Practical case: Use of NTC for temperature sensor

Materials

• 1 × NTC thermistor, 10 kΩ at 25°C (NTC1)
• 1 × Fixed resistor, 10 kΩ, 1% (R1)
• 1 × Breadboard and jumper wires
• 1 × 5 V source (USB 5 V from a microcontroller board or bench supply)
• 1 × Digital multimeter (DMM) for voltage measurement
• Optional: 1 × Microcontroller board (e.g., Arduino) to read A0
• Optional: Thermometer, cup with ice water, warm water, plastic bag (to keep NTC dry)

Wiring guide

• Place R1 and NTC1 in series on the breadboard.
• Connect the top of R1 to +5 V.
• Connect the bottom of NTC1 to GND.
• Join the bottom of R1 and the top of NTC1 together; this mid‑point is the sensor output node.
• Connect the DMM positive probe to the output node and the DMM negative probe to GND to read Vout.
• Optional: Connect the output node to the microcontroller analog input A0 and GND to GND.
• Abbreviations used below:
• Vout: voltage at the divider mid‑point relative to GND (measure with DMM).
• A0: microcontroller analog input pin connected to Vout.

Schematic

                 +5 V
                  │
                  ● V_5V
                  │
                ┌─┴─┐
                │   │     R1 = 10 kΩ (resistor fijo)
                │   │
                └─┬─┘
                  │────● V_NTC ───────────────> medir con DMM (V)
                ┌─┴─┐
                │   │     NTC = 10 kΩ @25°C (termistor)
                │   │
                └─┬─┘
                  │
                  ● GND
                  │
                 GND

Measurements and tests

• Baseline (room temperature):

  • Set DMM to DC volts. Measure Vout (●) to GND.
  • With R1 = 10 kΩ and NTC1 ≈ 10 kΩ at 25°C, expect Vout ≈ 5 V × 10k / (10k + 10k) ≈ 2.5 V.

• Warm test (NTC warmed by fingers or warm water):

  • Warm NTC1 gently (hold between fingers or put the bead—sealed in a small plastic bag—into warm water).
  • Expect NTC resistance to decrease, so Vout decreases below 2.5 V.

• Cold test (ice water):

  • Cool NTC1 (bagged) in ice water.
  • Expect NTC resistance to increase, so Vout rises above 2.5 V.

• Calculate R_NTC from Vout:

  • Use R_NTC = R1 × Vout / (5 V − Vout). Example: if Vout = 3.3 V, R_NTC ≈ 10k × 3.3 / (5 − 3.3) ≈ 19.4 kΩ.
  • This lets you map voltage to resistance for basic temperature estimation.

• Optional ADC check (if using A0):

  • Read ADC counts N (0–1023 at 10‑bit). Convert Vout ≈ 5 V × N / 1023.
  • Compare computed Vout to the DMM reading to validate wiring.

Common mistakes

• Misplacing R1 and NTC1 so they are not truly in series; ensure only one mid‑point node exists.
• Using a very different R1 value than the NTC’s 25°C value; sensitivity around room temperature worsens.
• Measuring current with the DMM across the node in current mode; this will short the node. Measure voltage (V) only.
• Floating grounds between the DMM/microcontroller and the circuit; always share GND.

Safety notes

• Keep the thermistor dry when using water; seal it in a plastic bag.
• Avoid hot water or heat guns that exceed the NTC’s rated temperature.
• Do not exceed 5 V if you connect A0; follow your board’s limits.

Improvements and next steps

• Add a 100 nF capacitor from Vout to GND to reduce noise before the ADC.
• Use a 1% resistor (already listed) to reduce measurement error.
• Calibrate: record Vout at two known temperatures (ice bath ~0°C, ambient, and warm water) to build a simple lookup table.
• For better accuracy, apply the Beta or Steinhart–Hart equation using the NTC’s datasheet parameters.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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