Sensors & isolation
Thermistor
A resistor whose resistance changes a lot with temperature. NTC thermistors drop in resistance as they warm up and are cheap, accurate temperature sensors; PTC types rise sharply and are used as resettable fuses.
Pins
- Either end
- : No polarity. For sensing, put it in a voltage divider with a fixed resistor of the same value.
NTCs come as tiny glass or epoxy beads (fast, used in probes) or discs printed with their value: "103" = 10 kΩ at 25 °C.
What to look for inside
- A 10 kΩ NTC thermistor in a divider across 5 V while its temperature rises from 0 °C to 80 °C and back.
- In an NTC, warming up frees more charge carriers in the material, so resistance falls and more current flows (faster electrons).
- The microcontroller doesn’t see the current directly: it reads the divider voltage and converts it to a temperature.
The numbers that matter
| Spec | Typical |
|---|---|
R25 Resistance at 25 °C. | 10 kΩ (most common) |
B value How steep the curve is. You need it to calculate the temperature. | 3380 to 4250 K |
Tolerance Sets how accurate the reading can be. | ±1 % to ±5 % |
Where you’ll use it
- Temperature sensing in 3D printers, chargers, thermostats and battery packs.
- Limiting the switch-on current surge of power supplies (big NTCs).
- Resettable fuses (PTC).
Common mistakes
- Passing a large measuring current through it. It warms itself up and reads high.
- Treating the curve as a straight line. Use the B-equation or a lookup table.
Lessons that use it
No lesson measures temperature yet. Lesson 5.2 uses a PTC thermistor as a resettable fuse.
Go deeper
The B-parameter model (T in kelvin).
Turned round to get temperature from a measured resistance.