Thermistor

The thermistor is a straightforward device that changes its resistance with temperature. This can be used for many purposes.


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The name thermistor is a shortening of the words thermal resistor. It is essentially a thermally sensitive resistor, giving a change in resistance for a change in temperature.

Thermistors can be used in many ways, enabling the temperature of the medium surrounding the device or the device itself to alter its resistance. This can then be detected by the equipment and used for everything from broad temperature sensing to overload cut-outs and many more ideas.

Thermistors are found in many circuits and equipment, providing a simple and cost effective but effective method of basic temperature sensing.

Thermistor circuit symbol

The thermistor is recognised within circuits by its own circuit symbol. The thermistor circuit symbol uses the standard resistor rectangle as its basis and then has a diagonal line through it which has a small vertical section.

Thermistor circuit symbol
Thermistor circuit symbol

The circuit symbol shown above is the most widely used. Other types may be seen but generally they follow a similar approach - typically using the old resistor symbol of a zig-zag line as the basis with the same line through it as used with the more conventional rectangular resistor.

Thermistor types

There are a number of ways in which thermistors can be categorised into the different thermistor types. The first is dependent upon the way they react to heat. Some increase their resistance with increasing temperature, while others exhibit a fall in resistance.

It is possible to use a very simplified equation for the curve of a thermistor to expand this idea:

Δ R = k   Δ T

Where
    ΔR = change in resistance.
    ΔT = change in temperature.
    k = first-order temperature coefficient of resistance.

In most cases the relationship between temperature and resistance is non-linear, but over small changes a linear relationship can be assumed.

For some thermistors the value of k is positive, whereas for others it is negative and accordingly it is possible to categorise thermistors according to this aspect of their performance.

  • Negative temperature coefficient (NTC thermistor)   This type of thermistor has the property where the resistance decreases with increasing temperature, i.e. k is negative. The term NTC thermistor is widely used in datasheets and component data.   . . . . Read more about the NTC thermistor
  • Positive temperature coefficient (PTC thermistor)   This type has the property where the resistance increases with increasing temperature, i.e. the value of k is positive.   . . . . Read more about the PTC thermistor

In addition to the nature of the resistance change, thermistors can also be categorised according to the type of material used. Typically they use one of two materials:

  • Metallic compounds including oxides etc.
  • Single-crystal semiconductors

How thermistors were first developed

As early as the nineteenth century people have been able to demonstrate the variation of a resistor with temperature. These have been used in a variety of ways, but many suffer from a comparatively small variation over even a large temperature range. Thermistors generally imply the use of semiconductors, and these provide a much larger resistance variation for a given temperature change.

Of the two types of material used for thermistors, the metallic compounds were the first to be discovered. The negative temperature co-efficient was observed by Faraday in 1833 when he measured the resistance variation with temperature of silver sulphide. However it took until the 1940s before metallic oxides became available commercially.

With the work that was undertaken into semiconductor materials after the Second World War, crystal germanium thermistors were studied, and later silicon thermistors were investigated.

Although there are two types of thermistor, the metallic oxides and the semiconductor varieties, they cover different temperature ranges and in this way they do not compete.

Thermistor structure & composition

Thermistors come in a variety of shapes and sizes, and they are made from a variety of materials dependent upon their intended application and the temperature range over which they need to operate. In terms of their physical shape they can come as flat discs for applications where they need to be in contact with a flat surface. However they can also be made in the form of beads or even rods for use in temperature probes. In fact the actual shape of a thermistor is very dependent upon the requirements for the application.

Metallic oxide thermistors are generally used for temperatures in the range 200 - 700 K. These thermistors are made from a fine powder version of the material that is compressed and sintered at high temperature. The most common materials to be used for these thermistors are Manganese oxide, nickel oxide, cobalt oxide, copper oxide and ferric oxide.

Semiconductor thermistors are used for much lower temperatures. Germanium thermistors are more widely used than their silicon counterparts and are used for temperatures below 100 K, i.e. within 100 degrees of absolute zero. Silicon thermistors can be used at temperatures up to 250°K. Above this temperature a positive temperature coefficient sets in. The thermistor itself is made from a single crystal which has been doped to a level of 1016 - 1017 per cubic centimetre.

Thermistor applications

There are many different thermistor applications - they are found in many applications. They provide very cheap, yet effective elements in circuits and as such they are very attractive to use. The actual applications depend upon whether the thermistor is a positive or negative temperature co-efficient.

  • Applications for negative temperature coefficient thermistors:
    • Very low temperature thermometers:   They are used as resistance thermometers in very low-temperature measurements.
    • Digital thermostats:   These thermistors are also commonly used in modern digital thermostats.
    • Battery pack monitors:   NTC thermistors are also used to monitor the temperature of battery packs while charging. As modern batteries such as Li-ion batteries are very sensitive to overcharging, the temperature provides a very good indication of the charging state, and when to terminate the charge cycle.
    • In-rush protection devices:   NTC thermistors can be used as in-rush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purpose designed for this application.
    Read more about the NTC thermistor

  • Applications for positive temperature coefficient thermistors:
    • Current limiting devices:   PTC thermistors can be used as current limiting devices in electronic circuits, where they can be used as an alternative to a fuse. Current flowing through the device under normal conditions causes a small amount of heating which does not give rise to any undue effects. However if the current is large, then it gives rise to more heat which the device may not be able to lose to the surroundings and the resistance goes up. In turn this gives rise to more heat generation in a positive feedback effect. As the resistance increases, so the current falls, thereby protecting the device.
    Read more about the PTC thermistor.

Thermistors can be used in a wide variety of applications. They provide a simple, reliable and inexpensive method of sensing temperatures. As such they may be found in a wide variety of devices from fire alarms to thermostats. Although they may be used on their own, they may also be used as part of a Wheatstone bridge to provide higher degrees of accuracy.

Another thermistor application is as temperature compensation devices. Most resistors have a positive temperature co-efficient, their resistance increasing with increasing temperature. In applications where stability is required, a thermistor with a negative temperature co-efficient can be incorporated into the circuit to counteract the effect of the components with a positive temperature co-efficient.

Thermistor specifications

Although thermistors have a basic resistance specification, other parameters like the temperature coefficient are very important.

The parameters specified in the datasheets include the basic resistance, tolerance on the basic resistance, Β tolerance on Β thermal dissipation factor, maximum power dissipation and operating temperature range.

  . . . . Read more about thermistor specifications & parameters.



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