Basic Characteristics of PTC Thermistors
Resistance-Temperature Characteristics
The relationship between resistance and temperature is shown in the figure below: the resistance value decreases slightly or remains constant between room temperature and the Curie point; however, once the Curie point is exceeded, the resistance increases sharply. The Curie point (Tc) is defined as the temperature at which the resistance doubles relative to its value at 25°C.
Leveraging this characteristic, it is possible to detect overheating abnormalities in a circuit when the temperature exceeds a specified threshold and to automatically disconnect the circuit. Since the resistance of a PTC thermistor increases sharply with temperature, the temperature of a circuit or a specific area can be simply monitored without the need to convert the temperature data into digital form.
Current–Voltage Characteristics (Static Characteristics)
The relationship between current and voltage is shown in the figure below, illustrating the correlation between the applied voltage across a PTC thermistor and the steady-state current flowing through it under conditions of 25°C in still air. This plot depicts the relationship between applied voltage and steady current when the internal heat generation within the PTC thermistor balances with external heat dissipation. The curve exhibits both a peak current point and a region of constant output power. The maximum current that can pass through the device is also referred to as the tripping current; this value is sufficient to raise the thermistor’s temperature to its Curie point. This current defines the current-limiting range for applications of PTC thermistors as overcurrent protection devices.
Leveraging this characteristic, PTC thermistors can be designed for applications such as:
● Constant-temperature heating and heating applications: no complex or additional control circuitry required;
● Overcurrent protection application: It can prevent excessive current from flowing through other electronic components in the circuit, thereby providing overcurrent protection.
Current–time characteristic (dynamic characteristic)
The current–time characteristics are shown in the figure below, illustrating the relationship between current and time before internal heat generation and external heat dissipation reach equilibrium. A key feature is the initial surge of a large inrush current, followed by a sudden and sustained decay to a lower steady-state current.
Leveraging this characteristic, it can be used for auxiliary starting of refrigeration compressors and for low-voltage auxiliary starting applications.
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