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Electronic Parts

Discrete Components

Resistors

Thermistors — Resistance That Changes With Temperature

A thermistor is a resistor made to change resistance significantly with temperature. The name comes from thermal resistor. Thermistors are widely used for temperature sensing, compensation, current limiting, and protection.

NTC and PTC
TypeWhat happens as temperature rises?Common use
NTC — negative temperature coefficientResistance decreases.Temperature sensing, measurement, compensation, inrush-current limiting.
PTC — positive temperature coefficientResistance increases.Overcurrent protection, self-regulating heaters, motor and transformer protection.
The words negative and positive refer to the direction of resistance change with temperature—not to electrical polarity. Most thermistors are two-terminal non-polarized parts.
Thermistor schematic symbols

Two common drawing styles are shown below. One uses the rectangular IEC-style resistor symbol and the other uses the zigzag resistor style. In both cases, the diagonal mark indicates that the resistance changes with temperature.

THERMISTOR — COMMON SCHEMATIC SYMBOLS rectangular resistor style zigzag resistor style temperature-dependent resistor temperature-dependent resistor
The basic thermistor symbol identifies a temperature-dependent resistor. Schematics may add NTC, PTC, a part number, or other marking to tell which temperature characteristic is intended.
A thermistor used as a sensor
NTC THERMISTOR AS A TEMPERATURE SENSOR +5 V NTC temperature-dependent VOUT fixed resistor

The thermistor and fixed resistor form a voltage divider. As temperature changes, the thermistor resistance changes and so does the divider voltage. A comparator, microcontroller ADC, meter circuit, or other analog stage can then interpret that voltage.

Resistance is not usually linear with temperature

A thermistor may be marked with a resistance such as 10 kΩ at 25 °C. That does not mean its resistance changes by the same number of ohms for every degree. Thermistor curves are normally quite non-linear.

For an NTC thermistor: temperature ↑   →   resistance ↓
Inrush-current limiting

A power NTC thermistor can also be placed in series with a power supply. When cold, it has relatively high resistance and limits the initial charging current into large capacitors. As current heats it, its resistance falls and normal operation continues with less voltage drop.

A thermistor used for inrush limiting can become very hot during normal operation. Its hot resistance, surge-energy rating, cooling time, spacing, and fault conditions must all be considered.
Self-heating

A sensing thermistor can warm itself because current through it produces heat. That can make it measure its own heating rather than only the surrounding temperature. Temperature-measurement circuits therefore usually keep sensing current small.

Next: Photoresistors (LDRs).