the NTC thermistor
/ N-T-C THERM-iss-ter /
The NTC thermistor is the more common cousin of the PTC: a resistor whose resistance falls smoothly and steeply as it warms up — 'negative temperature coefficient' meaning resistance down as temperature up. It is the small bead or disc that serves as the accurate, cheap temperature sensor inside thermostats, battery packs, engine sensors, and 3D-printer hot ends, and as the inrush-current limiter that tames the switch-on surge of a power supply.
NTC ceramics are semiconducting transition-metal oxides, most often spinels of manganese with nickel, cobalt, or copper (broadly the Mn-Ni-Co-O family). Conduction happens by small-polaron hopping: the spinel holds the same metal in two oxidation states on neighbouring octahedral sites — manganese as both Mn3+ and Mn4+, for instance — and an electron hops from one to the other. That hop must be thermally activated over an energy barrier, so raising the temperature makes hops far more frequent and the resistance drops exponentially. The behaviour follows R = R0 times exp(B / T), where T is absolute temperature and B is the characteristic 'B constant' (typically 3000 to 4000 kelvin) that sets how sharply the resistance changes; a part is specified by its resistance at 25 degrees C (its R25) and its B value.
That exponential sensitivity is the NTC's great strength for temperature measurement — a small temperature change produces a large, easily read resistance change, more sensitive than a metal resistance thermometer over a modest range. As an inrush limiter, a power-supply NTC starts cold and resistive to block the turn-on surge, then self-heats and drops to low resistance so it wastes little power in normal running. Two honest caveats: the response is strongly nonlinear, so accurate thermometry needs calibration and a linearising fit such as the Steinhart-Hart equation, not a straight line; and the very current you use to measure the resistance heats the bead (self-heating), which can bias the reading unless the sensing current is kept small.
A lithium-battery pack watches its own temperature with a tiny NTC bead pressed against the cells: as the pack warms during fast charging, the bead's resistance drops along its known R = R0 exp(B/T) curve, and the charger reads that change to slow down or stop before the cells overheat.
The NTC's resistance falls exponentially with temperature via thermally activated electron hopping — cheap, sensitive, but nonlinear enough to need calibration.
An NTC's resistance is not a straight line in temperature. Treating it as linear introduces large errors; accurate use requires a fit such as the Steinhart-Hart equation and attention to self-heating from the measuring current.