Several Common Applications of PTC Thermistors
Release Date:
2022-07-26
A PTC thermistor is a type of positive temperature coefficient thermistor, meaning its resistance increases as the temperature rises. These resistors are typically made from barium titanate and, depending on the manufacturing process and application requirements, are formed into components with specific geometries. We often leverage three key characteristics: the resistance–temperature relationship, the voltage–current relationship, and the current–time relationship. The differences in these characteristics stem from variations in packaging, which are tailored to suit different operating conditions.
A PTC thermistor is a type of positive temperature coefficient thermistor, meaning its resistance increases as the temperature rises. These resistors are typically made from barium titanate and, depending on the manufacturing process and application requirements, are formed into components with specific geometries. We often leverage three key characteristics: the resistance–temperature relationship, the voltage–current relationship, and the current–time relationship. The differences in these characteristics stem from variations in packaging, which are tailored to suit different operating conditions.
1. Overvoltage, overcurrent, and overload protection
PTC-based protection against overvoltage, overcurrent, and overload is widely employed. When a PTC thermistor is connected in series within a circuit, it can limit excessive current—whether due to a short circuit or other causes—and even completely block the current, thereby protecting the associated components or circuitry. Its primary applications include overcurrent and overheating protection in power transformers, various chargers, and a wide range of instruments and meters. This represents the most common and extensively observed application of PTC thermistors, particularly in power equipment.
A PTC thermistor is connected in series within the circuit as an overcurrent protection device, thereby protecting the transformer. When a short circuit occurs in a component or load for various reasons, resulting in excessive current, the PTC thermistor comes into play to limit the current and prevent it from becoming too high. It is important to note that when selecting the maximum non‑reactive current and operating voltage, factors such as the rated zero‑power resistance, the operating temperature range, and the component package must also be taken into account. As a general guideline, the maximum operating current should be set at approximately 1.5 times the normal operating current of the circuit.
2. Starting assistance for motors, drives, compressors, etc.
When an electric motor is started, it must overcome its own inertia. For example, in single-phase starting, a relatively large starting torque is required. Once the motor reaches normal operating speed, the required torque is much smaller; at this point, an auxiliary circuit can be connected to the motor—this can be regarded as an auxiliary power source. Analogously, just as an electric vehicle requires greater power when climbing a slope, a PTC thermistor can be connected in series to form such an auxiliary circuit.
For example, in a refrigerator compressor, the resistance of the start resistor increases upon startup, energizing the start winding while the main winding continues to operate. As the resistance rises significantly, the start winding effectively opens, allowing the compressor to run normally.
PTC thermistors are used not only in switch-mode power supplies but also in many other applications.
3. Demagnetization applications for color televisions, cathode-ray tubes, and similar devices
The function of a PTC thermistor is to eliminate the influence of spurious magnetic fields, such as color distortion and geomagnetic interference, on the color picture. In conventional color televisions, the automatic degaussing circuit typically consists of a degaussing coil and a degaussing resistor. The PTC degaussing element operates based on its current–time characteristics.
After the demagnetization circuit is connected in series, the initial resistance of the PTC element is low, resulting in a large initial current through the demagnetizing coil and generating a strong alternating magnetic field. As the PTC demagnetizing element heats up, its resistance increases, causing the current in the circuit to gradually decrease and the alternating magnetic field in the coil to weaken, thereby achieving the purpose of demagnetization. This product series is used in the demagnetization circuits of color televisions, color CRT monitors, and other CRT devices.
In addition to the three functions mentioned above, PTC thermistors also offer a variety of other applications, including temperature sensing and control, motor overheating protection, constant-temperature maintenance for soldering irons, filament preheating with delayed start, and constant-current compensation for LEDs.
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