How Does a Thermistor Work?: A Basic Explanation

For home appliances, accurate temperature measurement is essential in a wide range of applications. One of the most commonly used components for temperature sensing is the thermistor. Known for its sensitivity and reliability, the thermistor plays a crucial role in appliances we interact with daily, such as refrigerators, ovens, dishwashers and laundry appliances.

But how does a thermistor work, and why is it so widely used? In this article, we'll delve into the principles behind thermistors, explore how they function, and examine their applications in various appliances.

What is a Thermistor?

A thermistor, short for "thermally sensitive resistor," is a type of resistor whose resistance changes significantly and proportionally with temperature.

Thermistors are made from ceramic or polymer materials that respond to temperature fluctuations by altering their electrical resistance. This sensitivity makes thermistors ideal for precise temperature measurements. They’re essential components in every home appliance that relies on controlling temperature of air or fluids such as water to operate properly.

There are two main types of thermistors:

• Negative Temperature Coefficient (NTC) Thermistors: In these, resistance decreases as temperature increases.
• Positive Temperature Coefficient (PTC) Thermistors: Here, resistance increases as temperature rises.

Each type has distinct properties suited to specific applications, but the underlying principle remains the same: temperature changes influence the material's resistance.

The Science Behind Thermistors

Thermistors operate on the principle of semiconductor conductivity. The materials used in thermistors are typically ceramics composed of metal oxides, which exhibit unique electrical properties at different temperatures.

Key Principle: Resistance and Temperature

For NTC thermistors, as temperature increases, more charge carriers become available in the semiconductor material, reducing its resistance. This relationship allows NTC thermistors to provide a predictable and measurable change in resistance, which is ideal for temperature sensing. NTC thermistors are most effective in applications where gradual temperature increases need to be monitored.

In contrast, PTC thermistors experience a rise in resistance with an increase in temperature. These thermistors are often used for overcurrent protection or self-regulating heating applications, where high temperatures signal a need to restrict current flow.

How Does a Thermistor Work in Practical Terms?

A thermistor’s design and properties make it simple to use with electronic circuits. Here’s a step-by-step breakdown of how a thermistor is typically used in appliance temperature-sensing applications:

• Placement in Circuit: A thermistor is placed in a circuit from the electronic control board. Supply and return wires connect the thermistor to the circuit board. The circuit board sends low voltage DC current through the supply wire and monitors the DC voltage through the return wire.
• Temperature Change Detection: When temperature changes, the thermistor’s resistance also changes. In an NTC thermistor, an increase in temperature causes a drop in resistance. For a PTC thermistor, the resistance would increase.
• DC Voltage Change: This resistance shift causes a change in the current and DC voltage detected by the control board through the return wire. This DC return voltage is measured and interpreted by the electronic control board to determine temperature at the thermistor.
• DC Voltage Signal Interpretation: The control system interprets the thermistor circuit's return DC voltage translates it into temperature data, which can be displayed on a screen or trigger specific responses in the circuit, such as turning on a fan when temperatures rise or shutting down an overheated device.

Advantages and Limitations of Thermistors

Advantages

• High Sensitivity: Thermistors precisely measure actual temperature in the their vicinity so the electronic control board gets accurate temperature data.
• Quick Responsiveness: Thermistors respond quickly to temperature changes, allowing for real-time monitoring.
• Compact Size: They are small and easy to integrate into various electronic devices.
• Cost-Effectiveness: Thermistors are relatively inexpensive, making them suitable for mass-market applications. This also helps reduce the cost of appliance repairs.

Limitations

• Non-linear Response: Thermistors have an exponential resistance-temperature proportional relationship, requiring calibration for accurate readings.
• Limited Temperature Range: Most thermistors are designed for a specific temperature range and may not perform accurately outside of it.
• Susceptibility to Environmental Conditions: Humidity and external contaminants can affect some thermistor types, impacting their reliability over time. Wire harness failures such as broken or damaged wires can also cause inaccurate temperature readings.

Testing Thermistor Accuracy

The tech sheets and service manuals for appliances have temperature/resistance charts or DC voltage/temperature charts to help technicians check the accuracy of thermistors. The internal logic of electronic control boards in some appliances will also detect thermistor problems.

When you see appliance symptoms that point to thermistor problems and you want to diagnose the problem yourself, refer to the technical information for your appliance regarding testing of components including thermistors. You can often find the tech sheet with thermistor testing information in the console or behind the bottom front toe panel of an appliance. You can also find technical information for appliances online.

Here’s a DIY repair advice video that provides practical tips on troubleshooting thermistor problems in refrigerators and provides details on testing a thermistor:

If you test a thermistor and it’s bad, replace it.

If you’re not completely confident in performing DIY diagnosis and repair on your own, schedule appliance repair service and we’ll send a Sears Home Services technician to your home to fix the problem.

Also, scheduling professional appliance maintenance every year will give us an opportunity to check the thermistors and other components in your appliances to help keep them operating smoothly and efficiently. Annual preventive maintenance will also improve appliance longevity.

Sears Home Services knows thermistors and how they work in home appliances. Trust us to help you keep your appliances working smoothly and lasting longer.