Ouch! Watch out for power supply memory buttons

What is it about power supplies?  They are so very boring, until suddenly they bring excitement. It’s always the wrong kind of excitement, too…

It was a quiet day at work. I was using a GW Instek PST-3201 bench power supply, a nice unit with three 30 volt/1 amp outputs. It was purring along at 12V and a couple dozen milliamps, powering a rare and expensive prototype while I did firmware development. Then, when I reached over to grab something, I bumped the “RECALL ↑” button. Bang! The supply’s relays clacked, and suddenly all three outputs were configured for maximum output. The supply pushed a full 30 V, with a 1 A limit, into my circuit.

Needless to say, I switched the output off as fast as I realized what happened. Thank heavens for the clatter of those relays, which told me something unexpected had happened. With the output off, it was time to check for damage.

To my surprise, there was none. The power supply wasn’t plugged directly into the prototype — instead it went through a test board that interfaced a PC to the device under test. By very fortunate coincidence, the wimpiest chips on that board were rated for a hefty 36V, and some were rated for as high as 42V. (Those “wimpy” chips weren’t wimpy in any real sense — they were some LT1970 high-power op amps, more than capable of hefty output of their own.)  The interface board was undamaged. Better yet, none of the high voltage made it to that precious prototype.

As I breathed a sigh of relief, I looked for the cause of the accident. It turns out this supply came from the box with half of its memories set for 30 V / 1 A and the other half set for 0 V / 0 A. Hitting “RECALL ↑” took it to one of the 30 V memories.

This could have been much worse. It was sheer luck that all of the parts I picked for that interface board had maximum ratings higher than 30 V.

Needless to say, if you happen to use GW Instek PST-3201 supplies or the closely related PST-3202, it would be wise to check the memories and set them all to zero volts. That’s what I did as soon as I realized I was not going to have to tell my boss I had blown the prototype.

Updated 3/24/12: Corrected the name of the button at fault. It is “RECALL ↑”, not “MEMORY ↑”.