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Understanding Contact Temperatue Sensors
  /thermistors
 

It seems that Seebeck almost discovered semiconductors. When we study semiconductor physics we learn about three energy levels referred to as bands: the valence band, the conduction band, and the gap between them (depletion band). Thermal excitation of a pure semi-conductor (the addition of heat energy) causes more carriers to be available for conduction, a phenomenon referred to as intrinsic conduction. At low temperatures the conductivity is very low because of the shortage of intrinsic carriers, so the material is usually doped with additional carriers. Their contribution is known as extrinsic conduction.

Seebeck was working with metal conductors and would have seen only the wire resistance increasing with temperature, although he also saw an increase in voltage resulting from carriers moving from the valence to the conduction bands. The characteristics of the metal conductors in conjunction with the observed changes in thermoelectric voltages so thoroughly complicated the situation as to result in his noting the effect without explanation. The effect bearing his name is much more pronounced with semiconductors than with conductors.

A thermistor is a semiconductor with all its material at the same temperature. As a consequence, no difference of potential exists. Thermistors are mixtures of metal oxide semiconductor materials formulated to exhibit specified characteristics. The relationship between the resistance of the thermistor and its temperature is a combination of its intrinsic and extrinsic characteristics.

Intrinsic conduction can be expressed as:

(2)

where:

= temperature in kelvin

= reference temperature in kelvin

= a constant of the material that represents the change in as a function of temperature

Attempts to modify equation (2) for extrinsic conduction resulted in several empirical equations that serve to further complicate calculations and are therefore rarely used. Current practice is to determine polynomial equations to fit the characteristics of the particular thermistor as follows:

(3)

(4)

An accepted practical version, the Steinhart–Hart equation (5), is used by thermistor producers.

(5)

With the introduction of the transistor, temperature compensation of electronic circuits became a much more important task than it had been with vacuum tubes. The latter required heaters to function and tended to become stable after a short warm-up period. Transistors require no warm-up and their dynamic characteristics are very much affected by temperature. Thermocouples are not well suited for this purpose because the measurement must be relative to a fixed cold-junction temperature, making the implementation impractical. Into the 1970s RTDs were made of wire. Platinum wire RTDs were often considered too costly and some RTDs were made of nickel or copper wire or foil instead. Today the RTD cost comparison is reversed because platinum is available in high-performance, low cost thin film and efficient wire designs.

The leakage current associated with semiconductors was often used in temperature stabilization circuits. It is a consequence of the Seebeck effect on a reverse-biased semiconductor junction, and the development of the thermistor was a natural offshoot of that technology. Using homogeneous material (no junction) controlled to yield specific resistance vs. temperature characteristics, thermistors were developed that could provide accurate temperature measurement. Early thermistors produced in 1960 cost as much as $40.00 each, but found use in many applications because of their great sensitivity and small size.

Thermistors are restricted to measuring temperature over only a portion of their limited operating temperatures because of the dynamic range required if they are connected via an amplifier. A typical thermistor having a resistance of 14,000Ω at 0°C will have <500Ω at 100°C (dynamic range of 28:1). The excitation current must be kept low to avoid self-heating; if the thermistor is made larger to increase its mass and thus decrease self-heating concerns, response speed is sacrificed. At the time thermistors were invented, RTDs were made by winding small-diameter wire on a mandrel. The thermistors were smaller, faster, and more rugged, and quickly became much lower in cost.

 

 

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