Selecting an open LED protector for light string products

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 typically connected in series in strings, if one LED in the string fails open, all LEDs in the string will go out. (If one LED shorts, the rest of the string will be largely unaffected.) What is needed is some way to keep the rest of the string functioning if one LED fails open, while protecting against other electrical faults.

Fortunately, there is such a device: the open LED protector. The protector can be connected in parallel with each LED in the light string, and it can bypass the LED that has failed to open circuit to keep the rest of the light string working. The protector will also work when an overvoltage occurs in the circuit and causes an overcurrent in the light string, or when the electrodes are connected reversely.

Figure 1 When an LED has an open circuit fault, the LED open circuit protector starts to conduct.

Limit its voltage drop to 1.3V and keep the rest of the string functioning normally.

Designers have tried many devices to do this—such as Zener diodes, metal oxide varistors, and silicon-controlled rectifiers—but none are as good as open-circuit LED protectors, which are designed specifically for this application. As shown in Figure 1, when an LED fails open, the voltage across it attempts to rise to the constant-current output voltage of the power supply. The open-circuit LED protector senses this voltage rise and begins to conduct, limiting its voltage drop to around 1.3V to keep the rest of the light string operating normally.

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. LED open circuit protectors introduced in previous years can provide open LED and lightning surge protection, but they are not very good for ESD events and reverse polarity. The new generation of LED open circuit protectors can provide these protections at the same time. Designers should choose this device based on the possible fault conditions and needs in the application.

ESD and lightning protection

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

Lightning surges have a rise time of 8 to 10 microseconds (Figure 2), while ESD events have a rise time of only a few nanoseconds (Figure 3), depending on the detection method. Early LED open circuit protectors could only protect against the former, but not the latter. A new generation of LED open circuit 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 groups).

Figure 2: Lightning impulses have a rise time of 8 to 10 microseconds.

Reverse polarity protection

The most common way LED light strings are affected by reverse polarity is through incorrect string placement or orientation during field installation: The LED light string is powered in reverse.

Figure 3 ESD events have a rise time of a few nanoseconds.

The protector can handle ESD at ±15kV air discharge and ±8kV contact discharge.

events, and ±40A 5/50 ns EFT events (fast pulse trains).

Some models of open-LED protectors (such as the Littelfuse PLED 5 series) have a reverse-bias turn-on voltage of 1.1 to 1.6V (Figure 4), which is much less than the reverse breakdown voltage of a typical high-power LED (in contrast to other devices, their reverse-bias turn-on voltage is similar to their forward-bias turn-on voltage).

Figure 4 Littelfuse PLED 5 LED open circuit protector has a 1.1 to 1.6V reverse

Biased on-voltage, can provide 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.

Selecting an open circuit LED protector

Before choosing a suitable LED open circuit protector, the following three preliminary evaluations need to be done:

• Cost: What is the protection plan?

LED protection devices, such as the Littelfuse PLED 9 series through 18 series, can reduce costs by allowing the use of a single device to protect two, three, four, or five LEDs. The trade-off is that if one LED fails, all LED lights protected by the device will go out. For applications such as street lighting, this is likely acceptable. But if the problem of dark spots in a light string needs to be solved (such as in illuminated advertising signs), designers should use the PLED5 series or PLED6 series, which can protect each individual LED.

• Nature of the hazard: Is the LED installed outdoors and potentially subject to nearby lightning strikes? Did the personnel performing the installation ensure that proper ESD protection measures (grounding, etc.) were used, or were they not adequately trained on these precautions?

If these problems are likely to occur, please use a protector that provides lightning strike, EDS and electrode reverse polarity protection, such as PLED5.

• Application importance: Would a full or partial failure of an LED string endanger human life and property, or would it simply be an inconvenience?

For critical installations, use a protector such as PLED5 or PLED6 in parallel with each LED in the string.