Designing a Voltage Divider Circuit with an NTC Thermistor: A Step-by-Step Guide
An NTC thermistor by itself doesn't output a voltage a microcontroller can read — it just changes resistance. Turning that resistance change into a signal an ADC (analog-to-digital converter) can actually measure is almost always done with a simple voltage divider circuit: one fixed resistor, one thermistor, and a couple of design decisions that determine how accurate and linear your temperature readings will be.
This guide walks through building that circuit from scratch, the math behind it, and the mistakes that quietly hurt accuracy in an otherwise correct design.
The Basic Circuit
A thermistor voltage divider is built from two components in series between your supply voltage and ground:
- A fixed reference resistor (Rfixed)
- The NTC thermistor (Rthermistor)
The junction between them — the midpoint of the divider — connects to your ADC input. As the thermistor's resistance changes with temperature, the voltage at that midpoint changes too, and your microcontroller reads that voltage to calculate temperature.
There are two common arrangements:
Thermistor on top (VCC → Thermistor → ADC → Rfixed → GND) — Output voltage increases as temperature increases (since NTC resistance drops, less voltage is dropped across it).
Thermistor on bottom (VCC → Rfixed → ADC → Thermistor → GND) — Output voltage decreases as temperature increases.
Neither arrangement is inherently better — the choice usually comes down to which polarity is more convenient for your firmware or matches convention used elsewhere in your codebase.
The Voltage Divider Formula
For the thermistor-on-bottom configuration:
V_out = VCC × (R_thermistor / (R_fixed + R_thermistor))
For the thermistor-on-top configuration:
V_out = VCC × (R_fixed / (R_fixed + R_thermistor))
Once you measure V_out through the ADC, you rearrange the formula to solve for R_thermistor, and then use the thermistor's resistance-temperature relationship (typically the Beta equation or a Steinhart-Hart equation) to convert that resistance into an actual temperature value.
Step 1: Choose Your Fixed Resistor Value
This is the single most important design decision in the whole circuit, and it's the one most often done carelessly. The goal is to choose Rfixed so that the voltage divider is most sensitive across the temperature range you actually care about.
A common starting rule of thumb: set Rfixed equal to the thermistor's resistance at the midpoint of your expected temperature range. For example, if you're measuring 0°C to 50°C and the thermistor's resistance at 25°C (the midpoint) is 10kΩ, start with a 10kΩ fixed resistor.
This centers the divider's most sensitive region — where V_out changes most per degree — right where you need the most resolution, rather than off at one extreme of your range.
Step 2: Check ADC Resolution Across Your Range
Once Rfixed is chosen, calculate V_out at both ends of your expected temperature range and confirm the ADC has enough resolution to distinguish meaningful temperature steps. A 10-bit ADC on a 3.3V reference gives roughly 3.2mV per step — if your divider only swings 200mV across your entire temperature range, you're only getting a fraction of the ADC's available resolution, and your temperature readings will be coarse and steppy.
If resolution is too coarse, either:
- Adjust Rfixed to increase the voltage swing across your range, or
- Use a higher-resolution ADC, or
- Add an op-amp stage to amplify the divider output before the ADC (common in precision applications)
Step 3: Account for Self-Heating
Current flowing through the thermistor generates a small amount of heat inside the component itself — self-heating — which can introduce measurement error, especially at low-power, low-resistance configurations where more current flows through the divider.
To minimize this:
- Use a higher-value fixed resistor where possible to limit current
- Only power the divider when taking a reading, rather than leaving it continuously energized (common in battery-powered designs)
- Check the thermistor's datasheet for its dissipation constant, which tells you how much self-heating to expect per milliwatt of power dissipated
Step 4: Convert Resistance to Temperature
Once you have Rthermistor from the voltage reading, convert it to temperature using either:
The Beta equation (simpler, adequate for many applications):
1/T = 1/T0 + (1/B) × ln(R/R0)
Where T0 is 298.15K (25°C), R0 is the thermistor's resistance at 25°C, and B is the beta value from the datasheet.
The Steinhart-Hart equation (more accurate across wider temperature ranges, uses three calibration constants specific to the thermistor):
1/T = A + B×ln(R) + C×(ln(R))³
For most appliance, HVAC, and general industrial applications, the Beta equation is accurate enough. Steinhart-Hart is worth the added complexity mainly in precision applications spanning a wide temperature range, where Beta-equation error becomes noticeable at the extremes.
Common Design Mistakes to Avoid
- Choosing Rfixed based on convenience (a resistor already in the BOM) rather than matching it to the thermistor's midpoint resistance — this quietly reduces sensitivity across your actual operating range
- Ignoring self-heating error in low-power or battery-sensitive designs, especially with low-resistance thermistors
- Using the Beta equation across a very wide temperature range where Steinhart-Hart would meaningfully reduce error
- Not accounting for tolerance — a thermistor's stated resistance and beta value both carry tolerance (see our guide on thermistor tolerance and resistance grades), which introduces some irreducible error unless calibrated per unit
- Skipping a pull-up/pull-down verification — confirming the ADC pin's input impedance is high enough not to load down the divider and skew the reading
Getting the Right Thermistor for Your Circuit
A well-designed voltage divider still depends on starting with a thermistor whose R25 value, beta value, and tolerance are well matched to your circuit and application. Zentriad supplies NTC thermistors with verified resistance and beta specifications, and our team can help recommend the right R25 value to pair with your chosen fixed resistor for maximum sensitivity across your target temperature range.
Need help selecting a thermistor to match your circuit design? Contact Zentriad's engineering team for datasheets or design support.