Why are NTC thermistors non ohmic?

Answer 1

Any temperature-sensing device whose resistance is a function of temperature is inherently nonohmic; i.e., does not display a constant resistance with varied current flow or varied applied voltage. This is due to the fact that current flow will cause the device to heat and do its job which is to change resistance. An NTC device could conceivably "run away" with a constant applied voltage; as current increases it heats up and its resistance goes down resulting in further current increase. Conversely, a PTC device could conceivably run away with a constant applied current; as it heats up its resistance increases resulting in a higher voltage drop across it, which produces more heating, etc. For this reason, when using a thermistor it is a good idea to measure its resistance with a very sensitive (low sensing voltage and current) ohmmeter to minimize spurious heat input.