Many modern appliances use a computer to control the various loads in the appliance. But computers are also used for temperature monitoring and sensing. Typical temperature sensors used in modern appliances are NTC thermistors (refrigerators, dryers, washers, dishwashers) and RTDs (resistive temperature devices--used in ovens). If you're a member here at Appliantology, you can watch this webinar recording on Appliance Temperature Sensing Devices & Technology for a deep dive into how the computer uses these devices to monitor and control temperature. 

Sometimes when we're troubleshooting a temperature control problem in a computer-controlled appliance, we need to determine whether the temperature sensing device is out of calibration per the specifications OR the temperature sensing circuit on the computer is malfunctioning. Or, for some other troubleshooting reason, we want to be able to test the computer's temperature sensing circuit to see how it responds to changes in sensed temperature.

How can we do this? We could change the temperature of the sensing device (thermistor or RTD) and see how the computer responds. Ice water, heat gun, etc. That can be a hassle. A simple way to do it instead is to replace the temperature sensing device with a Resistor Substitution Box and then dial in different resistances corresponding to different temperatures. 

Why does this work? Because whether it's a thermistor or RTD, these devices work by changing their internal resistance with temperature according to specifications. If you know those specs-- the resistance of the device at a specific temperature-- then you can use a Resistor Substitution Box to temporarily replace the temperature sensor and dial in different resistances (corresponding to different temperatures) to see if the computer is responding correctly to these changes. Super cool, right?

Let's watch this in action on an electric oven.