Composite thermistor is a combination of thermal coupling, The VDR varistor and PTC thermistor close to and packaged into one. Its main application in power meters and other power supply in the switching power supply and transformer primary At the same time to achieve the current, voltage integrated protection. Which solves the difficulty of using a single PTC thermistor in matching with the transformer. Over-voltage over-current state of the PTC thermistor protection instrument can not work properly, And cryogenic instrumentation exception PTC may not be protected And so on.