User:JamesForrest

POSITOR

A Positor is a thermistor element having a characteristic of a Positive Temperature Coefficient of electrical resistivity (commonly referred to as a PTC, but also as a Positor). The characteristics are that the device has a low resistance at low temperature and a high resistance at high temperature. This is generally useful for controlling the start-up conditions of machinery requiring an initial high current (low resistance) but reducing slowly to a lower running current (high resistance due to temperature rise from starting current).

It is also used in cathode ray tube televisions for degaussing (i.e. demagnetising) the tube's electrodes. The purpose is to remove any magnetism from magnetised components (a magnetised component will distort the TV tube image). Degaussing is achieved by applying the alternating current (AC) to a coil around the tube (usually from the domestic 50Hz supply) via the Positor, poducing an initial high current hence a strong magnetic field to the tube, which then slowly decreases to a very low level as the positior warms up. The reducing alternating field reduces the permanent field by applying similar opposite but diminishing polarity fields.