Thermistors

Types

NTC (Negative Temp Coefficient)

Resistance decreases with temperature. Used as temperature sensor, inrush limiter.

PTC (Positive Temp Coefficient)

Resistance increases with temperature. Used as self-resetting fuse, heater.

Beta equation (approximate)

R(T)

= R₀ · exp(β · (1/T − 1/T₀))

1/T

= 1/T₀ + (1/β) · ln(R/R₀)

β typical

3 000 – 4 500 K for common NTCs

T₀

298.15 K (25 °C reference)

Steinhart-Hart (more accurate)

1/T

= A + B·ln(R) + C·(ln(R))³

A, B, C

Coefficients from calibration at 3+ temperatures

Common NTC values

Circuit notes

• Use a voltage divider with a stable reference resistor and measure with an ADC.
• Self-heating: I²·R warms the thermistor — minimize current (< 1 mW).
• Non-linearity: pre-compute or use a lookup table for accurate temperature.
• For wide range, use log-amp or choose β close to your operating range.