What happens inside a thermistor when temperature changes?

In one line

The rule

The resistance of a thermistor changes with temperature, usually decreasing as temperature increases (for NTC thermistors) or increasing as temperature increases (for PTC thermistors).

There is no single formula for all thermistors, but a common one for NTC (Negative Temperature Coefficient) thermistors is:

R = R₀ × exp[B × (1/T - 1/T₀)]

where:

• R = resistance at temperature T (in kelvin)
• R₀ = resistance at reference temperature T₀ (in kelvin)
• B = material constant (in kelvin)
• exp = exponential function

Let's understand it simply

Imagine you are riding your cycle up a hill. When the road is rough (high resistance), it is hard to pedal. When the road is smooth (low resistance), you go faster. A thermistor acts like a road that gets smoother or rougher depending on the weather (temperature).

Inside a thermistor, there are tiny particles (atoms) that make it harder or easier for electric current to pass. When the thermistor gets hotter, these particles move more. In most thermistors (NTC type), this movement actually makes it easier for current to flow — so resistance drops. In some special thermistors (PTC type), the opposite happens: resistance goes up as temperature rises.

How it works

1. Thermistor is made of special materials (like metal oxides) that are sensitive to heat.
2. At low temperatures, the atoms in the thermistor do not move much. Electrons (tiny charges) find it hard to move, so resistance is high.
3. When temperature increases, atoms vibrate more. In NTC thermistors, this vibration helps electrons jump around, so resistance drops.
4. In PTC thermistors, the material is made so that more vibration blocks the path of electrons, so resistance rises.
5. The change in resistance is smooth and predictable, so you can use it to measure temperature.

The story behind it

The word "thermistor" comes from "thermal resistor." The first thermistors were made in the 1930s by Samuel Ruben, an American inventor. He was looking for a way to make batteries better and found that some materials changed resistance with temperature. This discovery led to the use of thermistors in many devices, from radios to modern refrigerators.

Where you see it in real life

• Electric iron: A thermistor senses when the iron is hot enough and helps turn it off to avoid overheating.
• Car engine temperature sensor: It tells the dashboard how hot the engine is, using a thermistor.
• Digital thermometers: The probe you put under your tongue has a thermistor inside.
• Refrigerators: A thermistor checks if the inside is cold enough and controls the cooling.
• Battery chargers: Thermistors prevent batteries from overheating by sensing their temperature.
• Geysers (water heaters): They use thermistors to cut off power when water is hot.

Why it matters

• Safety: Thermistors protect devices from overheating, which can prevent fires.
• Accurate temperature measurement: They are used in medical and scientific tools.
• Automatic control: Many machines use thermistors to work only at safe temperatures.
• Everyday appliances: From your kitchen to your car, thermistors help things run smoothly.

Worked example

Let's say you have an NTC thermistor with R₀ = 10,000 Ω at T₀ = 298 K (25°C), B = 3500 K. What is its resistance at T = 318 K (45°C)?

Given: R₀ = 10,000 Ω, T₀ = 298 K, B = 3500 K, T = 318 K
Step 1: Write the rule   R = R₀ × exp[B × (1/T - 1/T₀)]
Step 2: Calculate (1/T - 1/T₀) = (1/318) - (1/298) ≈ 0.003145 - 0.003356 = -0.000211
Step 3: Multiply by B: 3500 × (-0.000211) ≈ -0.739
Step 4: Find exp(-0.739) ≈ 0.478
Step 5: Calculate R = 10,000 × 0.478 = 4,780 Ω

So, at 45°C, the resistance drops to about 4,780 Ω.

Common mistakes

• Thinking all thermistors behave the same: Some decrease resistance with heat (NTC), some increase (PTC).
• Forgetting to use kelvin: Always use temperature in kelvin (K), not Celsius, in the formula.
• Mixing up resistance and current: Thermistors change resistance, not current directly; current changes as a result.
• Assuming resistance changes instantly: There can be a small delay as the thermistor heats up or cools down.

Remember it

Thermistor = "thermal resistor" — its resistance changes with temperature, like a road getting smoother or rougher as the weather changes.