Switch stuck in closed position

Question

I'm repairing and old board that had a problem with its power switch, a C&K NE182UEE6AMP DPDT switch for 250V/6A (https://www.ckswitches.com/media/1409/ne18.pdf). I found a replacement and soldered in, only to find the new switch then got stuck in the on position the first time is was switched on. There is nothing wrong mechanically with the switching mechanism, it seems to work fine. My only conclusion is the contacts somehow got welded together on that first switch. I see a few other discussions regarding this happening to relays with inductors in the circuit that cause a current spike welding the contacts closed on switching events.

The circuit in question is just mains power, through this switch, to several AC-DC power supplies for providing rail voltages to some further circuitry. The only odd thing in the circuit is a ~0.5uF capacitor between the mains active and neutral at the switch input (not really sure why its there). However, I can't imagine that could provide the current to weld the contacts as the circuit is powered up. Each of the AC-DC power supplies seems to be working fine, and in fact the whole system now works with the switch stuck on, but not ideal...

Any suggestions?

Answer 1

Switch contact welding is caused by high inrush current at switch-on. In power supplies the high inrush current is caused by charging capacitors.

You would need to select a switch or a relay, from 'high-inrush current' models, considering the TV rating mentioned in the data sheet.

The TV rating, as per UL and CSA standards, is an assessment of the inrush current withstanding capability of switch or relay contacts as follows:

TV3 - 51A, TV4 - 65A, TV5 - 78A and TV8 - 117A, TV10 - 141A, TV15 - 191A.

TV5 and TV8 rated switches, intended for use as 'Power On' devices, are available.