What is the difference between NTC and PTC thermistors?

In one line

The rule

NTC (Negative Temperature Coefficient) thermistors decrease their resistance as temperature rises, while PTC (Positive Temperature Coefficient) thermistors increase their resistance as temperature rises.

Let's understand it simply

Imagine you are riding your cycle up a hill. For NTC thermistors, as the day gets hotter (temperature rises), the hill becomes less steep (resistance drops), so it gets easier to pedal. For PTC thermistors, as the day gets hotter, the hill gets steeper (resistance rises), so it gets harder to pedal. So, NTC gets easier with heat, PTC gets harder.

How it works

1. Both NTC and PTC are made from special materials that react to heat.
2. NTC thermistors: When they get hot, their atoms move more, letting electric current flow more easily (resistance drops).
3. PTC thermistors: When they get hot, their structure changes so it is harder for current to pass (resistance increases).
4. This change is used in circuits to sense or control temperature.

The story behind it

Thermistors were developed in the 1930s when scientists wanted a cheap, simple way to measure temperature using electricity. The word "thermistor" comes from "thermal resistor." They are now used in many devices, from geysers to fridges.

Where you see it in real life

• NTC thermistor in a geyser: Senses water temperature, tells the heater when to switch off.
• NTC in a mobile phone battery: Stops the battery from overheating by cutting off charging if it gets too hot.
• PTC thermistor in a fan heater: Acts as a safety switch—if the heater gets too hot, resistance rises and current drops, preventing overheating.
• PTC in tube lights (starter): Helps start the light, then increases resistance to stop the starter circuit.

Why it matters

• Used in temperature sensors in cars, ACs, and fridges.
• Protects devices from overheating (safety feature).
• Helps control circuits that need to react to temperature changes.
• Questions about NTC and PTC thermistors come in board exams and competitive exams like JEE.

Common mistakes

• Mixing up which one increases or decreases resistance with heat: Remember, NTC = Negative = resistance goes Down.
• Thinking all thermistors behave the same: NTC and PTC are opposites.
• Forgetting that both are used for different purposes in circuits.

Remember it

NTC = "Negative" = resistance goes Down with heat. PTC = "Positive" = resistance goes Up with heat.