What Is a Thermistor and How Do You Use One?

Detailed Explanation

NTC thermistors are among the most cost-effective temperature measurement components available: a 10 kΩ NTC in a 0402 package costs a few cents, requires no power management, and reads out directly on any MCU ADC pin. The trade-off is that the resistance-to-temperature relationship is highly non-linear and requires careful circuit design and calibration to achieve good accuracy. For higher-temperature or wider-range applications beyond a thermistor's practical limit (typically −40°C to +150°C), see PT100 and RTD sensors or how thermocouples work. See the Temperature Sensors topic for the full set of temperature-sensing guides.

NTC vs PTC Thermistors

NTC (negative temperature coefficient): Resistance decreases as temperature rises. Most temperature measurement applications use NTC thermistors. Made from sintered metal oxides (manganese, nickel, cobalt oxides). Available in resistance values from a few ohms to several megaohms at 25°C. The most common value is 10 kΩ at 25°C.

PTC (positive temperature coefficient): Resistance increases as temperature rises. Available as polymer PTC (gradually increasing resistance, used in resettable fuses/polyfuses) and ceramic PTC (sharp resistance jump at a specific transition temperature, used as self-regulating heater elements or overcurrent protection). Less used for measurement; the resistance-temperature curve is not smooth enough for precision sensing.

The Voltage Divider Measurement Circuit

A thermistor cannot be read by an ADC directly, since the ADC reads voltage, not resistance. The simplest interface is a resistor voltage divider:

Vcc ─── R_fixed ─┬─── Vout → ADC
                 │
              R_NTC (thermistor)
                 │
               GND

Vout = Vcc × R_NTC / (R_fixed + R_NTC)

Or with the thermistor at the top and fixed resistor at the bottom:

Vout = Vcc × R_fixed / (R_fixed + R_NTC)

Both variants work; the choice affects which end of the temperature range gives the highest ADC reading.

Example calculation (10 kΩ NTC, 10 kΩ fixed, 3.3 V supply, 12-bit ADC):

Note the strongly non-linear output: a 25°C change from 0° to 25° moves the ADC by 1066 counts, while the same 25° change from 50° to 75° moves it by only ~450 counts. At higher temperatures, resolution degrades because the thermistor's resistance change per degree becomes smaller.

Converting ADC Reading to Temperature

Step 1: ADC → resistance

From the ADC reading (N), recover R_NTC:

R_NTC = R_fixed × (ADC_max / N − 1)

where ADC_max is the maximum ADC count (4095 for 12-bit).

Step 2: Resistance → temperature using the Beta equation

T (Kelvin) = 1 / (1/T0 + (1/B) × ln(R/R0))

where:

• T0 = 298.15 K (25°C nominal)
• R0 = nominal resistance at T0 (e.g. 10,000 Ω for a 10 kΩ NTC)
• B = Beta constant from datasheet (e.g. 3950 K for many 10 kΩ NTC types)
• R = measured R_NTC

To convert to Celsius: T_C = T − 273.15

Beta equation accuracy: ±0.5°C over a 50°C range is typical. For wider ranges, use the three-parameter Steinhart-Hart equation:

1/T = A + B·ln(R) + C·(ln(R))³

The Steinhart-Hart coefficients A, B, C are provided in thermistor datasheets or can be calculated from three resistance/temperature data points. This achieves ±0.01°C accuracy over a 100°C range.

Lookup Table vs Formula

For resource-constrained microcontrollers (8-bit MCU with limited floating-point), compute accuracy vs speed trade-offs exist:

• Steinhart-Hart formula: most accurate, requires floating-point arithmetic.
• Beta equation: simpler floating-point calculation, ±0.5°C.
• Lookup table: store (ADC code, temperature) pairs at 1–5°C intervals, interpolate between nearest entries. Requires ROM space (100–200 bytes for ±1°C interpolation over 0–100°C), no floating-point needed.

For typical embedded applications (Arduino, STM32, ESP32), the Beta equation with floating-point arithmetic is the standard approach. It executes in a few microseconds on any modern MCU.

Optimising the Voltage Divider Fixed Resistor

For maximum voltage resolution across the measurement range, set R_fixed equal to the geometric mean of the thermistor's resistance at the range endpoints:

R_fixed = √(R_at_T_min × R_at_T_max)

This places maximum sensitivity at the centre of the measurement range. For most applications, selecting R_fixed ≈ R_NTC at the operating midpoint temperature is practical.

For the ADC input voltage divider circuit, especially when the thermistor is read by a high-impedance MCU ADC, the Thevenin equivalent source resistance of the divider is R_fixed ∥ R_NTC. This should be kept below ~10 kΩ to avoid significant ADC input charge-transfer errors on SAR ADCs with typical sample capacitances of 5–20 pF.

Sensor Interface PCB Design

• Switch power to reduce self-heating: Drive the top of the voltage divider from a GPIO configured as output, not from a permanent power rail. Turn on the GPIO, wait 1 ms for settling, sample the ADC, then turn off the GPIO. This eliminates continuous current flow and self-heating during idle periods. See how do you budget for error and calibrate a temperature sensor? for sizing self-heating error against the rest of the measurement's error budget.
• Add an RC filter on the ADC input: A 100 Ω series resistor + 100 nF capacitor directly at the ADC pin (cutoff ≈ 16 kHz) filters switching noise from the GPIO turn-on transient and any EMI picked up by thermistor wires. This also forms part of an anti-aliasing filter as described in sensor signal conditioning basics.
• Use short, twisted-pair cabling for remote sensors: For thermistors mounted off-PCB (attached to a heatsink, inside a product enclosure), twisted-pair wire with a shield reduces capacitive pickup of switching noise. Keep the cable below 1 m where possible to minimise cable capacitance at the ADC input.
• Precision fixed resistor: Use a 1% or better metal film or thin-film SMD resistor for R_fixed. A 5% carbon film resistor adds ±5% to the resistance value, which translates to a significant temperature offset error.

For sensor interface circuit design as part of a complete product, Zeus Design's electronics engineering team provides full sensor integration support from schematic through to production.

Design Considerations

• Check the thermistor's rated temperature range: Most 10 kΩ NTC thermistors are rated −40°C to +125°C, but low-cost versions may only be rated to +85°C. Use a rated 105°C or 150°C type if the sensor will operate near high-power components. Verify the maximum continuous temperature against the datasheet.
• Match the B constant to your actual thermistor: The nominal B value printed on a reel label or distributor page is sometimes the B25/85 value (calculated between 25°C and 85°C), while your application may require the B25/50 or B25/100 value. Using the wrong B constant introduces a systematic offset across the measurement range. Confirm which temperature range the datasheet B constant applies to.
• For tight accuracy, calibrate: Even with the correct Steinhart-Hart coefficients, individual thermistor tolerances are typically ±1% in resistance at 25°C (±0.25°C). For medical, HVAC, or refrigeration applications where ±0.5°C or better is needed, perform a one-point or two-point calibration against a reference thermometer and store offset/gain correction in non-volatile memory.

Common Mistakes

• Using the wrong B constant: The B value from a distributor's search result is often the B25/85 constant. If you're measuring in the 0–40°C range, the B25/50 constant (usually lower) gives a more accurate curve. Always read the full datasheet and confirm the B constant definition.
• Forgetting the non-linearity when mapping ADC range to temperature: If you need to report temperature in 1°C increments using a fixed lookup table, the table must be denser at the cold end (where resistance changes rapidly) and can be sparser at the hot end. A uniform ADC-count-to-temperature lookup based on a linear assumption will show large errors at extreme temperatures.
• Using a high-impedance fixed resistor value (>100 kΩ) to reduce self-heating: High-value resistors do reduce self-heating current but make the divider output impedance very high, potentially too high for an SAR ADC to sample accurately without extended acquisition time. Either use the extended acquisition time feature (if available on the MCU) or use a unity-gain op-amp buffer between the divider and the ADC input. See what is an op-amp? for buffer circuit details.