NiCR Wire changes its resistance when heated in my Test Setup between 0.0006 and 0.0009 Ohm.
I now want to heat the nice Wire via PWM an an N-Channel Mosfet (already works). and from time to time check the heat by reading its resistance to calculate back to temperature to o check the resistance to avoid burning the coil away.
Any ideas on this, I tried to use an opamp as an differential amplifier in the PWM pause but up to now I didn't succeed in getting somehow useful Output for the Arduino ADC.
I tried to use an opamp as an differential amplifier in the PWM pause but up to now I didn't succeed in getting somehow useful Output for the Arduino ADC.
No use mentioning this without posting the diagram.
You may get better results using an Atmega MCU that can measure the differential voltage between two analog pins. This excludes the Atmega 328, but most of the others are OK. Those MCUs also have internal amplifiers.
However the variation in resistance is so tiny that I suspect you will need a sophisticated external circuit to detect the change and convert it to a voltage range that the Arduino ADC can work with.
I suspect you did not understand what MarkT explained in post #7 here: http://forum.arduino.cc/index.php?topic=283445.msg3736161#msg3736161
The better approach is to monitor the power applied to estimate temperature rise and resistance. Trying to measure resistance is extermely difficult when dealing with a milliohm device with a relatively low temperature coefficient of resistance.
Depending upon your operating voltage and current, you can use something like the INA219 breakout boards to calculate the power directly.
But a few milliohms with a few amps flowing through it will give a reasonable signal. Measure while PWM is on, not off.