I have an old 60's to 70's vintage SCR variable speed drive that I thought would make a good DC power supply for powering simple circuits on a breadboard. I had a simple circuit made up that included a relay taken from salvaged electronic equipment. Once power was applied to the circuit the relay began to chatter. I took the relay out of the circuit and tested it with a 9V battery. It "clicked" indicating proper operation. As well, I verified that the contacts opened and closed with a DMM.

I tried the relay with the SCR VSD again this time out of the circuit. Once the power was applied it chattered like before. I tried a different relay and it did the same. I tried varying the voltage ever so slightly +/- and it made no difference. I then tried using a different straight 12VDC supply. Both relays functioned properly. So why does the SCR VSD cause these relays to chatter? I assume it has to do with the design. The previous owner was a knife maker who used this SCR VSD for DC motor speed control.

Is the DC power not "clean" enough for my application? Would a bridge rectifier and capacitor make a difference? Any suggestions would be appreciated.


A malfunction at the junction
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Dwayne