Light-emitting diodes ( LEDs ) are fragile devices that are vulnerable to heat, mechanical shock, electrostatic discharge, and lightning, especially when used outdoors. Due to the increased use of LED strings for lighting and backlighting, R&D engineers are required to pay more attention to the reliability of LED strings. High-brightness LEDs, due to their sapphire substrate, are very sensitive to voltage transients caused by nearby lightning strikes. Even in home applications, LED strings still require electrostatic discharge (ESD) protection to ensure long-term, reliable operation of the entire assembly. In the absence of such protection, if one of the LEDs in the series fails and the circuit is disconnected, all other LEDs will be turned off.
LED lighting system protection
Many protection devices are available for power and LED drives , and there are many publications that can guide you through the selection. The circuit in Figure 1 is an example demonstrating switching power supply protection in an LED street lighting system. In this circuit, the AC fuse provides basic fire protection to deal with major system failures that may cause overcurrent conditions, but must be able to tolerate surges from 3KA to 6KA without breaking. In the event of a failure of downstream components, DC fuses can be used for fast overcurrent protection in DC-DC converters or LED drive circuits.
Figure 1: Examples of LED street lighting circuits, including protection devices associated with switching power supplies (SMPS).
On the AC input side of the circuit, it is also necessary to handle overvoltage events and voltage transients. They are usually caused by nearby lightning strikes, but they can also be generated by transient state changes on the power line. A typical protection device for overvoltage conditions is a metal oxide varistor (MOV) that can be combined with a transient voltage suppressor (TVS). Circuits used for power protection also need to be isolated from the ground to prevent possible shock hazards (all of which are included in IEC/UL 60950-1, UL 1449 and IEC/UL 6500). Figure 2 shows the solution to meet these requirements. This design incorporates a metal oxide varistor with a transient voltage suppressor.
Figure 2: Example of a power input overvoltage protection scheme for LED lighting.
In addition, overvoltage protection should also be considered for LED driver chips. Proper decoupling capacitors, along with the TVS unit for the line driver's rated supply voltage, will provide a very stable design. Some LED driver manufacturers include circuitry that senses an open LED string, but the circuit should not be confused with the circuit that protects the string, or if the LED fails, keep the LED string in operation.
Single LED protection
The bypass protection inside the LED string (Figure 3) allows the string to continue to operate when a single LED fails. Helps protect the LED driver by limiting any excess current or voltage across the entire LED string.
Figure 3: Protecting a single LED in a string of LEDs with an appropriate bypass protector not only keeps the LED string still illuminable, but also helps protect the LED driver from overcurrent and overvoltage (possibly due to LED failure).
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