PTC resistor measurement and connections check

Hello,

I'm designing a device which must be able to measure a PTC resistor and check if the connections (welds) of this resistor are good by applying 24VDC to it during a few seconds.

The PTC resistor is used as a fluid heater and its resistance at ambiant temperature is within 3.5 and 5.5Ω.

The resistance measurement at ambiant temperature is done at a low 2VDC voltage in order to avoid self-heating.

A picture of the circuit is attached.

I need your mind about it and other possible solutions.

Thank you.

Johann

hi Johann,
Looked over the Sim circuit.
Can you give a quick explanation on how you are expecting the circuit to function.

It helps if you attach your LTspice asc file and any unusual models.

Since you are not trying to heat the sensor I think I would make the current sense resistor .1 ohms instead of .01 ohms. This would give you a larger signal to work with to minimize errors.
I'm thinking the bottom circuit is to detect the open welds when the 24 volts is applied. But the way it is configured the op amp will always see 24 volts (minus the fet drop) so I'm not sure how this is supposed to work. I think I would just give it a short pulse with the 24 volts and check the resistance against the low current reading. A careful selection of the gain of the amplifiers would probably let you use the same ones and just move the a to d back a stage when you apply the 24 volts.