What Is a Positive Temperature Coefficient (PTC) Thermistor?
A PTC thermistor is an electronic component whose resistance value increases sharply at a certain temperature.
PTC stands for positive temperature coefficient, meaning that there is a positive correlation between temperature and resistance. Thermistor is derived from the term “Thermal Sensitive Resistor”. A thermistor is a component that can measure temperature from the difference in electrical resistance when a resistive element is brought into contact with the object to be measured. It is characterized by the use of a metal oxide semiconductor as the resistive element.
An overcurrent protection device using a positive temperature coefficient (PTC) thermistor is also called a resettable fuse. Tube and blade fuses use alloy elements that need to be replaced once they blow, so the principle is different from that of common fuses.
When an excessive current flows in a circuit, a resettable fuse limits the current value by causing the positive temperature coefficient (PTC) thermistor to become hotter and its resistance to increase. When the cause of the excessive current is removed, the positive temperature coefficient (PTC) thermistor decreases and its resistance becomes lower. Therefore, it is used as a resettable fuse.
Uses of Positive Temperature Coefficient (PTC) Thermistors
PTC positive temperature coefficients (PTC) thermistors are used in electronic circuit boards and a wide range of other applications, from home appliances to industrial applications. The following are examples of positive temperature coefficient (PTC) thermistor applications:
- Overcurrent and overload protection for smartphones, etc.
- Overload protection for small motors
- Electric carpets and heaters
- Overload protection for motors and power semiconductors
PTC positive temperature coefficient (PTC) thermistors have a low resistance value when the temperature is low, but when overheated due to excessive current, the resistance value increases rapidly. This mechanism allows them to be used for overcurrent protection in electrical circuits. Circuits in which PTC thermistors are used include heater circuits and electronic communication devices.
Protection against overheating must also be considered when using power semiconductors and motors. This is where positive temperature coefficients (PTC) thermistors are used. To protect against overheating, a positive temperature coefficient (PTC) thermistor is used by physically thermally coupling the PTC thermistor to a heat sink or motor winding for heat removal.
The positive temperature coefficient (PTC) thermistor is used to detect overheating by increasing its resistance, and the circuit is configured to stop operation or shut off the power supply.
Principle of Positive Temperature Coefficient (PTC) Thermistors
PTC thermistors have three major characteristics:
1. Resistance Temperature Characteristics
The resistance value is almost constant from room temperature to the Curie point (temperature at which the resistance value is about twice that of room temperature), and increases rapidly after the Curie point. This property allows the device to detect overheating of an electronic circuit quickly and limits the circuit current, thereby preventing failure of electronic components.
2. Static Characteristics (Voltage-Current Characteristics)
Until the Curie point is exceeded, the current rises in proportion to the rise in voltage. Once the Curie point is exceeded, the resistance of the thermistor itself increases, and the current value decreases. In other words, a constant power is maintained. The voltage-current characteristic is characterized by a mountain-like graph.
3. Dynamic Characteristics (Current-Time Characteristics)
The current-time characteristic of a PTC thermistor is characterized by the fact that the current value decreases over time. This characteristic allows a large current to flow immediately after the power is turned on, and is used for motor startup and other applications.
Types of Positive Temperature Coefficient (PTC) Thermistors
PTC thermistors can be classified into two types: polymer-based and ceramic-based.
1. Polymer Type
Polymer-based PTC thermistors are manufactured by mixing conductive particles with polymer materials, such as polyethylene. Carbon black and nickel are used as conductive particles.
They are also known by the acronym PPTC. Resettable fuses are also called poly switches, but the name is derived from the polymer. The positive temperature coefficient (PTC) thermistor is used only for overcurrent protection.
2. Ceramic-Based
PTC thermistors are positive temperature coefficient (PTC) thermistors made of ceramic with a small amount of rare earth elements added to barium titanate. Curie temperature can be adjusted by adjusting the amount and type of additives. They are used not only for overcurrent protection but also for demagnetization circuits and self-regulating heating elements.