Thermistors are resistors made from materials that have large values of temperature coefficient. Both PTC and NTC types are produced for applications that range from temperature measurement to transient current suppression. Figure shows some representative types. Miniature thermistors either in bead form or in glass tubes are used for temperature measurement, using a bridge circuit and are also used for timing circuits and in stabilizing the amplitude of sine wave oscillators.
Thermistors are self-heating if the current through them is allowed to exceed the limits laid down by the manufacturers, so the current flowing in a bridge-measuring circuit must be carefully limited. Larger thermistor types, with lower values of cold resistance (measured at 20◦C) are used for current regulation, such as circuits for degaussing colour TV tubes, controlling the surge current through filament light bulbs, or reducing the speed of fan when a set temperature is reached. The general form of graphs of resistance plotted against temperature is that shown in Figure and the formula for finding the resistance at any temperature is shown in the following section and example.
The graph shows the ratio RT/R25 (where RT is the resistance at any temperature and R25 is the resistance at 25◦C) plotted against temperature, and is a curve. A logarithmic plot of resistance against (1/temperature) can be more useful than one of resistance against temperature because it gives straight line characteristics that are easier to interpret and extrapolate, but such graphs are more difficult to extract useful information from.