led protection

Posted on Feb 5, 2014

All optical transmitters will contain circuitry to limit the current to a value that will be safe enough to protect the LED during short-term accidents. Red, Red-Orange, and Amber Luxeon I LEDs have a much lower peak current rating - about 550 mA. At this lower current, R1 would need to be above 2. 5 ohms and R2/R3 would be adjusted accordingly.

led protection
Click here to download the full size of the above Circuit.

If a precise value of R1 (around 2. 2 ohms) can be selected, R2 and R3 can be eliminated and the "adjust" terminal can be connected directly to the "load" side of R1. This lower current/voltage also means that the total drop of this circuit will be in the 3. 5-4 volt range at the limiting current. Philips is apparently phasing out the Luxeon I, III, and V lines in favor of the lower-power Luxeon Rebel devices. Since I have not used those other devices, the techniques described here may not directly apply unless the maximum current is appropriately adjusted. For the time being, however, the Luxeon III devices are still available from various sources. The Luxeon Rebel devices have a peak current of 700 milliamps, so a 1. 8 ohm resistor in place of R1 (with R2 omitted and the "ADJ" terminal connected to the "load" side of R1) would be appropriate. With "ultra high-power" LEDs - that is, those with maximum peak currents in the 10`s of amps - the following methods may not directly apply as the LM317 and similar 3-terminal regulators simply can`t handle the current! In those cases, current-limited supplies, a slightly more elaborate overcurrent protection circuit and plain, ordinary care and common sense are your best defenses against accidentally "killing" an LED! The simplest current limiter is a series resistor. While simple, it`s somewhat costly in terms of power dissipation and its effectiveness (both in preventing damage...

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