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More information would help - including what you intend to do with the information. If you need to design this yourself for a school or personal project we can help guide you so that you get something from the experience. If all you want to do is copy - I've seen plenty of this described here and elsewhere- will just take a little digging. I have nothing at hand at the moment.
Proportional control decribes, more or less, that the output of the controller will be proportional to the difference between your goal or setpoint and the actual measured value - in this case temperature. Unless there is some bias or offset then if the setpoint temp and actual temp are the same then the output it zero. As the difference between the setpoint and actual increases the output will increase or decrease.
Once you have the output from the controller you'll need to do something with it. How you accomplish that depends on what you are trying to power.
well why don't you use a PT-100? you van use it in a wheatstone bridge configuration, and the voltage will be proportional to the temperature. Then you can add PWM to control one or two relays, directly connected to 240 VAC.
Thanks for your replies Steve and David.
Steve: the control you describe is absolutely correct for what I have in mind.
The controller will control a 240VAC resistive heating element, an electric oven element to be exact. 2400Watts maximum power.
What I vaguely had in mind was an IC555 using the duty cycle output pin to drive a Triac. The 555 lengthens or shortens the length of the 'on' period in the duty cycle controlled from one of the voltage input pins. This input might come from an op amp (if necessary) or straight from the voltage drop across some kind of temperature responsive resistor device.
What I would like is a simple device that runs at 240VAC so I don't have to build in a complex low voltage DC regulated power supply.
I would also like it variable in the temperature that it controls.
I would also like it to be 'universal' in that I could use the same circuit to drive heating elements of say, 50Watts to 2000Watts. I could use it for controlling a drying/varnish baking oven, water jacket temperature for a PCB etching tank or even my offive laminator (which is out of action due to its temperature controller being US.
Any imformation would help.
David: I don't quite understand what you are getting at, what is a PT100 or a PWM? I know what a Wheatstone bridge is though.
well a PT100 correspond to PTC, it is a resistor that varies its value with temperature, and a pwm is a phase withd modulation. It is used to simulate an analog signal but "pulsing" a discrete actuator