Andrew, the difficulty you are having is because Ohms law does not apply directly to PTC materials. The thing controlling the resistance is temperature only, given a certain minimum value. These materials have a kink in the ohmic value, known as a knee. It is fairly sharp and if you plot resistance against temperature you will see a fairly linear response to some temperature and then a fairly sharp increase in resistance to temperature slope, which may get to be almost constantly very high. This means that the only thing controlling the circuit current at the operating temperature is the temperature, Ohms law goes out of the window. Draw a little sketch graph of resistance v temperature as described to see this clearly.
Andrew, the difficulty you are having is because Ohms law does not apply directly to PTC materials. The thing controlling the resistance is temperature only, given a certain minimum value. These materials have a kink in the ohmic value, known as a knee. It is fairly sharp and if you plot resistance against temperature you will see a fairly linear response to some temperature and then a fairly sharp increase in resistance to temperature slope, which may get to be almost constantly very high. This means that the only thing controlling the circuit current at the operating temperature is the temperature, Ohms law goes out of the window. Draw a little sketch graph of resistance v temperature as described to see this clearly.
