View Full Version : ID vintage thermocouple

August 12th, 2015, 10:07 PM
I'm trying to refit a 3B2/1000-80's power supply with a custom bridge board to an ATX supply. The last signal I need to deal with is the thermal overload sense. It's a single wire running to one terminal of the pictured device below. The black lead on the other terminal is tied to chassis ground via one of the two mounting screws. When I check resistance across the terminals, I see a dead short. When I try and pass current through it limited by a resistor, the resistor is dissipating all the potential. So it doesn't behave like a thermal sense resistor or diode. Does anyone have any ideas what it is or if it might be defective?


August 12th, 2015, 10:35 PM
Any writing on it at all? Could be a one shot thermal fuse, or a resettable thermal fuse which would be closed until triggered then open. That can just be bypassed with a jumper.

A thermocouple of any kind uses two dissimilar metals welded together to create a voltage that changes with temperature. That does not look like any thermocouple to me, and the device reading it would be fairly complex for it to work and would need both wires as an input.

August 13th, 2015, 12:20 AM
Do you have any clue as to what the wire is hooked to? That might give a clue.

August 13th, 2015, 03:08 AM
It's difficult to make out traces running under components on the original PSU board. However the best I can tell it forms the lower half of a resistor divider from 5V. The upper resistor is 10 ohms with a .1u across it. The output of which runs into a MC14584 Schmitt trigger inverter. So it does appear to be a resistor from the use case. I just don't get much resistance in a cold environment. Tonight I might try and heat it up a smidge with a hot air gun. Seems like an awfully power wasteful way to sense thermal overload. However wasteful is very relative next to the overall scope of this machine.

August 13th, 2015, 09:43 AM
The fact that this feeds a schmitt trigger means that any proportionality isn't part of the picture, so probably not a T/C or thermistor I really wonder if it's not simply a make-break thermal switch. The physical configuration would certainly seem to imply that.

August 13th, 2015, 01:11 PM
Thanks for the input Chuck. I'll connect an ohm meter across it and do a slow controled temp rise with an IR preheater.

August 13th, 2015, 02:31 PM
That looks an awful lot like a common bimetal thermal switch. Warm it up and it should break the connection to ground.

August 13th, 2015, 06:38 PM
Confirmed. Once I heated it up, I heard a click and then open circuit. Once it cooled back down, another click and short circuit again. Thanks for all the input!