Thermistor

- an overview of the thermistor - a thermally sensitive resistor used in many electronics applications

The name thermistor is a shortening of the words thermally sensitive resistor. This describes the action of the thermistor particularly well. Today, thermistors are used in a wide variety of devices from temperature sensors through to providing temperature compensation in electronic circuits. As such thermistors are widely used in electronic, although they are obviously not as commonly used as ordinary resistors, capacitors and transistors.

Thermistor categories

There are a number of ways in which thermistors can be categorised. 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. Accordingly it is possible categorise them accordingly:

• Positive temperature coefficient (PTC)   Where the resistance increases with increasing temperature



• Negative temperature coefficient (NTC)   Where the resiatnce decreases with increasing temperature

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

History and development

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. Themistors 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 themistors were investigated.

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

Structure and 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 10^16 - 10^17 per cubic centimetre.

Thermistor applications

Thermistors 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 (PTC) or negative (NTC) temperature co-efficient.

1. Applications for negative temperature coefficient (NTC) thermistors:



• Very low temperature thermometers:   NTC thermistors are used as resistance thermometers in very low-temperature measurements.



• Digital thermostats:   Thermistors are also commonly used in modern digital thermostats.



• Battery pack monitors:   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 inrush-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.





2. Applications for Positive temperature coefficient (PTC) 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 loose 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.

Summary

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 used for thermistors 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.