NTC/PTC Thermistor Temperature Tool
Convert thermistor resistance to temperature, check a voltage divider, or decode an ADC reading. Use a Beta approximation for an NTC, or your device’s resistance–temperature table for an NTC or PTC sensor.
Two sensors rated at the same resistance can have different temperature characteristics. Start with the exact part number, curve data, and measurement conditions.
Calculate a sensor operating point
Choose your model and the value you know. Temperature means sensor-body temperature; the tool does not automatically correct a reading to ambient temperature.
Temperature & circuit results
Enter your sensor data to calculate.
Measurement details
View calculated lookup table
| °C | Ω | Vout | ADC code |
|---|
Resolution is not accuracy
The displayed °C/LSB is a local ideal estimate. Sensor tolerance, curve fit, resistor tolerance, ADC errors, wiring, leakage, self-heating, and thermal contact are not combined into an accuracy specification.
NTC and PTC need different models
Beta for a first estimate
The Beta equation approximates an NTC curve using a reference resistance and temperature. Use the specified Beta interval; a single coefficient is not a precision fit for every temperature. For tighter work, use part-specific tables or a validated Steinhart–Hart fit. Vishay model guidance.
Use the sensor’s own curve
“PTC” does not define one universal equation. Silicon sensing devices and switching ceramics behave differently. A ceramic PTC curve may reverse direction outside its rising region; that is why this tool accepts only a strictly monotonic segment. TDK PTC characteristics.
Keep curve and circuit aligned
Some silicon PTC sensors are polarized and bias-dependent. TI’s TMP61, for example, requires positive terminal polarity and an R–T table calculated for its actual bias conditions. Do not reuse that table unchanged after changing the bias. TMP61 datasheet.
Check the divider connection
The same thermistor produces opposite voltage trends when moved from the bottom to the top of a divider.
Balance sensitivity and self-heating
- A fixed resistor close to the thermistor resistance at one target temperature maximizes local voltage sensitivity for a fixed supply.
- Higher divider resistance reduces excitation current, but increases source impedance. Check the ADC’s acquisition time and input leakage.
- The tool reports thermistor dissipation, the source resistance Rfixed ∥ Rtherm, and local voltage sensitivity.
- Self-heating rise is estimated from power divided by the dissipation factor. Mounting and the surrounding medium affect that factor. Vishay selection guidance.
A rail reading can mean an extreme temperature, open circuit, short circuit, incorrect bias, or ADC saturation. Check the circuit before interpreting it.
Equations and calculation limits
Resistance is in ohms, temperature is in degrees Celsius unless marked K, and logarithms below are natural logarithms.
NTC Beta conversion
R = R₀ × exp[B × (1/Tᴋ − 1/T₀ᴋ)]
T°C = 1 / [1/T₀ᴋ + ln(R/R₀)/B] − 273.15
dR/dT = −B × R / Tᴋ²
Positive B is used only for an NTC. Readings outside the entered temperature window are rejected. Input limits prevent extreme numerical values; they do not qualify any physical sensor.
R–T table interpolation
T(R) = T₁ + (T₂ − T₁) × (R − R₁)/(R₂ − R₁)
Adjacent points are interpolated linearly in resistance, not log-resistance. Duplicate temperatures, flat resistance segments, reversals, and out-of-range readings are rejected. Use sufficiently dense data where the curve bends sharply. The plot’s logarithmic axis does not change the interpolation method.
Divider and thermal estimates
Top sensor: Vout = Vₛ × Rfixed / (Rfixed + R)
I = Vₛ / (Rfixed + R)
Ptherm = I² × R
Estimated rise = Ptherm(mW) / δ(mW/°C)
Power is evaluated at the calculated sensor resistance. This is not a coupled thermal equilibrium or warm-up simulation. For resistance-only measurements, the divider outputs describe the entered circuit, not the meter’s unknown excitation.
Explicit ideal ADC convention
Code = clamp(floor(Vout / LSB), 0, N − 1)
Decode center: Vout ≈ (code + 0.5) × LSB
Local °C/LSB ≈ LSB / |dVout/dT|
For a non-rail code k, the modeled voltage bin is [k × LSB, (k + 1) × LSB). The temperature envelope is shown only if both bin boundaries lie within the supported circuit and curve range. Real ADC offset, gain, transition placement, noise, and nonlinearities require the device’s transfer function.
Thermistor measurement questions
Why does a 10 kΩ sensor give the wrong temperature?
The nominal resistance alone does not identify the curve. Confirm NTC versus PTC, reference temperature, Beta interval or table, units, and divider orientation. A missing ordering suffix can identify a different resistance tolerance, curve, or assembly.
Can I use a resettable fuse as a precision temperature sensor?
This tool is for a specified, single-valued sensing curve. It does not model fuse trip, switching hysteresis, inrush limiting, or heater behavior. Those applications require device-specific electrical and thermal characteristics, not just an R–T conversion.
Does the table mode support a Steinhart–Hart sensor?
It can use resistance–temperature points produced by a validated Steinhart–Hart fit or supplied by the manufacturer. It does not fit coefficients or evaluate a Steinhart–Hart equation directly. Use enough points to keep interpolation error within your measurement budget.
Should the ADC reference equal the divider supply?
If they share the same actual source, the ideal divider ratio is ratiometric and common supply variation cancels in the code ratio. Equal nominal values from two independent sources do not provide that cancellation. Reference noise, settling, and sensor bias dependence still need review.
Why is the decoded temperature slightly different?
A code identifies a voltage interval, not an exact voltage. This tool decodes its center. The resulting temperature need not equal the temperature that generated the code. The displayed bin envelope excludes sensor and circuit tolerances and is not a total uncertainty band.
Can I reduce heating by pulsing the divider?
Possibly, but allow enough electrical settling before sampling and check the thermal time constants. The continuous-power estimate here is not an average-power or pulsed thermal model. Do not divide its predicted rise by duty cycle without validating the thermal behavior.
Continue your sensing design
Need the exact thermistor or sensing IC?
Send the part number or BOM, resistance and curve requirements, package, tolerance, quantity, and required date. YURUNOX can review the sourcing requirement and available supply options.
Technical references
- Vishay — Selecting NTC Thermistors, document 33001: Beta and Steinhart–Hart models, measurement power, and dissipation factor.
- Texas Instruments — TMP61 datasheet: silicon PTC behavior, polarity, and bias-dependent R–T data.
- TDK / EPCOS — PTC thermistors, general technical information: zero-power curves, switching behavior, and self-heating.