Sensorless control circuit for heater

[kikerts]

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!

 

[kam1787]

I don't see how you could use the heater resistance, except perhaps to turn it on

https://hypertextbook.com/facts/2004/DeannaStewart.shtml

 

[kikerts]

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.

 

[chuckey]

What is the temperature coefficient of resistivity of your indium heater. My idea is first to set up your heater as one arm of a Wheatstone bridge, whose output feeds amplifier that drives a relay. Now if you apply your AC from the secondary of a floating transformer, you can heat the heater, you will need a great deal of filtering to keep the 100V of AC out of the 1mV of DC that your bridge is trying to detect, but if you can get it right, when the bridge becomes unbalanced, the relay pulls or drops out and the power is taken of the heater. I think you want to energise your bridge from some thing like 5 KHZ and AC couple amplifiers.
Frank

 

[kikerts]

Thanks for recommendation!
The temperature change by 1 degree Celsius is aproximetly 1 ohm resistivity change.
I think I have idea how to use this circuit:
https://sicimatic.blogspot.com/2013/10/electronic-smart-heater-controller.html I can use a thermo-stable resistor between NEUTRAL and one ITO heater lead and measure the voltage across this resistor and with precision AC to DC converter(**broken link removed** Page 18) filter out the DC component and give this signal to comparator, instead of LM35 temperature sensor output. What did you think about this version?

 

[chuckey]

The problem is that if the mains increases in level, the current increases, your circuit then changes the temperature. You can do it but your have to have a sample of the mains to compare if the rise to due to mains or temperature - You are back to a Wheatstone bridge, energised by the mains. You have to switch off the bridge entirely else you will be comparing the current when the heater is off with the mains volts, which are then equal and the heater never comes on again! Sounds a bit fraught to me :roll:
Frank

 

[kam1787]

The problem is that if the mains increases in level, the current increases, your circuit then changes the temperature. You can do it but your have to have a sample of the mains to compare if the rise to due to mains or temperature - You are back to a Wheatstone bridge, energised by the mains. You have to switch off the bridge entirely else you will be comparing the current when the heater is off with the mains volts, which are then equal and the heater never comes on again! Sounds a bit fraught to me :roll:
Frank

quite so, even if you turn it off you would have to wait quite some time to measure ambient temp