This is entry #6 of my 52 Project 2012: Foundational Electronic Components Crash Course series.
What is a thermistor?
A thermistor is a resistor whose resistance changes based on temperature.
How does a thermistor work?
A thermistor is simple device made of a special kind of semiconductor material whose electrical properties (particularly resistance) changes either directly or inversely with changes to the temperature of the material. Thermistors are built with a negative temperature coefficient (NTC), or a positive temperature coefficient (PTC). NTC thermistors decrease resistance with increasing temperature, and PTC thermistors increase resistance with increasing temperature.
One obvious use of a properly calibrated thermistor is to detect the temperature of a device. By supplying the thermistor with a known voltage input and measuring the changed output voltage, you can calculate how hot or cold the thermistor is. Another common use for PTC thermistors is to create extra resistance in devices that are getting too hot, in order to prevent overheating. A PTC thermistor in the power supply of a circuit will automatically reduce power in the circuit as the temperature increases.
Thermistor ratings
Although thermistors have many detailed ratings, there are only a few main ones that are most commonly considered. One of these is the nominal resistance which may be anything from one ohm to many megohms, just like regular resistors. Of course, this is not a fixed value, since thermistors vary their resistance by design, but they are built to be centered around a specific value.
Another important rating is the temperature range in which they are designed to operate. Some have very wide temperature ranges (a few hundred degrees), while some are rated for a relatively small range—perhaps only 50 degrees. A thermistor with a large variance in its resistance but covering only a small temperature range can give more precise results over that range. Which ranges and nominal resistance values you want depends largely on your project and its intended environment.
Finally, similar to resistors, thermistors have a tolerance rating which indicates the expected margin of error. Thermistors often have tolerance ratings of 5% or 10% and can be bought very inexpensively at this level. More precise components, such as those used in thermostats which require very accurate temperature detection and control, go all the way down to 0.2% or even 0.1% tolerance, and cost much more (though still not usually more than a few dollars).
For a description of this 52 Project series and links to the all existing entries, go to the 52 Project 2012: Foundational Electronic Components Crash Course page.