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What are Thermistors?
Thermistors are semiconductor devices that are used to measure temperature. The name comes from a combination of the words "resistor" and "thermal". Thermistors have an electrical resistance that is proportional to temperature. From a general physics course on electricity and magnetism, you may have learned that this is a property typical for all conductors. For example, devices such toasters, heaters, and light bulbs operate on this principle. Thermistors are different in that they are created to deliberately exploit this effect, and hence are more temperature sensitive than usual.
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The Mathematical Model
The basic mathematical model used for thermistors is the Steinhart-Hart equation, discovered by oceanographers I.S. Steinhart and S.R. Hart. In its simplest form it is:
1/T = a + b(ln R) +c(ln R)3
where T is the temperature, a, b, and c are coefficients that are measured, ln is the natural log, and R is the resistance in ohms.
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Thermistors are used in science and engineering applications. They are also useful in medicine as clinical temperature sensors or as probes during surgery. There are two types: PTC (Positive Temperature Coefficient of Resistance) and NTC (Negative Temperature Coefficient of Resistance).
NTC thermistors have temperatures that vary inversely with resistance such that as the temperature increases, the resistance decreases, and vice versa. They are very often used for temperature control and indication, and for current suppression. Common materials used in their construction include oxides of materials such as nickel, manganese, copper, iron, and cobalt. Some are also made from silicon and/or germanium. They are usually packaged in an epoxy, and are the most common type of thermistor.
PTC thermistors are the opposite of NTCs in that they have a resistance that increases with rising temperature and decreases with falling temperature. They are used to protect circuits from overload, and can function as thermal switches or as ordinary thermometers. PTCs are constructed using semiconductors combined with ceramics or polymers.
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Advantages and Disadvantages
Thermistors are small, very stable, are long-lasting, and are usually accurate to within +/- .05% to +/- .02% . This makes them superior to thermocouples and other devices that measure temperature. The disadvantage is that like typical semiconductors, they are non-linear, and so this effect must be compensated for when building circuits. Also, unlike thermocouples, they cannot be used at very high temperatures.
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For an experiment you can do if you have the proper equipment and are experienced with circuits, try PraticalPhysics.org's The Effect of Temperature on a Thermistor, where you place a thermistor and a thermometer into a beaker of hot water, and then compare and plot their results as the water cools. Remember to observe the basic safety rules for handling circuits.
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Physics for Scientists and Engineers by Douglas Giancoli
Physics For Scientists and Engineers by Raymond Serway
Thermistors from Amwei.com
Thermistor experiment from PracticalPhysics.org
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Detailed Thermistor Tutorial by Henry Sostmann and Philip D. Metz (1295K PDF file)
Capgo Thermistor Calculator (Java applet)
BetaTHERM's Resistance Chart (scroll to the bottom)