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Wednesday, 16 October 2019

Thermistor vs Varistor

Thermistor :

Thermally sensitive resistor is called thermistor. The temperature coefficient of semi conductors covers wide range 5% per degree C for NTC thermistors to 60% / °C for PTC thermistors. This property leads to their primary application as temperature sensors. Negative temperature coefficient (NTC) thermistors are those whose resistance decreases with increase in temperature. Positive temperature coefficient (PTC) thermistors are those whose resistance increases with the increase in temperature.

The NTC thermistors are made from the oxides of certain metals such as Copper, Manganese, Magnesium, Zinc and Titanium. Most often a mixture of several oxides is used to obtain the requisite property. They are prepared by ceramic techniques in the form of discs, short rods and bits. The properties will depend not only on those of the constituents and their purity but also on their fineness to which they have been grounded, the pressure of the press moulding and the firing condition. They are used to compensate for change in resistance of electrical circuit relays, valves etc., caused by the variation of the temperatures. They are extensively used to measure power in very high frequency test sets.

PTC thermistors are currently made out of two types of materials. The first type uses the semi conductors germanium and silicon and possess a low temperature co-efficient of resistance in the region of 0.7/°C. The second type of material is based on semi conducting Barium titanate and possesses a large temperature coefficient of resistance of the order of upto 70% per-°C. These are frequently called switched PTC thermistors due to the use of the large resistance change that occurs over a small temperature range to switch off over heated equipment. One of the major application of thermistor is for over temperature protection of electric motors. Normal practice is to embed one thermistor in each phase winding, then to connect the three thermistors in series to 2 transisterised control circuit operating power cut-out relay. The second important applicant is as a current limiter in T.V degauging circuit.

Varistor :

In this type the resistance of the resistor varies with the applied voltage. These are divided broadly into two main classes.

(a) Devices with symmetrical (non rectifying) voltage versus current characteristics.
(b) Those with non symmetrical voltage versus current characteristics.

The area of contact devices made from copper oxide, selenium belong to non symmetrical varistor category whereas the device from silicon carbide fall into symmetrical varistor category. The current relation for these is I = KVn where

I is the current.
n is the non linearity exponent dependent on the quality of the material.
K is constant dependent on the quality of the material.

Silicon carbide obtained by heating a mixture of quartz, sand with carbon electrically to a temperature of 2000°C is the prime composition of the varistor and disc moulds are made from the mixture of Silicon carbide, graphite and glass. Elements are made by baking these mould discs. These materials are used to protect high voltage equipment and power lines from over voltages and lightning surges in the form of lightning arresters. In electronic field they serve as voltage stabiliser, arc suppression, motor speed control etc.

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