NTC Thermistors

NTC Thermistors (Negative Temperature Coefficient) are resistive sensors for which the ohmic value decreases when temperature raises. Such variation follows an exponential curve known under the name of STEINHART-HART equation. It is as follows :

Where : R = Resistance in Ohm e = 2,718281828 T = Temperature in °K. A, B, C variable parameters for each kind of RT Curve. Two kinds of NTC thermistors are present on the market: American thermistors, following MIL - T 23648 -C spec. They are unique for their high accuracy and long term stability, as they don't contain neither Carbon, nor Cobalt and Gas. They are used for high precision measurements. Thermistors from other countries in the world, aren't so accurate and used for temperature compensation or as surges guards (current killers) because it's possible to make them in big sizes. American thermistors are very small, from an O.D. of 0.004" (0,1 mm), with very thin leads (0.4 mil or 10µm). The main used NTC for temperatures probes have an O.D. of 0.077" (2.0 mm) with an interchangeability of ± 0.2°C. over a temperature range up to 100° C.