What are Littelfuse PLED devices and how do they compare?
Where are Littelfuse PLED devices typically used, and what do they do?
Littelfuse PLED open-LED protection devices are often incorporated into the circuit designs of high-power LEDs in such applications as roadway lights and outdoor LED advertising display signs.
PLED devices are connected in parallel with each LED in a series string. If one LED should fail open-circuit then the PLED connected to it will turn on and carry the current that would have gone through the failed LED. This keeps the rest of the string operating, so that only a single LED goes dark instead of the whole string.
In the off state, a PLED draws only a few microamps so it does not affect the circuit. When triggered it carries the full current of the string with a voltage drop of about 1.3 volts.
PLED devices help protect the LED from surges induced by nearby lightning strikes and ESD events. In addition, PLEDs contain reverse current diodes that will protect the LED string if the power supply is connected with reverse polarity.
PLED devices are typically connected across each LED in a string, but units are available to connect across two or even three LEDs. This reduces the cost of protection but causes more than one LED to go dark if one fails.
How do they compare to other circuit protection technologies?
Littelfuse PLED open-LED protection devices are often incorporated into the circuit designs of high-power LEDs in such applications as roadway lights and outdoor LED advertising display signs.
PLED devices are connected in parallel with each LED in a series string. If one LED should fail open-circuit then the PLED connected to it will turn on and carry the current that would have gone through the failed LED. This keeps the rest of the string operating, so that only a single LED goes dark instead of the whole string.
In the off state, a PLED draws only a few microamps so it does not affect the circuit. When triggered it carries the full current of the string with a voltage drop of about 1.3 volts.
PLED devices help protect the LED from surges induced by nearby lightning strikes and ESD events. In addition, PLEDs contain reverse current diodes that will protect the LED string if the power supply is connected with reverse polarity.
PLED devices are typically connected across each LED in a string, but units are available to connect across two or even three LEDs. This reduces the cost of protection but causes more than one LED to go dark if one fails.
How do they compare to other circuit protection technologies?
- A PLED provides open LED protection; an SCR does as well, but a zener does not, as string current will overload it and shorten its life.
- A PLED provides reverse polarity protection, while an SCR and a zener do not.
- A PLED and a Zener both provide ESD/lightning protection, but an SCR does not.
| Feature | PLED | SCR | Zener Diode |
|---|---|---|---|
| LED open circuit protection | Yes | Yes | No |
| Reverse polarity protection | Yes | No | No |
| ESD/Lightning/ EFT protection | Yes | No | Yes |
| Response time | ~100ns | ~5µs | - |
| Major disadvantages |
Large device size Typically requires a resistive voltage divider network to set the trigger voltage Variation of triggering voltage versus temperature is quite large Reverse blocking voltage is too high and so cannot provide reverse polarity protection |
When an LED becomes open circuited, the Zener Diode must conduct all of the current. The Zener diode is not rated for this and so its life will be short |
