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PTC Elements as Self-Regulating Heaters

By applying a voltage across a PTC thermistor, current will flow and begin to heat the PTC material.  The PTC element rises in temperature until it reaches its Curie point, when its resistance will increase exponentially.
The increase in PTC resistance limits the through current until the PTC heater reaches a state of equilibrium.  If heat is withdrawn from the PTC, its resistance will drop allowing more current to flow through the PTC until a steady-state condition is reached again.  Conversely, if the PTC element temperature rises, its resistance will increase accordingly reducing the through current until a further state of equilibrium is achieved.
Thus a PTC thermistor can operate as a self-regulating heater.