How do you test if a thermistor is NTC or PTC?

In one line

The rule

A thermistor is a type of resistor whose resistance changes with temperature. If its resistance decreases as temperature increases, it is called a Negative Temperature Coefficient (NTC) thermistor. If its resistance increases as temperature increases, it is a Positive Temperature Coefficient (PTC) thermistor.

Let's understand it simply

Imagine you are holding a thermistor in your hand. A thermistor is like a special wire whose resistance (how hard it is for electricity to flow) changes when it gets hot or cold. Some thermistors (NTC) let electricity flow more easily when they get hot. Others (PTC) make it harder for electricity to flow when they get hot. It’s like a water pipe: NTC is like a pipe getting wider with heat, PTC is like a pipe getting narrower with heat.

How it works

1. Take a multimeter (a device that measures resistance).
2. Connect the two wires of the thermistor to the multimeter.
3. Note the resistance value at room temperature.
4. Warm the thermistor gently (use your fingers, a hair dryer, or dip it in warm water).
5. Watch the resistance reading:

• If resistance goes DOWN as it gets warmer, it is NTC.
• If resistance goes UP as it gets warmer, it is PTC.

The story behind it

Thermistors were invented in the 1930s when scientists wanted a simple way to measure temperature using electricity. They discovered that some materials change their resistance a lot with temperature. This property was very useful for making cheap and quick temperature sensors.

Where you see it in real life

• Electric kettles: NTC thermistors sense water temperature to turn off the kettle at the right time.
• Refrigerators: NTC thermistors help control the cooling cycle by sensing inside temperature.
• Iron box (clothes iron): PTC thermistors help keep the iron at the right heat.
• Battery chargers: NTC thermistors protect batteries by sensing overheating.
• Automobile engines: NTC thermistors measure coolant temperature for engine control.

Why it matters

• Helps in making temperature sensors for many machines.
• Used for safety: prevents overheating in devices.
• Needed for precise temperature control in fridges, ACs, and more.
• Questions about NTC and PTC thermistors are common in board exams and entrance tests.

Common mistakes

• Thinking all thermistors behave the same way — remember, NTC and PTC are opposite.
• Not waiting long enough for the resistance to change after heating.
• Using too much heat (like a flame), which can damage the thermistor.

Remember it

NTC = "Negative" means resistance goes DOWN with heat. PTC = "Positive" means resistance goes UP with heat.