Thermistors are a special kind of resistor. Unlike conventional resistors which are designed to have little or no variation with temperature, thermistors are made to be especially sensitive to temperature changes. Because they are inexpensive, rugged and easy to use you'll find them in a wide variety of applications. Thermistors can be found in digital thermostats, chargeable battery packs, automobile engine controls, and many other systems that require reliable, fast-responding temperature sensors.
A typical thermistor responds to temperature changes as shown in this graph. Notice the large resistance change over a useful range of temperatures.
In the example shown in the graph, the resistance at -10 degrees C is 20 times that at +100 C. This quality makes the thermistor very useful in a wide variety of applications, both for measuring temperature directly and in circuits needing compensation for temperature changes. Also, notice that the resistance decreases with temperature. In other words, the thermistor modeled here has a negative temperature coefficient (NTC). This is by far the most common type of thermistor. There exist thermistors with positive temperature coefficients (PTC) but they are not widely used.
All NTC thermistors have the same general shape to their response curves. However, depending on the material and fabrication process used by the manufacturer these devices can possess a wide variety of sensitivities and operating ranges. For most applications, it's necessary to convert the measured resistance to actual temperature. The manufacturer may supply this information in the form of a chart, but if not, or if you are dealing with an unknown device, your next step will require that you use the Steinhart-Hart equation to determine the resistance of the thermistor at a given temperature. This equation is accurate to better than 0.1%
The key to using this equation is to find the three coefficients; A, B and C. With these in hand you can simply enter the coefficients into the program presented here. Next, enter the resistance of the thermistor and click the button labeled "Click here to show temperature". If the manufacturer doesn't supply these coefficients, this program can generate them from some observations you can make. This requires you to measure the resistance of the unknown thermistor at three different temperatures (the more widely spaced the better). Enter the three temperature-resistance pairs of data into the program, click the "Calculate coefficients" button and it will calculate the coefficients and enter them into the coefficient boxes for you. Click here to download the Thermistor program