How the NTC voltage divider calculator works
The simplest way to read an NTC thermistor is a voltage divider: the thermistor and a fixed series resistor between the supply and ground, with the middle point going to an ADC input. As the temperature changes, the thermistor’s resistance changes and so does the voltage at the middle point.
The calculator finds the NTC’s resistance from the Beta model, then the output voltage, its slope in mV per degree, and the temperature step that one ADC code represents. It does this at your operating temperature and across your whole range, and it suggests the series resistor that gives the largest output swing.
Choosing the series resistor
The output swing between two temperatures is largest when the series resistor equals the geometric mean of the NTC’s resistance at those temperatures. For a 10 kΩ, β 3950 thermistor used from 0 to 100 °C, R is 33.6 kΩ at 0 °C and 698 Ω at 100 °C, so the best series resistor is about 4.84 kΩ; the nearest standard values are 4.7 kΩ (E24) and 4.87 kΩ (E96).
A series resistor equal to R₂₅, 10 kΩ here, gives the steepest slope at 25 °C but less swing towards 100 °C. Use the geometric mean when the whole range matters, and R at the temperature you care about most when one point matters.
Which way round
With the NTC to ground the output falls as the temperature rises; with the NTC to the supply it rises. The resolution is the same either way. Choose the orientation that keeps a broken or shorted sensor recognisable: with the NTC to ground, an open thermistor pulls the input to the supply and a short pulls it to ground, both outside the normal range.
Resolution and the ADC
One ADC code is the reference voltage divided by 2^N. Divided by the slope it gives the temperature step: 0.022 °C at 25 °C for the example on a 12-bit converter, but coarser at the ends of the range, where the curve flattens. The calculator shows the coarsest point. If the ADC uses the divider’s supply as its reference, the measurement is ratiometric and supply changes cancel; leave the reference empty for this case.
Self-heating
The current through the divider heats the thermistor. The temperature rise is the power in the NTC divided by its dissipation constant, which the datasheet gives in mW/K for still air or for the mounting it was measured in. At 3.3 V with 10 kΩ + 10 kΩ the NTC dissipates 272 µW; with 1.5 mW/K that is 0.18 K. A larger series resistor, a lower supply or powering the divider only while measuring reduces it.
For the resistance–temperature relation itself, and Steinhart–Hart coefficients, use the NTC thermistor calculator. The ADC resolution calculator covers other sensors on the same converter.
Frequently asked questions
Why not make the series resistor very small for more current?
More current means more self-heating, and a small series resistor moves the swing to one end of the supply. The swing is largest at the geometric mean; departing from it costs resolution.
How accurate is the Beta model?
Close to T₀ it follows a real thermistor well; the further from T₀, the larger the deviation, which over 0 to 100 °C can reach about 1 K. Use Steinhart–Hart or the manufacturer’s R–T table in the firmware for accuracy, and this calculator for the circuit design.
Should I add a capacitor?
A capacitor from the output to ground, close to the ADC, supplies the charge for the ADC’s sampling capacitor and filters noise. Check the settling with the anti-aliasing filter calculator, using the parallel resistance of the divider as R.