Introduction to Thermistor

A thermistor is a type of resistor with resistance proportional to its temperature. Thermistors are special solid temperature-sensing element composed of sintered semiconductor material that behave like temperature-sensitive electrical resistors which exhibits a large change in resistance proportional to a small change in temperature. No surprise then that their name is a contraction of “thermal” and “resistor”. There are basically two broad types, NTC-Negative Temperature Coefficient, used mostly in temperature sensing and PTC-Positive Temperature Coefficient, used mostly in electric current control.

Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting over-current protectors, and self-regulating heating elements.

Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range.

Thermistors are one of the most accurate types of temperature sensors. Thermistors have an accuracy of ±0.1°C or ±0.2°C depending on the particular thermistor model. However thermistors are fairly limited in their temperature range, working only over a nominal range of 0°C to 100°C.

Finished thermistors are chemically stable and not significantly affected by aging.

Of the many choices for temperature sensing – thermocouple, RTD, thermistor – the thermistor serves up one big advantage: a large signal. This means you don’t have to amplify it as much as the other sensors (and amplify all that goes with the signal such as noise, offsets, drifts, etc.) But, as we’ve come to expect, for every advantage there’s at least one disadvantage; the thermistor is highly non-linear.

We’ll have to move beyond the basic RLC components to emulate the thermistor’s non-linear behavior. But, once the equation describing a thermistor’s resistance versus temperature is known, creating a model is easier than you think. To get there, we just need a simple trick of modeling a resistor without using a resistor. Then, an equation that describes the resistance as a function of temperature will be put in the model.

Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:

ΔR = kΔT

where
ΔR = change in resistance
ΔT = change in temperature
k = first-order temperature coefficient of resistance

Thermistors can be classified into two types depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a k as close to zero as possible, so that their resistance remains nearly constant over a wide temperature range.

Thermistor Circuit Symbol


Thermistor (NTC) Resistance

How does a thermistor behave versus temperature? As temperature increases, its resistance gets smaller according to the R vs. T equation

R = Ro exp( Beta/T – Beta/To)

Where
R = Thermistor resistance at T (K)
T = Thermistor temperature (K)
Ro = Nominal resistance at To (K)
To = Temperature where Ro is measured
Beta = Thermistor material constant

This exponential equation describes a very non-linear curve that decreases with increasing temperature. For this reason, you may see the thermistor referred to as a Negative Temperature Coefficient (NTC) device.

Thermistor Characteristics


Thermistor Applications

  • NTC thermistors are used as resistance thermometers in low-temperature measurements of the order of 10 K.
  • NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purposely designed for this application.
  • NTC thermistors are regularly used in automotive applications. For example, they monitor things like coolant temperature and/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard.
  • Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging.