Sensorless control circuit for heater

[Kristaps]

Hello!

I would like to make control circuit for heater. Heater function is to protect device from too low temperature(<0 C). My idea is to make it without temperature sensor and it sense temperature only from the heater resistance changes. The heater works with AC about 70-100V. Is it posible if resistance changes for 10 celsius amplitude only for about 10 ohms? Some variants I find in internet but in these circuit are used temperature sensors:
https://sicimatic.blogspot.com/2013/10/electronic-smart-heater-controller.html
https://www.final-yearproject.com/2009/02/hvac-thermostat.html#.UxeoEvl_u3S
What did you can suggest?
Thank you!

 

[Reloadron]

I suggest using a circuit like those you posted. There is a reason that temperature control circuits use sensors. I can't think of any easy way to use the heating element as a temperature sensor, none likely to work anyway.

Ron

 

[ChrisP58]

I think that measuring the temperature by the delta of the heating element is going to be rather difficult, and will require much more circuitry than a simple thermistor or other sensor would take.

1) You will need a calibration step to establish the baseline resistance at some reference temperature. (so you need a sensor anyway)
2) If you're trying to measure the resistance by current/voltage, you will probably find that the significant information is several orders of magnitude smaller than the measured values.
3) When the element is on, the measurement will only show the temperature of the element itself. If you need to know the present ambient, you will need to turn off the heater long enough for it to cool down to ambient. And you won't know when that is unless you have a separate temperature sensor.

 

[Kristaps]

Thank you for response!
My application is to make indium tin oxide thin film heater. The reason why I don't want to use sensors is that I need to measure the actual temperature of thin film not the surround environment temperature, and if I use some sensor it also restrict visibility what is a disadvantage because I need my heater to be transparent as maximum.

 

[Misterbenn]

ah so this is some kind of window de-mist application....

 

[Kristaps]

ah so this is some kind of window de-mist application....

Yes, something like that.