Positive temperature coefficient transition sensor

A device for sensing transition of a current limiting positive temperature coefficient resistive element disposed in an alternating current path into a state of high resistance, comprising:

• a first optoisolator having a first light emitting diode and a first phototransistor optically connected electrically with said first light emitting diode, said first phototransistor conducting when exposed to light emissions from said first light emitting diode, said first light emitting diode coupled in parallel with said positive temperature coefficient element;
• a second optoisolator having a second light emitting diode and a second phototransistor optically coupled with said second light emitting diode, said second phototransistor conducting when exposed to light emissions from said second light emitting diode, said second light emitting diode connected electrically in parallel with said positive temperature coefficient element at an opposite polarity to that of said first light emitting diode,
• wherein said phototransistors are connected electrically in parallel in a sensing circuit, and a transition of said positive temperature coefficient element into a state of high resistance causes forward biasing of at least one of said first and second light emitting diodes, thereby causing at least one of the first and second phototransistor to conduct, for signalling the transition.

A device for sensing transition of at least one of three current limiting positive temperature coefficient resistive elements into a state of high resistance, individual ones of said three positive temperature coefficient resistive elements being disposed in three separate phase lines of a three-phase alternating current transmission line, the device comprising:

• an optoisolator pair for each of the three phase lines, said optoisolator pair having first and second light emitting diodes and first and second phototransistors responsive to said first and second light emitting diodes, respectively, said first and second light emitting diodes being connected electrically in parallel at opposite polarity, and in parallel with one of the three positive temperature coefficient elements for the three phase lines, said first and second phototransistors for each of the three phase lines being connected electrically in parallel with one another in a sensing circuit, the first and second light emitting diodes for each of the three phase lines being forward biased upon transition of a corresponding one of the positive temperature coefficient elements to a high resistance state.