NTC Thermistor Divider

Formula

Description

The simplest NTC thermistor interface circuit is a voltage divider with a fixed resistor. As temperature increases, the NTC resistance decreases, changing the output voltage. The fixed resistor value is chosen to maximize sensitivity at the temperature range of interest, typically equal to the NTC resistance at the center of the desired measurement range. The output voltage is then measured by an ADC and converted to temperature using the Steinhart-Hart equation or a lookup table. This circuit provides a low-cost temperature measurement solution with good accuracy.

Variables

• Vout — Voltage at the divider output (V)
• Vcc — Supply voltage (V)
• Rntc — NTC thermistor resistance at current temperature (Ω)
• Rfixed — Fixed series resistor (Ω)

Practical Notes

Place the NTC in the upper position (connected to Vcc) for a positive temperature coefficient of voltage, or in the lower position (connected to ground) for negative. Choose Rfixed = Rntc at the midpoint of the measurement range for best linearity. Self-heating must be controlled by limiting current through the thermistor. A 10kΩ NTC at 25°C with 3.3V supply and 10kΩ fixed resistor dissipates only 0.27 mW, causing negligible self-heating.