What Is a Thermistor (NTC / PTC)?
A thermistor is a resistor whose resistance changes markedly with temperature. It has two main types: NTC and PTC. It's common in temperature-sensing and protection applications.
NTC vs PTC Difference
| Type | Resistance as temperature rises | Typical use |
|---|---|---|
| NTC | Decreases | Temperature sensing, inrush protection |
| PTC | Increases | Overcurrent/overheat protection |
How Does It Work?
The semiconductor material's conductivity changes with temperature. In an NTC, resistance drops as temperature rises; in a PTC it's the opposite, and at a certain temperature the resistance rises sharply.
Where Is It Used?
NTC: temperature sensing with a voltage divider and limiting the inrush current in power supplies. PTC: self-limiting heaters and overcurrent protection (like a resettable fuse).
Beta (B) Value and Linearity
An NTC thermistor's temperature–resistance curve is defined by its Beta (B) value. A microcontroller converts the measured resistance into a real temperature using this value and an equation (e.g. Steinhart-Hart); because the curve is not linear.
Common Mistakes
- Confusing NTC/PTC: the reversed curve makes the circuit behave differently than expected.
- Assuming it's linear: the thermistor curve isn't linear; it's corrected with a table/equation.
- Forgetting self-heating: if too much current flows through it, the thermistor heats itself and corrupts the reading.
Frequently Asked Questions
Where is an NTC used?+
Does a PTC act like a fuse?+
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