What is a Thermistor?

What are thermistors? What are they used for? From the basics and common questions about these vital electronic components to the detail of how they work, join us as we take a deep dive into thermistors.

A thermistor is an electronic component that changes the resistance in its circuit as the temperature it is sensing goes up or down. Its name comes from thermal (pertaining to heat) and resistor (a standard electronic component that limits current flow).

There are two main types of thermistors - negative temperature coefficient (NTC) and positive temperature coefficient (PTC).

With NTCs, a higher temperature means less thermistor resistance and a lower temperature means more resistance. NTCs are by far the most common type of thermistor.

PTCs have the opposite effect – a higher temperature will result in a higher resistance, and a lower temperature will mean lower resistance.

Since dissimilar electrically conducting materials have differing resistances at different temperatures, the thermistor is a relatively simple device in physical terms. As the bead of the thermistor gets warmer or cooler, the current flowing through its circuit is impeded in line with the variable resistance. Now, it’s a case of measuring the current, which will be reduced as resistance increases, and since the original current is known, the difference can be used to measure temperature.

The complicated part is choosing which materials to use, as they all react to temperature in their own ways. That’s good because it means manufacturers can create a range of thermistors to act over different temperature ranges. It also means that NTCs and PTCs can be created, as they react to heat in opposite ways.

NTC thermistors are made from oxides of metals, typically chromium, manganese, cobalt, iron or nickel. PTCs use titanium compounds, usually with barium, strontium or lead. These compounds are then usually encapsulated in an epoxy or glass layer for protection and consistency.

In terms of a circuit, the thermistor is essentially a variable resistor that is automatically controlled by temperature, rather than a knob or slider.

Thermistors are used whenever the temperature is important, which means they can be installed in food preparation equipment, chemical manufacturing processes, engineering applications, refrigeration devices and a host of other places. They might simply give a temperature readout to allow people to monitor and act upon temperature changes. Alternatively, they can be part of automated control systems, that is, where the measured temperature activates cooling or heating via computer or electronic control of valves, pumps, inlets, flow rates and so on.

Thermistors are calibrated by placing them alongside pre-calibrated temperature sensors in a controlled environment (an enclosed, insulated, fluid filled-tank or gas-filled chamber is best), then adjusting the temperature and noting the thermistor’s response over a range of different temperatures. You’ll only need to perform the tests within the expected range of temperatures the thermistor will be used for, with perhaps a small amount above and below. Knowing the range of operating temperatures will determine the exact type of thermistor you buy.

It’s important to recognise that thermistors do not follow a linear path, so knowing just two temperature/resistance pairings will not be sufficient to interpolate the rest of the values over a range of temperatures. The relationship is a curved form, which means you’ll need at least five (but the more the better) points to plot the curve on your thermistor graph and predict the points in between. Also, there will be a range of temperatures where the temperature/resistance curve will be steeper, and therefore less precise over small temperature changes. If you’re calibrating a thermistor for a very temperature-sensitive application, make sure you choose one that’s most accurate within that range.