All these characteristics will have to be considered for a high-performance design. Be sure to look at Vishay’s document “PTC Explanation of Terms”, which describes the numerous PTC parameters, from Ts to the voltage dependent resistance (VDR) effect and trip and non-trip currents. If you don’t have the time to dedicate to studying, the second approach is to go directly to a component’s datasheets. For example, you might read articles about the Heywang model and immerse yourself in the interesting principles of the grain boundary’s resistivity changes, coupled with the Curie temperature effect. If you need to use a PTC in your application, there are two classic ways to explore. However, in practice the phenomena underlying a PTC’s behavior are extremely complex and the equations describing them are seldom decipherable. Thus, at a low ambient temperature the PTC lets the normal current flow into the circuit, and above the Ts point it acts as an open circuit.
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