Temperature Controlled Soldering Station Using 12Vdc Glow Plug

Hie. I am trying to make a high power temperature controlled soldering station using a diesel engine glow (heater) plug as the soldering iron's element, however I am not so sure of the electronic schematic and also how to measure temperature of the glow plug since it accepts 12V dc. I am also trying to use a 400VA 220/12V transformer and a rectifier as part of my tool. 

As it happens I have a glow plug soldering iron but it is not temperature controlled. It IS pretty heavy duty and you could probably solder gutters with it (but you wouldn't use it for SMD work I can think of two ways you could control the temperature.  The first is to use a type K thermocouple - these are cheap and easy to get and you can get ICs that will condition them and provide a linear voltage output proportional to the temperature. You could use this to switch the power and also to display the temperature.  Type K thermocouples work up to around 1000 degrees C so your main hassle would be to mechanically couple the thermocouple to the glow plug or soldering bit to get an accurate temperature reading.The other possible way (and don't take this as gospel as I have not tried it) would be to measure the resistance of the glow plug and - providing it varies with temperature as most resistors do - use that to derive a temperature value.  You'd have to have some sort of PWM for the power control anyway, so you could maybe measure the resistance in the off times of the PWM.  This would be more difficult as you'd be on your own design-wise (using a thermocouple you could use "reference designs" to some extent) but if you're up for some design work it could be a nice project.  You'd probably have to get a thermocouple thermometer to calibrate the glow plug resistance / temperature curve as a start.Anyone else got any other ideas?

One other comment.  a glow plug is just a resistor so you need not rectify your transformer output - you can just apply AC to the glow plug.  You could use a thyristor or a triac to switch the AC to the glow plug.  You can use what's called Zero Voltage Switching - you switch the thyristor / triac on as the voltage crosses from - to + or vice versa, so you only get complete half cycles on the glow plug.  This minimises any interference from switching higher voltages.

Hi Retzvad-When I first read your question I was thinking that this would be an odd way to make a soldering iron, but after I looked into it I see what you're up to.  I found a Youtube video HERE , but instead of implementing closed-loop temperature control, they used a button on the handle driving a MOSFET, so you would use it more like a trigger-type soldering gun than a traditional soldering iron. Looks absolutely practical to me!-Rick