NTC Thermistor Calculator
Calculate NTC thermistor resistance at any temperature using Beta and Steinhart-Hart equations. Design accurate temperature-sensing circuits.
Common NTC Thermistor Values @ 25°C
| Resistance @ 25°C | Beta (typ) | Common Use |
|---|---|---|
| 1 kΩ | 3950 K | High-sensitivity sensing |
| 10 kΩ | 3950 K | Standard (Arduino, etc.) |
| 100 kΩ | 3988 K | Low-power sensing |
Temperature Sensing Circuit Tips
- Use voltage divider: ADC reads V_out = V_cc × R_therm ÷ (R_series + R_therm)
- Keep excitation current <100 µA to minimize self-heating
- Add noise filtering (0.1 µF capacitor) on analog input
- Calibrate at two reference points (ice bath 0°C, boiling water 100°C) for best accuracy
- Use Steinhart-Hart for ±0.5°C precision across wide ranges
Frequently Asked Questions
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An NTC (Negative Temperature Coefficient) thermistor is a resistor whose resistance decreases as temperature increases. Made from metal oxide ceramics, NTCs offer high sensitivity (large resistance change per degree) and are used in temperature sensing, thermostats, and thermal protection.
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The Beta equation relates resistance to temperature: R(T) = R₀ × exp[β × (1/T − 1/T₀)], where R₀ is reference resistance at T₀, β is the Beta value in Kelvin, T is absolute temperature. Beta is typically 3000–4000 K for common NTCs.
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A more accurate model: 1/T = A + B×ln(R) + C×(ln(R))³, where A, B, C are calibration coefficients. This is more accurate over wide temperature ranges than Beta equation but requires three reference points to calculate coefficients.
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Use Beta for quick calculations and narrow ranges (<50 °C). Use Steinhart-Hart for precision across wide ranges (−20 to +100 °C) or when ±0.5 °C accuracy is required. Datasheet may provide both or recommend one.
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Beta is listed in the thermistor datasheet or can be calculated from two temperature/resistance points: β = ln(R₁/R₂) ÷ (1/T₁ − 1/T₂). Common values: 3435 K (3.3 kΩ @ 25 °C), 3950 K (10 kΩ @ 25 °C), 3988 K (100 kΩ @ 25 °C).
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When current flows through a thermistor, its resistance dissipates power (P = I²R), causing the element to heat above ambient. This error is called self-heating. Use low excitation currents (<100 µA) to minimize self-heating.
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NTC response is nonlinear, especially at extreme temperatures. Linearize using a parallel resistor across the thermistor (Linearization resistor) to produce more constant slope, or use digital compensation (software).
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Common values: 1 kΩ @ 25 °C, 10 kΩ @ 25 °C, 100 kΩ @ 25 °C. Higher resistance reduces self-heating but requires higher impedance measurement circuits. Lower resistance draws more bias current but offers faster response.
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Standard NTCs are rated −40 to +125 °C. Specialized high-temp types go to +200 °C or beyond but are more expensive. Check datasheet for continuous and peak rating (some allow brief excursions above continuous limit).
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Use a digital multimeter on the Ω range. For high-impedance thermistors (>100 kΩ), avoid finger contact on leads (body resistance affects reading). Measure at stable ambient temperature to minimize self-heating error.
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Aging: Resistance drifts ~1–3% per year due to ceramic crystallization and moisture absorption. High-temperature operation accelerates aging. Use quality thermistors (Class A tolerance) and periodic recalibration for precision instruments.
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Typical tolerance: ±5% @ 25 °C means resistance at 25 °C varies by ±5%. Tolerance widens at temperature extremes (nonlinear). For critical applications, buy matched pairs or select low-tolerance (Class A, ±1%) thermistors.