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During this ongoing pandemic, I, like a lot of people, began doing a lot more cooking. I haven’t jumped on the sourdough band wagon (yet), but I have started making some of my own bread along with pizza dough.

Thermistors: Tiny Devices Helping Home Appliances Keep Their Cool (or Heat)

Chuck Ross
During this ongoing pandemic, I, like a lot of people, began doing a lot more cooking. I haven’t jumped on the sourdough band wagon (yet), but I have started making some of my own bread along with pizza dough. The pizza dough is a particular favorite because it comes together quickly and doesn’t need quite as much rising time as regular bread – but it does need a hot oven, with some recipes calling for 500 degrees F. Also like a lot of people during the last almost-two-years, I’ve upgraded my appliances in the last six months. One of my favorite features of my new range is that the oven now tells me how hot it is as it’s preheating, instead of just letting me know when it has hit its setpoint. So, now I have a decent idea how long before that pizza dough can head into the oven.

Researching temperature sensing for this article got me wondering how this process worked, and the answer in one word is “thermistor.” I had heard this word before, but just classified it as one of the vast number of electronic bits and bobs that repair pros throw around when they come to look at your freezer that isn’t freezing anymore. It turns out that these small, inexpensive devices are helping to control most temperature-related devices in our homes, from freezers and fridges to HVAC thermostats and – yes – ovens, using the properties of electrical resistance.

The word thermistor is just a shortening of “thermal resistor,” and it refers to the way electrical resistance through these devices changes with its surrounding temperature. They come in two varieties. Negative temperature coefficient models, like those in many household appliances, feature resistance that decreases as temperature rises. Positive temperature coefficient units used in fuses, for one example, do just the opposite, increasing their resistance as temperature rises. In both cases, the resistance changes are non-linear, meaning they occur more rapidly than the rate at which the temperature is increasing. They’re also finely tuned, so a specific level of resistance can be a very accurate reflection of the surrounding temperature.

Along with simply measuring temperature, thermistors also can be built into control systems – so the same device that’s feeding my oven display might also be supplying that data to the trip unit that tells the oven to stop heating when the desired temperature is reached. And a similar design is built into freezer operations, telling the compressor to kick on when temperatures rise too high. So, you could say thermistors are one of the few examples when it’s beneficial to run both hot and cold.
Photo courtesy of Littelfuse
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