Battery Eliminator Power Supply Kit. This kit was a very basic power supply and was rugged enough to charge car batteries if necessary. It was built around a variac which output 0 to 20 volts AC. The output of the variac was rectified, filtered and fed unregulated to the output terminals. A switch was available to cut the voltage in half and double the current. In those days we still had a fair number of cars with 6 volt electrical systems. At the time I was servicing a lot of audio electronics and as a result there were many car radios and stereos coming across my bench. I even had the opportunity to service some car radios that had tubes and mechanical inverters. The EICO served the purpose of a bench power supply for this type of equipment very well. Almost nothing was very voltage sensitive so I would hook up the equipment and crank up the variac until the meter said 12 volts and proceed to track down the problem in the circuitry.
Fast forward 45 years or so and while the EICO still has some good uses it falls short of a cheap modern bench supply. For example if I dial up 12 volts on the EICO and put a 3 amp load on its back it promptly drops to 8 volts. Of course I can turn up the variac to compensate for this but why should I.
A year ago I bought this little circuit for a project that never materialized. It is a 32 VDC in 0 to 30 VDC output buck converter. It can output 12 amps according to the seller though for $8.42 I will trust it to give me 5 Amps and be happy. I have removed the small 50K trimmer and installed a 10 turn 50K Pot. I decided to build a replacement for the EICO around this circuit board.
I took a look around my shop and the dental service business and found the following items. I found an old Cavitron 660 of about the same vintage as the EICO. A cavitron is a device the causes a tip to vibrate at 25 kHz and when applied to a patients tooth under a flow of water will remove all the nasty build up of plaque. It was no longer functional but the case and chassis was in good shape and about the size I wanted. I also found a transformer for a Pelton Crane dental light that had 4 primary taps and a secondary that changed from 22 VDC to 25 VDC depending on which primary tap is selected. After experimenting the 22 volt tap was selected as this gave me almost exactly 30 volts DC after rectification and filtering. I also found some good filter caps from a Siemens X-Ray machine and a General Electric X-Ray machine.
My plan was to produce a good low internal resistance 30 volt supply to power the buck converter. I was originally going to install a cheap LED volt / ammeter display but when this smoked I went back to my roots and decided to use nice analog gauges for the Voltmeter and Ammeter. My next challenge was to find a suitable front panel. The 1/8 inch aluminum face plate of the cavitron was tempting but the existing holes could not be accommodated so I decided to use a black opaque plexiglass. This would look nice and be within the technology of my tools ability to modify to my requirements.
Here is a picture of the faceplate as it is taking shape.
Top and bottom views of the completed chassis.
Here is the finished product. I can set the voltage where ever I want it between 0.6 and 30 volts. If I drop a 3 or 4 amp load across the output terminals I can't just barely see the voltage needle quiver. I have looked at the ripple with my scope and under my normal bench project loads it is negligible. Obviously as the current goes up the ripple becomes more significant. The fun part is that it is made of old dental equipment parts salvage and a few contributions from China with the rest supplied by our favorite supply house.
Thanks for looking John