Tricky Circuit

NTC + R

A circuit feels temperature

Warm a sensor and watch temperature turn into voltage.

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NTC + R · NTC thermistor
NTC — Negative Temperature Coefficient / thermistor. NTC means Negative Temperature Coefficient: resistance falls as temperature rises. A thermistor is a temperature-sensitive resistor. Here it forms a divider with a fixed 10 kΩ resistor.
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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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A circuit feels temperature

Illustrative NTC: 10 kΩ at 25 °C, beta 3950 K. No self-heating or thermal delay.

R(T) = 10000 × exp[3950 × (1/(T + 273.15) − 1/298.15)]. The formula uses absolute temperature in kelvin. Warming reduces the upper resistance, so Vout rises. Real sensors need calibration.

Difficulty

  1. Build · A circuit feels temperature

    Power on at 25 °C. Observe 10 kΩ and 2.50 V.

    Need a hint?

    25 °C → 10 kΩ

  2. Explore · A circuit feels temperature

    Warm the sensor to 50 °C. Compare its resistance with 25 °C.

    Need a hint?

    50 °C → 3.59 kΩ

  3. Solve it · A circuit feels temperature

    Set the sensor to 40 °C and observe an output near 3.27 V.

    Need a hint?

    40 °C → 3.27 V

Extra reading and circuit recipes

A circuit feels temperature

Negative does not mean negative resistance

NTC means Negative Temperature Coefficient: resistance decreases as temperature rises. A thermistor is a temperature-sensitive resistor. Its resistance remains positive; “negative” describes the direction of the change, not a minus sign on its resistance.

Placement changes the direction

Here the sensor is above the fixed resistor, so warming raises the output voltage. If their places were exchanged in a real divider, warming would lower the output. A temperature-dependent resistance alone does not decide whether the output rises or falls.

In this circuit, what happens to output voltage when the sensor warms from 25 °C to 50 °C?

  1. It rises
  2. It falls
  3. It stays at 2.50 V
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The upper sensor resistance falls from 10 kΩ to about 3.59 kΩ. The lower 10 kΩ then gets a larger share of the 5 V supply: output rises from 2.50 V to about 3.68 V.

Discover a curved response

Power on and compare 20 °C, 30 °C and 40 °C. Record output at each setting. The voltage increases, but equal 10-degree steps do not give equal voltage steps: the sensor and divider together make a nonlinear response.