Littlefuse briefly discusses the protective effect of LED open circuit protector

High-power LEDs are increasingly being used in traffic lights, illuminated billboards, streetlights, airport runways, railroad crossings, medical/surgical lighting , and traffic signals. They have a long lifespan (high-power white LEDs typically have a lifespan of 35,000 to 50,000 hours) and low power consumption. However, LEDs can also be damaged by lightning strikes, overvoltage caused by electrostatic discharge (ESD), overheating, and even reverse power supply.

Because LEDs are usually connected in series in a light string, if one LED in the light string fails and becomes open circuit, all LEDs in the string will go out. Littelfuse offers LED open circuit protectors. The protector can be connected in parallel with each LED in the light string, and it can bypass the LED that has an open circuit failure to keep the rest of the light string working. The protector also works when an overvoltage occurs in the circuit and causes an overcurrent in the light string, or when the electrodes are reversed.

When an LED fails to open circuit, the LED open circuit protector will start to conduct, limiting its voltage drop to 1.3V and keeping the rest of the series connection working.

To achieve this, designers have tried many devices, such as Zener diodes, metal oxide varistors, and silicon controlled rectifiers, but none of them are as good as LED open circuit protectors, which are designed specifically for this application. As shown in Figure 1, when an LED fails to open circuit, the LED open circuit protector senses this voltage rise and begins to conduct, limiting its voltage drop to about 1.3V to keep the rest of the light string working properly.

LED open circuit protectors are available with different features and voltage ratings. Some protectors are designed to protect a single LED in a light string; others allow designers to protect two, three, four or five LEDs with a single device to save costs. Littelfuse's new generation of LED open circuit protectors can also provide ESD protection.

ESD and Lightning Protection

ESD can enter the LED light string during assembly or field installation. Lightning can enter the LED light string in two ways: a nearby lightning strike may cause transients in the power supply, which can couple to the LED driver and enter the light string; or a nearby lightning strike may cause electrical transients on the LED driver power rail and also on the lines connected to the LED light string.

The new generation of open LED protectors, such as the Littlefuse PLED 5 series, can handle ESD events at ±15kV air discharge, ±8kV contact discharge, and ±40A 5/50 ns EFT events (fast pulse trains).

Reverse polarity protection

When a LED string equipped with PLED5 is reverse powered, all protectors will start to conduct, shunting the current on the protected LEDs to protect them from damage. After the reverse polarity is corrected, the protectors will turn off and the LEDs will operate normally.