Thermistor

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The great advantage of thermistors is their high sensitivity. They are used mostly for room temperature measurement up to moderately high temperatures. They are popular in research and in medical applications, e.g as electronic medical thermometers.

A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the resulting voltage drop.

There are two basics types of thermistor, PTC and NTC:

  • A PTC (Positive Temperature Coefficient) device shows an increase of resistance when temperature increases. PTC thermistors are temperature-dependent resistors made from barium titanate and should be chosen when a drastic change in resistance is required at a specific temperature or current level.
  • A NTC (Negative Temperature Coefficient) device shows a decrease of resistance when temperature increases. NTC thermistors consist of a semiconductor. They are made from mixtures of oxides of manganese, cobalt, copper or nickel. They operate at temperatures from -150°C to 600°C. For temperatures of the order of 700 °C, devices utilising zirconia doped with rare earth oxides can be used. NTCs should be chosen when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with high sensitivity.

Thermistors are commercially available in a wide variety of shapes: bead, disc, chips and probes. The most common form is a bead with two wires attached. The bead diameter can range from about 0.5mm to 5 mm.

   The small size of thermistors makes them very adaptable and they are often included in electronic circuits. They can be encapsulated in epoxy resin, or glass or be painted. The thermistor is simple, strong and very reliable.

Temperature dependence of the resistance

The resistance of a CTN thermistor falls exponentially with increasing temperature. It is negative and non-linear, as expressed by the following relation:

where R0 is the nominal resistance measured at the absolute temperature T0 (often taken at 25 °C) and B is a constant for the particular thermistor material. The non-linear nature of the resistance may be undesirable and this can be offset by using two or more thermistors in series or parallel, packaged in a single device.

The temperature coefficient of resistance α, expressed in %/K (or %/°C), is defined as:

The coefficient α is positive for a PTC thermistor and negative for NTC thermistor.

The temperature can be determined from a thermistor using the Steinhart & Hart equation:

where R is the thermistors resistance in ohms, T the absolute temperature in K, and a, b and c are constants, normally supplied by the manufacturers but may be determined by calibration at three different temperatures and solving three simultaneous equations.

This equation is a close fit to practical devices but it does not always provide the precision required over the full temperature range. This can be corrected by fitting the Steinhart & Hart equation over a series of narrow temperature ranges and then 'splicing' these fits together to cover the required range.

Thermistor advantages and disadvantages

  • The great advantage with thermistors is the possibility of point measurements
  • Their high sensitivity allows thermistors to work over a small temperature range, compared to other temperature sensors, and to be very accurate (commonly better than 0.05°C and 0.1°C). The sensitivity of thermistors can be an order of magnitude greater than that of the resistance thermometer (RTD)
  • The most common range of temperature is between 0°C and 100°C. At higher temperature, they are subject to large drifts
  • The high resistivity of the thermistor negates the need for a four-wire bridge circuit
  • Thermistors are highly non-linear and not rugged, which limits their application
  • A source of error is due to the self-heating effects produced by an excessive bias current when the thermistor is powered up to give an output voltage signal. To reduce this source of error, it is necessary to use a current suited to the value of the measured resistance.

Because of their performance and moderate cost, thermistors are suitable for temperature measurement and control, temperature compensation, electronic, medical and many other applications.

References

  1. T D McGee (editor), Principles and methods of temperature measurement, John Wiley & Sons, ISBN 0 471-62767-4
  2. J Scholz, T Ricolfi (editors), Thermal sensors, VCH, vol 4, ISBN 3-527-26770-0
  3. G Bonnier, H Ronsin (editors), Thermistances CTN et autres thermomètres à semiconducteurs, Technique de l'Ingénieur R 2 580
  4. A Tong, Improving the accuracy of temperature measurements, Sensor Review, vol 21, n°3, 2001
  5. P R N Childs, J R Greenwood, C A Long, Review of temperature measurement, Review of scientific instruments, vol 71, n°8 2000
  6. J Yoder, Making contact with temperature, Flow Research, 2000


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