How do you test a thermistor in a circuit?

In one line

The rule

A thermistor is a type of resistor whose resistance changes with temperature.

There is no single formula for testing, but the resistance (R) of a thermistor changes as temperature (T) changes.

Let's understand it simply

Imagine you have a water pipe with a tap. If you turn the tap, water flows more or less. A thermistor is like a tap for electric current, but the handle is temperature. When the thermistor gets hot or cold, it lets more or less current flow. You can test it by seeing how much current flows when you touch it or heat it.

How it works

1. Switch off the circuit and find the thermistor.
2. Use a multimeter (a device that measures resistance) to check its resistance at room temperature.
3. Note the resistance value.
4. Gently heat the thermistor (rub it with your fingers or use a hair dryer).
5. Watch the resistance on the multimeter. For an NTC thermistor, resistance will decrease as it gets warmer. For a PTC thermistor, resistance will increase.
6. Let it cool and see if the resistance returns to the first value.

The story behind it

Thermistors were first made in the 1930s by Samuel Ruben. He wanted a simple way to measure temperature using electricity. The name comes from "thermal" (heat) and "resistor" (something that resists current).

Where you see it in real life

• Electric iron: Thermistor senses when the iron is hot enough and switches it off.
• Refrigerator: It checks if the inside is cold enough.
• Car engines: Monitors temperature to prevent overheating.
• Mobile phones: Stops charging if the battery gets too hot.
• Digital thermometers: Measures body temperature quickly.

Why it matters

• Thermistors protect machines from overheating.
• They help in automatic temperature control.
• Used in many everyday gadgets.
• Understanding them helps in electronics, robotics, and engineering courses.

Common mistakes

• Mixing up NTC (resistance decreases with heat) and PTC (resistance increases with heat) thermistors.
• Not disconnecting power before testing, which can damage the multimeter or circuit.
• Touching the metal parts of the multimeter probes, which can affect the reading.
• Not waiting for the thermistor to cool down before checking if it returns to its original value.

Remember it

Thermistor = "Thermal resistor"; test by checking resistance change with temperature.