We are developing a tungsten heater (50-100Ω) with an integral 2-wire tungsten RTD (80-120Ω) for closed loop control, with the control being done by a custom microcontroller board to-be-developed. It has occurred to me that, just like the RTD, the heater element itself will change resistance according to its temperature.
We plan on controlling the heater with phase-angle AC power; if I can get the RMS voltage and current for the heater (which a quick internet search indicates would be pretty straightforward) it seems pretty easy to take their ratio to get resistance, and derive the temperature based on a defined temperature coefficient of resistance.
Of course the heat element will be the hottest part of the heater (when on anyways), but accuracy is not a concern, just precision (still the precision tolerance is fairly wide, ±10°C).
One tricky part is that we'll also need to monitor temperature while the heater is off, so there would need to be a solution for that as well, possibly a more typical RTD circuit. There would need to be a reliable means of switching between the two circuits to avoid frying the RTD circuit.
This heaters need to be fast - removing the integral RTD will reduce its mass which will speed response. It will also reduce the wire count from 5 to 3 (the extra being for ground).
Has anyone ever done this? Does this seem feasible? Is this even wise? What am I not considering that I should be?
We plan on controlling the heater with phase-angle AC power; if I can get the RMS voltage and current for the heater (which a quick internet search indicates would be pretty straightforward) it seems pretty easy to take their ratio to get resistance, and derive the temperature based on a defined temperature coefficient of resistance.
Of course the heat element will be the hottest part of the heater (when on anyways), but accuracy is not a concern, just precision (still the precision tolerance is fairly wide, ±10°C).
One tricky part is that we'll also need to monitor temperature while the heater is off, so there would need to be a solution for that as well, possibly a more typical RTD circuit. There would need to be a reliable means of switching between the two circuits to avoid frying the RTD circuit.
This heaters need to be fast - removing the integral RTD will reduce its mass which will speed response. It will also reduce the wire count from 5 to 3 (the extra being for ground).
Has anyone ever done this? Does this seem feasible? Is this even wise? What am I not considering that I should be?