BASIC PRINCIPLE

I appreciate that electronics enthusiasts don’t necessarily want to read about cooking and ironing. So, I’ll describe a device you might be more comfortable thinking about: a soldering iron. All of the aforementioned appliances and tools have one important thing in common that makes them different from, say, a hair dryer. Any thoughts?

The important common factor is that the thermal resistance between the heating element and the heated medium is much lower than the thermal resistance between the medium and the ambient world. Thus, if electric current stops flowing through the heating element, the temperatures of the element and the medium will equalize long before the medium loses much of its temperature via heat radiation and conduction.

Because the resistance of all the conductors used for constructing heating elements has some temperature coefficient, you can measure the temperature of the heating element by measuring its resistance and comparing it to its resistance at, say, 25°C. This brings us neatly to the basic idea behind this project.

First, you turn on the heating element for a while. Then, switch it off and wait for the temperatures to equalize. After that, you must measure the resistance and calculate the temperature. And then do it again: switch on, switch off, measure, and so on. It’s easy to see why this approach wouldn’t work with a hair dryer. The air forced through a hair dryer moves quickly and has poor heat conductivity.

I can hear you asking the obvious questions. How difficult is it to measure the element resistance? How much does it change with temperature? Wouldn’t a simple sensor be cheaper and easier to use? It depends on the application. I will address these concerns as I describe the soldering iron example.