12V Inverter Repair

[jheatac]

Hello,

I have a question for those of you who are smarter than I am... And when it comes to electronics, that doesn't take much! I am learning though.

I am working on 12VDC to 120VAC inverter... About 3kw. Cheap Chinese, I'm sure.

It was in protected mode when I got it. Found the quad op-amp responsible for protection, then the circuit tripping the fault.

I thought I had it figured out down to a bad thermistor used for overheat. The thermistor is over a hole in the board, on the cold air side of the board... Seems like you would want it on the discharge air side, but anyways.

I have figured out that if the resistance is less than 657 ohms, it will pull the circuit down and trip the op-amp allowing the inverter to function. The resistance of the thermistor at about 60F is about 140k... A far cry from what it takes to trip the circuit.

Now here is my real question.. Can a PTC thermistor fail and act like a NTC?

The thermistor out of circuit is about 140k at 60F... If I hold it in my fingers it will drop to about 112k... breathe hot air on it and I can get it down to about 85k... It seems relatively responsive. This would be a NTC, correct?

To fit the circuit, it would need to be PTC because an overheat condition would be greater that 657ohm.


Should I assume the thermistor is fine and troubleshoot what is forcing the other side of the op-amp high?

The inverter was functional after satisfying the protection mode... that is until I was playing with the bench transformer and overvoltaged the damned thing... Now I have mosfets out of it and have bigger issues...