1、 Define core function – automatic current limiting and overheating protection
What is the function of PTC thermistor? Its core function lies in the intelligent protection feature brought by the “positive temperature coefficient effect of resistance”. Unlike common NTC (Negative Temperature Coefficient) thermistors, PTC thermistors exhibit lower resistance values below a specific temperature point (Curie temperature point); Once the temperature rises above the critical point or the current flowing through it becomes too large, causing it to heat up, its resistance value will sharply increase by several orders of magnitude. This unique physical characteristic makes PTC thermistors mainly function as “self recovering overcurrent protection” and “overheating protection”. Understanding the function of PTC thermistor is the basis for selecting and applying it.
2、 Analysis of Mechanism of Action – Resistance Mutation for Protection
The implementation of PTC thermistor relies on its special polycrystalline ceramic material (usually barium titanate based) inside. Under normal working conditions (low temperature or low current), the material has a low grain boundary barrier and low resistance value, which has a slight impact on the circuit. But when abnormal situations occur, such as circuit short circuits or equipment overload, resulting in excessive current flowing through the PTC thermistor or abnormal increase in ambient temperature, its own temperature rapidly rises. Once the “switching temperature” of the material is reached, significant changes occur at the grain boundaries, causing a sudden increase in the potential barrier and resulting in a sudden surge in resistance, thereby limiting the fault current to an extremely low level, as if automatically “cutting off” the circuit. This is precisely the key mechanism of the “PTC thermistor effect” – using the sharp increase in resistance value to limit excessive current.
3、 Widely applicable scenarios driven by core functions
What is the function of PTC thermistor? In practical applications, its core functions of “current limiting protection” and “temperature sensing/control” are widely utilized:
*Overcurrent/Short Circuit Protection: This is the most common application of PTC thermistors. Connected in series in power input, motor winding, speaker coil, transformer, or electronic circuit, it can quickly respond in case of short circuit or severe overload, limit current, and protect subsequent expensive electronic components from burning out. For example, it is essential in protecting mobile phone chargers, computer power supplies, and small household appliance motors.
*Overheating protection/temperature sensing: Installed on the protected equipment (such as motor casing, battery pack, power device heat sink), when the equipment temperature is too high, the PTC thermistor resistance increases sharply, which can trigger the control circuit to cut off power or issue an alarm. It is also commonly used for constant temperature control of hair dryers and curling irons.
*Demagnetization circuit (TV/monitor): In the demagnetization coil circuit of early CRT monitors, PTC thermistors were used to initially demagnetize with low resistance and high current, followed by an increase in self heating resistance and automatic reduction of current.
*Delayed start: When the air conditioning compressor is started, the PTC thermistor is used to initially conduct the cold low resistance starting winding. After starting, the heating resistance value increases and the starting winding is automatically cut off.
4、 Unique advantages and selection points
To understand the function of PTC thermistor, it is necessary to recognize its core advantage: “self-healing”. After troubleshooting and temperature drop, its resistance value will automatically return to a low resistance state, without the need to replace it like a fuse, greatly improving the reliability and maintenance convenience of the equipment. Of course, to achieve the ideal “PTC thermistor effect”, selection is crucial, and attention should be paid to its rated voltage, holding current (IH), operating current (IT), maximum current (IMax), switch temperature (Ts), maximum resistance (Rmax), and packaging size parameters to ensure that it matches the characteristics of the protection circuit.
