"sensorless" soldering iron temperature control

[star882]

Could the resistance of the soldering iron be measured to determine its temperature? (Assume it's just a normal soldering iron without any fancy controls.) A PWM circuit can be used to control power and the resistance could be measured during the off cycle.

I'm designing a "magic box" that the soldering iron plugs into that makes the iron temperature regulated, without having to add a temperature sensor. Just calibrate it whenever a different iron is used.

Taking it a step further, it could potentially be used for any heating appliance without motors or electronics.

[Magura]

A simple way of measuring the temperature, would be to simply measure the current draw of the heating element.

Magura

[star882]

Originally posted by Magura
A simple way of measuring the temperature, would be to simply measure the current draw of the heating element.

Magura

Main problem is having to account for supply voltage variations. It would be more complex than putting a current regulator in parallel with the PWM transistor and S&Hing the voltage across the load during the PWM off period.

It would work nicely if the supply is regulated, though. In that case, S&H the current during the PWM on period.

[Elvee]

It could be a good idea, but with the usual materials of the heating element, it will be difficult: these alloys have in general a near zero tempco, with a sign inversion around 350°C. This makes the design of a controller very difficult.
On the other hand, self-regulating heating elements do exist: they rely on the PTC properties of special materials, and are used in hot melt guns for instance.
One could probably make a soldering iron based on such an element.