I have a puzzling problem involving a thermostat.
I'm using an attic-fan thermostat (from Home Depot:
Initially I used the tstat to trigger a 12V power supply that ran a 12V fan. There was virtually zero current on the trigger line. The tstat worked like a charm for this, and I've been using it for about 6 months that way.
But the 12V fan wasn't really up to the job (for various reasons I need a LOT of suction power on this fan), and my 12V source (an old PC switching power supply) didn't like driving that kind of load, so I finally scrounged an old shopvac and hooked it up. Now the tstat switches the shopvac. The shopvac draws 5.3A, and the tstat is rated for 20A.
Here's the odd thing: when the projector heats up, the tstat trips and turns on the vac just like it's supposed to. But when the projector cools down, the tstat doesn't open. The vac keeps running, and running, and running... for as much as 4 hours one time when I was testing it. If I unplug the vac and plug it back in again, it still runs. But if I leave it unplugged for about 8 minutes, the tstat opens and then it's back to its normal cool-and-turned-off state.
What would cause the tstat to "stick" like that until I remove power from it for 8 minutes? It's conceivable the tstat has gone bad, but it was working just fine for triggering the PC power supply. It's very hard to get at so I don't want to rip it out unless I'm sure that's the problem. A friend suggested maybe the current was heating the tstat internally, and it couldn't turn off until you removed the current and it cooled off. But since the vac is only drawing 1/4 the rated current, I can't believe that's it.
Help!? Gary