NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) resistors, also known as thermistors, are types of resistors whose resistance changes significantly with temperature. The key difference between them lies in how their resistance responds to a change in temperature.
NTC (Negative Temperature Coefficient)
For NTC thermistors, the resistance decreases as the temperature increases. This inverse relationship is their defining characteristic.
Working Principle: NTC thermistors are typically made from semiconductor materials, such as sintered metal oxides like those of manganese, nickel, cobalt, and iron. In these materials, an increase in temperature excites electrons, moving them into a higher energy band where they can move more freely. This increased number of charge carriers leads to a decrease in the material's resistance.
Key Characteristics:
High Sensitivity: NTC thermistors are very sensitive to small changes in temperature.
Non-linear Response: The relationship between resistance and temperature is exponential, not linear.
Common Applications:
Temperature Sensing: Their high sensitivity makes them ideal for precise temperature measurement in a variety of devices, including digital thermometers and automotive systems.
Inrush Current Limiting: When a device is first turned on, there can be a surge of current. An NTC resistor, with its high initial resistance, can limit this surge. As the resistor heats up, its resistance drops, allowing the normal operating current to flow.
Temperature Compensation: They can be used to counteract the effects of temperature changes on other electronic components.
PTC (Positive Temperature Coefficient) Resistors
In contrast to NTC thermistors, the resistance of PTC thermistors increases as the temperature increases.
Working Principle: PTC thermistors are often made from polycrystalline ceramic materials like barium titanate. At a specific temperature, known as the Curie temperature, the material's properties change, causing a sharp increase in resistance.
Key Characteristics:
Switch-like Behavior: They exhibit a sudden, large increase in resistance at a specific temperature.
Self-Regulating: This sharp increase in resistance can limit the current flow, which in turn reduces the temperature, creating a self-regulating effect.
Common Applications:
Overcurrent Protection: PTC thermistors are widely used as resettable fuses. If the current exceeds a certain level, the thermistor heats up, and its resistance increases dramatically, limiting the current. When the fault is cleared and the thermistor cools down, its resistance returns to a low level.
Self-Regulating Heaters: The self-regulating nature of PTC thermistors makes them suitable for applications like glue guns and personal heaters, where they can maintain a constant temperature.
Time Delay Circuits: The time it takes for a PTC thermistor to heat up and increase its resistance can be used to create a time delay in a circuit.
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