25-10-2012, 02:29 PM
Thermistor
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A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteauof thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C
The current is measured using an ammeter. Over large changes in temperature, calibration is necessary. Over small changes in temperature, if the right semiconductor is used, the resistance of the material is linearly proportional to the temperature. There are many different semiconducting thermistors with a range from about 0.01 kelvin to 2,000 kelvins (−273.14 °C to 1,700 °C).
Most PTC thermistors are of the "switching" type, which means that their resistance rises suddenly at a certain critical temperature. The devices are made of a doped polycrystalline ceramic containing barium titivate (BaTiO3) and other compounds. The dielectric constant of this ferroelectric material varies with temperature. Below the Curie point temperature, the high dielectric constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance. In this region the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply. At even higher temperatures, the material reverts to NTC behaviour. The equations used for modelling this behaviour were derived by W. Heywang and G. H. Jonker in the 1960s.
PTC Thermistors Structure and Characteristics
Commercial PTC thermistors (abbreviated for Positive Temperature Coefficient of Resistance) fall into two major categories. The first category consists of thermally sensitive silicon resistors, sometimes referred to as “silistors”. These devices exhibit a fairly uniform positive temperature coefficient (about +0.77% /°C) through most of their operational range, but can also exhibit a negative temperature coefficient region at temperatures in excess of 150°C. These devices are most often used for temperature compensation of silicon semiconducting devices in the range of -40°C to +150°C.
The other major category, and the one that we shall concentrate on in this section, are referred to as switching PTC thermistors. These devices are polycrystalline ceramic materials that are normally highly resistive but are made semiconductive by the addition of dopants. They are most often manufactured using compositions of barium, lead and strontium titanates with additives such as yttrium, manganese, tantalum and silica.
These devices have a resistance-temperature characteristic that exhibits a very small negative temperature coefficient until the device reaches a critical temperature, that is referred to as its “Curie”, switch or transition temperature. As this critical temperature is approached, the devices begin to exhibit a rising, positive temperature coefficient of resistance as well as a large increase in resistance. The resistance change can be as much as several orders of magnitude within a temperature span of a few degrees.
Ceramic PTC Thermistors Resistance vs. Temperature Characteristic
PTC Thermistor Resistance vs. Temperature means the relation of zero-power resistance of PTC thermistor to PTC thermistor body temperature under a specified voltage. Zero-power resistance should be measured in super slot by using pulse power supply with low output impedance an stable output amplitude. Temperature rise of PTC thernistor induced by measuring current should be so limited that it could be ignored.The resistance of the PTC thermistor is composed of the grain resistance and the grain boundary transition resistance. Particularly in the hot state, the strong potential barriers are determining resistance. Higher voltages applied to the PTC thermistor therefore drop primarily at the grain boundaries with the result that the high field strengths dominating here produce a break-up of the potential barriers and thus a lower resistance. The stronger the potential barriers are, the greater is the influence of this "varistor effect" on resistance. Below the reference temperature, where the junctions are not so marked, most of the applied voltage is absorbed by the grain resistance. Thus the field strength at the grain boundaries decreases and the varistor effect is quite weak.