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.