Hi,
I had posted some info on the irons and their heaters a while back.
Basically they sell two different kinds, and yes they each have their own sensor and heater resistance.
The most interesting part is the heater resistance, because that's what makes the iron wattage rating what it is. The lower power ones have a higher resistance, and the higher power ones have a lower resistance. But also interesting is they are rarely rated for what they are sold as. Many of the "60 watt" irons come out to only 36 real watts. The other ones come up to only 50 real watts.
The 907A iron often sold as a 60 watt unit only measures 36 watts, and the iron with the yellow handle measures 50 watts.
That may still be enough for circuit board soldering though, so at least they do work. I just did not like the deception in the sales.
And yes, one has a thermocouple and one has a resistance sensor, so they are not compatible at all. The controllers for the thermocouple will rely on a voltage signal, while the controllers for the resistance type sensors will rely on a resistance measurement.
Either way though, these controllers are some of the simplest circuits i've ever seen. It takes very little to make one, so anyone can make a second soldering iron station with a few parts. The main part is the transformer, which has to be able to put out the full power of the iron which can be either 36 watts (true power) or 50 watts (true power). Alternately you could use a DC power supply, but i have not studied the effect of DC current on the longevity of the element vs AC current, and they are usually driven by AC current. I think they use AC though just to avoid the more expensive parts needed to build a DC power supply of the equivalent power rating.
The controllers can be very simple because they just rely on a single set point temperature. In other words, if we set it to 350 degrees C, then it always detects the level of voltage that represents that temperature, so there is no problem like linearity and the like. That only comes in if you want a true linear pot setting, which you dont need either. For example, if you turn the pot a little and measure the tip temperature and mark the dial, then turn it a little more and measure temp and mark the dial, you'd be just as well off as if you had a four digit temperature display.
Of course the thermocouple type need an amplifier to amplify the small thermo voltage generated, but it isnt too bad at these normally high temperatures used for soldering.
You could look up thermocouple voltages on the web and get an idea what to expect.
Alternately, just heat up the iron with a DC power supply and measure the voltage coming out of the connector for the sensor. The thermocouple voltage will increase as the temperature increases, and you can measure the tip temperature and correlate. Probably dont need cold junction compensation either because of the high temperatures involved in normal soldering. I would think the voltage would be around 0.5 volts at soldering temperatures, but a measurement would show the voltage for sure.
I think it is a very good idea to have at least 2 irons, with different tips, so you dont have to change tips. That's a good idea that i read elsewhere in this thread. Having a second iron controller would be nice too, in order to have them both up and running and ready to go.
The last time i made a small oven for testing electronic parts was probably 35 years ago, but all it took was a potentiometer, thermistor, voltage reference, op amp, and drive transistor (along with heater and power supply). Very simple circuit similar to this kind.