What Is a Thermistor and How Do They Work?
Image: Jeremy Cook | Thermistors come in a broad range of shapes and sizes
By: Jeremy Cook
The thermistor —short for thermally sensitive resistor—is a resistor that changes value with the temperature. Thermistors increase or decrease resistance with changing temperatures.
Thermistors are broadly divided into two classes:
• NTC Thermistor: Negative temperature coefficient–resistance decreases with increasing temperature
• PTC Thermistor: Positive temperature coefficient–resistance increases with increasing temperature
The positive or negative temperature coefficient label refers to the k term in the first-order behavior approximation:
∆R = k∆T
Here, R is resistance, T is temperature, and k is the coefficient that relates the two. In standard (non-thermistor) resistors, k is ideally as close to 0 as possible, maintaining a constant R over a wide temperature range.
This first-order approximation is only accurate over a limited temperature span. A more robust expression of resistance versus temperature is based on the Steinhart-Hart equation for semiconductor resistance, expressed as:
1/T = A + BlnR + C(lnR)3
In this equation, T (temperature) and R (resistance) terms match the first-order approximation. A, B, and C coefficients are adjusted to match a thermistor’s individual thermistor properties.
Beyond resistive behavior, thermistors come in a wide range of packages, from tiny SMD components that fit on a breadboard , to devices with leads attached for remote sensing duties. Thermistors may be used for temperature measurement or can be implemented for direct circuit control.
What Are Thermistors Used For?
Image: Jeremy Cook | Thermistors come in small and large packages.
Thermistor temperature sensing is accomplished with an NTC thermistor. For example, the temperature of a 3D-printer’s hot-end is typically measured with an NTC thermistor, generally with a rated ambient temperature (25ºC/77ºF) resistance of 100kΩ. The 3D-printer’s controller measures the thermistor’s resistance, which is correlated with temperature. This temperature data lets the printer provide the appropriate amount of power to melt PLA or other materials.
Besides sending data to a controller, thermistors can be used directly in-circuit in several ways:
• PTC: Current limiting for circuit protection. If excess current flows through a properly specified PTC resistor, heat is generated and resistance increases. This can thus act as a resettable fuse, inhibiting current flow when hot, and then allowing current flow in normal (cooler) conditions.
• NTC: Inrush current limiters. Resistance in these devices is initially high, but when a small amount of current heats up the thermistor, resistance is lowered over time. This lowered resistance allows more current to flow, ramping up in a controlled fashion.
• Actuation of Thermally Responsive Devices: It’s possible to control a cooling fan with an NTC thermistor, providing power when hot. A PTC thermistor could be used in a similar manner to actuate a heater when a space is too cold. While either can exhibit direct control, it may be advantageous to instead use a thermistor for measurement. A microcontroller , smart building setup, or other smart device can then be used to make more refined decisions.
Thermistors: Useful for Measurement and Circuit Protection
While thermistors can be divided up into NTC and PTC, perhaps a better way to look at these components is whether they are meant for measurement, or for direct action/circuit protection. Measurement thermistors often come with a cable for attachment to other electronics. “Direct action” thermistors, however, often come in small surface-mount packages for use with relatively small loads on circuit boards. They can also take the form of bigger through-hole style components for larger loads.
Thermistors present a number of useful design possibilities, and should be considered when dealing with temperature sensitive applications or electronics protection. They also tend to be quite cost-effective, helping to keep your project on-budget.
