PTC resettable fuse leakage current
Hi everyone, I have a question about the practical use of a PTC (positive thermal coefficient) resettable fuse. I've never used one before, but I'd like to add one to a DC motor circuit to prevent it from overheating when running continuously.
More features being used
I already have two motor protection features in place: current limiting and timed shutdown upon continuous current draw. Current limiting makes sure that the motor is not allowed to go anywhere near stall, before which the plastic gears in the gearbox become stripped due to the torque and the motor fails. The other feature: if there's current for a certain period of time, the microcontroller tells the motor driver IC to cut off power to the motor. I'm considering using a PTC as a third feature, just as an added safety measure to prevent the motor from heating up if anything goes wrong and current is allowed to just proceed through the coil for hours.
Background to question
I was looking at Littelfuse's PTC selection guide, and it mentions how that the resistance of the PTC increases greatly once the PTC is tripped. It also says that only a "low leakage level" of current of up to a few hundred milliamps is allowed through. However, when selecting PTCs on Digikey, I saw that the tripped resistance is only 2–4x the amount of the normal resistance, which is on the order of a few hundred milliohms to a few ohms to begin with.
What exactly can I expect for the PTC tripped leakage current? How is it calculated? Is it just a matter of using the tripped resistance with any other series resistances (motor), the source voltage, and Ohm's Law (V=IR)? Or is there some other phenomenon in play with PTCs where the current is limited to a certain range? I read that PTCs allow the leakage current through to keep it in its tripped state, but there was no word as to what this amount is or how it's calculated.
Thanks for any help!