LVR polymer positive temperature coefficient (PPTC) component application skills

*Protect linear transformer

Linear transformers are widely used in power conversion devices such as household appliances, building automation systems, portable electronic devices, rechargeable power tools, telephone answering machines, travel power supplies, cordless phones, and wireless phones. The power of these devices is often considered very reliable by the end user, and failure of these power supplies can result in prolonged work stoppages or need to be replaced.

*LVR device series can provide over current protection

Short-circuiting the secondary side of the transformer is a common fault in electronic circuit systems. This type of failure may be caused by an internal fault in the equipment or by a short circuit in the connected load. Such failures will cause an increase in the current in the secondary and primary coils, and the temperature of the coil will rise, which may result in a decrease in the insulation level and the occurrence of a fault. When accessing the line at the primary side of the transformer, the LVR device responds to the increased fault current and transitions to a high impedance state at the specified operating point, providing overcurrent protection for the system.

This limits the current at the primary side and also limits the secondary current. The secondary side electronics are protected from overcurrent damage. The LVR device remains in a high-impedance state until the fault is cleared and the device cools back to a low-impedance state. In most cases, in order for the LVR device to be reset, it may be necessary to clear the fault condition and disconnect the power to the device. Once the LVR device is reset, the device can resume normal device operation.

+LVR device can sense overheating

No exception. The LVR device is the same as a normal PolySwitch device. When the internal temperature rises to a certain level, the action also occurs, regardless of the magnitude of the current passed. This feature allows LVR devices to be used for both over-temperature protection and over-current protection. The overheating sensing element of the LVR device is particularly useful for situations where the current does not rise significantly and the fault causes the coil to heat up. By replacing the LVR device with a thermal fuse that is typically wrapped in a transformer, the dual protection capability of the LVR product can be fully utilized.

Take the following faults as an example for specific analysis:

One is the drawback of traditional thermal fuse protection methods. If the neutral connection is loose, the 240Vac voltage is applied to the 120Vac main circuit.

If in this state, the current added at the primary side of the transformer may not exceed the operating current value of the current protection device. This is the general case F in addition to the addition of an overcurrent protection device at the secondary side of the transformer, the reason for installing a thermal fuse at the primary side of the transformer. The thermal fuse senses the temperature rise of the transformer and stops the flow of current by opening the circuit when the temperature reaches a certain value. Although this can prevent the transformer from malfunctioning, it is required to replace the transformer component or its circuit board after the loose midline is repaired. This means that the equipment is out of service, repaired or replaced.

The second is to use LVR. The device replaces this thermal fuse. LVR devices are typically small enough to fit into the transformer in the same way as thermal fuses. Heating the transformer will cause the LVR device temperature to rise accordingly, causing the LVR device to operate. After the action, the current drops, preventing the transformer temperature from continuing to rise. Moreover, the LVR device not only operates due to overcurrent, but also may operate when the transformer is overheated. In an overcurrent fault condition, the LVR device will remain in a high impedance state until it is removed and the power supply is disconnected. The LVR device cools down. Once the LVR device is reset, the device will resume normal operation. Importantly, the LVR device not only helps protect the transformer and secondary electronics from overcurrent and overheating, it is also a resettable device. As a result, the equipment it protects usually does not require maintenance to resume operation. This is not possible with a single-purpose fuse protection solution. Therefore, if the device uses an LVR device, it can eliminate or reduce the high cost of maintenance and equipment outage, thereby saving the time and cost of the device user.

+ It should be noted that the ceramic positive temperature coefficient (CPTC) device is also a resettable protection scheme for transformers. But less than the PPTC of the LVR device family. Why is this? In the above project applications, the PVR of the LVR device series can limit the maximum temperature of the coil to a lower level and provide a lower surface temperature in the operating state than the ceramic positive temperature coefficient (CPTC) device. °C~120°C). Depending on the applied voltage, the surface temperature of the ceramic positive temperature coefficient device in the high resistance state can reach 180qc to 220 °C, which is not suitable for the overheat protection scheme of the coil insulation rating lower than the surface temperature of the ceramic positive temperature coefficient device. The ceramic positive temperature coefficient device is also more fragile, making it susceptible to damage due to shock, vibration, or thermal stresses caused by repeated heat and cooling in many transformer applications. LVR devices typically have lower resistance values ​​in the circuit and their capacitance values ​​are much lower and are less susceptible to frequency. Moreover, the volume is also smaller than the ceramic positive temperature coefficient device with a continuous current rating. The special features of LVR devices make it the preferred solution for overcurrent and overtemperature protection at the primary side of a linear transformer.