Thermistors are resistors that react to temperature changes. Two categories exist: thermally-sensitive silicon resistors and polycrystalline-ceramic materials.
Commercial PTC thermistors fall into two major categories. The first category consists of thermally sensitive silicon resistors. These devices exhibit a fairly uniform positive temperature coefficient (about +0.77 %/°C) through most of their operational range. They can also exhibit a negative temperature co-efficient region at temperatures in excess of 150 °C. These devices are most often used for temperature compensation of silicon semi conducting devices in the range of -60 °C to +150 °C.
The other major category is polycrystalline ceramic materials. They are normally highly resistive but made semi-conductive by the addition of dopants. They are most often manufactured using compositions of barium, lead and strontium/titanites with additives such as yttrium, manganese, tantalum and silica.
These devices have a resistance-temperature characteristic that exhibits a very small negative temperature co-efficient until the device reaches a critical temperature that is referred to as its ‘Curie’, meaning the 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.
The PTC thermistor has to be connected to a control circuit that can transform the resistance change to a control signal which is able to cut the mains current to the motor.
Many Grundfos pumps have PTC thermistor motor protection built-in, where this is a requirement for the application.