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Useful information on Thermistors

Useful information on Thermistors

 A thermistor is actually a resistor, whose resistance varies with the rise of the temperature of the environment. They have wide range of usage, which will be one of the subjects in this article. Their main assignment is to act as a thermal protection in the electric circuits. There are two main categories: silistors – they actually are sensitive silicon resistors, and the second type is called a switching PTC thermistor. They are made of compositions of barium, lead and strontium titanates with additives like yttrium, manganese, tantalum, and silica. They can be bought in every local electronics store and are relatively cheap

Production of switching PTC thermistor

It is known that a PTC thermistor comes as a disk, a chip, a lozenge, or can be surface mounted. The materials used for production are blended, milled, dried, and at last, crushed in powder. This material is calcined, and it is again milled with binders. The next step is to blend the binders into the mixture and fine powder is produced by drying the mixture. The powder is then formed and pressurized to the needed geometry. In order to form the body, the PTC thermistor must be sintered. The metalized contacts are applied to the surface through a process called painting, sputtering, or dipping. The switching characteristics and voltage ratings must be constant. Contaminations are not allowed, because they will cause changes in the thermal and electrical characteristics of the thermistor.

The transition temperature of the thermistor varies from 0-200 degrees. Insulation coating of a switching PTC thermistor does not interrupt any of its normal operations.

PTC thermistor properties

By the manufacturers’ data, every PTC thermistor has its own thermal and electrical properties for every field of application. Thermal properties are described through heat capacity, dissipation, and thermal time constant. The other type of PTC thermistor properties, are electrical properties, which are divided in several subcategories: Current-time characteristic, Resistance-temperature characteristic, Power and minimum resistance, Minimum resistor, Temperature coefficient of resistance, Transition temperature, Voltage dependence, Voltage rating and Voltage-current characteristic. It must be noted that PTC thermistor properties can change according to changes in the ambient temperature, radiation, dissipation constant and electrical parameters in the circuit.

PTC thermistor usage

PTC thermistors are used in many applications, such as current limiting devices – they can replace fuses. In this case, current passing through the them causes resistive heating. If the current is too large, the thermistor heats up and thus the resistance increases. They are also used as timers in most CRT displays and TV’s. A quick example will be explained. The display unit is switched on, and in that case, the current flows through the thermistor and the degaussing coil. This current flow heats the thermistor, and the degaussing coil shuts off the circuit in which it is included. This circuit is very cheap and reliable. It is naturally turned off, thanks to the characteristics of the thermistor. They are also used in electric circuits for motor and additional electronic equipment protection, or for liquid level and flow sensing.

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