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.
- 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.
- 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.
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