Laminator Laminex Minitoke not sensing the heating temperature

[Diego_GM]

Hi EEV community!

I am currently working with the repair of a Laminex Minitoke laminator. As the title says this laminator its not sensing the temperature, overheating the resistance and finaly burning the thermal fuses.

The lamiantor has 2 heating elements each one of them with 3 inside components; the heating resistance, one 216C thermal fuse and one more component that i think is the heat sensor (i think is a thermistor but not sure, picture attached).

I started removing the "thermistor" from the heating element to measure it and see if it is working properly, the problem is that the "thermistor" has a weird behavior. I tried measuring the resistance with a multimeter, at room temperature this component has a resistance of 1.5 Ohm aprox, i started applying hot air with a heating gun to increase the component temperature and see if the resistance changes. The problem is when a measure the cables coming out of this component in one polarity the resistance increases as the component heats (normal of a PTC), but if a measure it in the opposite polarity the resistance decrease when the temperature increases but at one point it starts giving negative resistance.

The previous described behavior confused me and i came to this forum to seek for assistance. Does someone know what this component could be or if this behavior is normal? Thank you in advance!

Greetings Diego.

[floobydust]

It seems to be a type J thermocouple, not a thermistor. If it is not an open circuit and not shorted to the case/thermowell, you can measure the small DC voltage mV they generate. This is what is confusing your ohmmeter when you switch polarity. You could swap the top and bottom sensor (connectors) and see if the problem moves. The voltage is very tiny so the connections must be clean, check the connectors at the main board, if they are oxidized or not making contact.
Does it give any temperature readings? I found this Minikote wiring, if it matches yours.

[Diego_GM]

Hi floobydust!

First of all, thanks a lot for you reply, it is of great help.

Indeed now that you say it, it must be a thermocouple and the negative resistance measured must be the small voltage generated.

Im going to do some more test to the thermocouples to conclude if they are good or bad (I'm thinking they are in good state).

The thermocouples (both of them) go to a control board, exactly to an op amp (LMC6462), maybe the op amp is the problem.

Again, thank you for you reply.

Greetings Diego.