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
| R @ 25 °C | Use |
|---|---|
| 1 kΩ | Low-value thermal cutoffs |
| 10 kΩ | Most common — 3D printers, HVAC, batteries |
| 47 kΩ | Low-power sensing |
| 100 kΩ | Very low-current sensing |
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.
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