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Thermistors |
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NTC
PTC |
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| PTC (Positive
Temperature Coefficient) Thermistors are temperature
dependent resistors which increase in resistance abruptly when a
certain temperature is exceeded. This characteristic makes it an excellent solution for overload protection. A PTC Thermistor connected in series with a load becomes high resistance in a fraction of a second as a result of the overload current flow heating the Thermistor. Once the disturbance has been removed, the PTC Thermistor will cool down enabling the load to receive current again. PTC Thermistors are also used as heating elements. As in conventional methods, they are heated by the current flowing through them, but they have the extra great advantage of self-regulation. Once a specified temperature or current is exceeded, resistance increases and the current flow and further heating is restricted until the PTC Thermistor has cooled down. Other applications include time delay functions and fluid level measurement which makes use of the change in self-warming properties of PTC Thermistors depending upon whether or not the Thermistor is surrounded by fluid or air. Ceramic-based PTC's have a better life expectancy than Polymer-based PTC's. They are able to handle thousands of turn-off situations and are able to take much higher loads. |
NTC (Negative Temperature
Coefficient) Thermistors, in
contrast, decrease in resistance as the temperature
increases. This change occurs more slowly than the abrupt NTC Thermistors are particularly suited to temperature measurement, typically between -60 degrees and +180 degrees C. Special devices can be used up to 1000 degrees C, for example to measure exhaust emission temperatures. The steep characteristic curves allow simple driving circuits to be used compared to other temperature sensors. NTC's are used in many items of
equipment such as
switched-mode power supplies or electric motors exhibit
excessive inrush currents when first switched on, where
without the NTC damage would be caused or there would
be inconvenience as fuses would continually need replacing.
The NTC Thermistor, with its relatively high cold resistance,
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