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Thread: LEDs. What gives?

  1. #31

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    Quote Originally Posted by Chuck(G) View Post
    Do you perhaps mean a "current limiter" or "ballast"?
    Similar in intent, but instead of worrying about current building over time, this is about a very low resistance being used to just stop the initial 'burst' of energy across the LED during power up before it can start to properly draw current on its own. For some reason in LED circles it's called a pull-down, even though what it does has NOTHING to do with the logic circuit equivalent. :/

    If you deal with any sort of high energy LED's like Cree Q or R series (like the Q5's and R6's I used when making custom bike headlights for the local PD) if you are trying to drive them directly without a proper voltage regulator, you NEED that type of resistor in place or you'll pop it on power-on eventually even if the voltage and amperage of the supply are within tolerances.

    Though that's why I always build using voltage regulators... my preference being for 4v to 9v that converts to the 3.5v that most of your high brightness LED's expect. Means you can hook up a pair of 18650's in series and run full brightness right up until almost zero power.

    Though the packs I made for the po-po were made from eight 18650's recycled from dead laptop batteries. Gives them like 70 hours runtime on a charge with three Q5's or two R6's at full brightness. The controllers I use offer full, 50%, and strobe.. I have one on my own bike with a single R6 and I charge it maybe once a month despite riding an hour every night. (for health reasons)

    SO many off the shelf answers skip that little tiny detail, resulting in GREATLY shortened product life.

    For example, again the typical dollar store three LED just dumps the battery power direct to the bulbs, meaning at least one bulb -- often all three -- usually burn out in a week of frequent use. Pop it open, add one cheap resistor and boom, they'll last forever.

    But to see that type of limiting missing in expensive high power applications? Disgusting -- but entirely to be expected.

    Side note, did you guys know that there's no such thing as white LED's? Well, there are but they're prohibitively expensive. The "white" LED's we do get in the commercial space are blue-UV with a yellow phosphor over it. The UV sets off the phosphor, and as all of us RGB nutters know, blue + yellow == white. A LOT of UV bleeds through the high brightness ones, which can be freaky at night as street signs are UV reactive for better daytime visibility... First time I rode down the street with three Q5's lighting the way, it was making street signs brightly glow all the way to the horizon!
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  2. #32
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    Sort of the old carbon disks inserted in the socket of incandescent lamps as a "bulb saver"; essentially a negative temperature coefficient resistance.

    The probably with the little minimalist LED flashlights is that just about anything you put in series with the LED to limit current is going to waste some power. In automotive and line-operated stuff, this probably doesn't matter, but when you've got only 3-4V to play with, it becomes a concern.

    There are/were ZnSe white LEDs which don't use phosphor; essentially the LED emits light of two colors (white isn't a color, but rather a combination of colors). But they had terrible efficiency and short lifetimes. The current thinking is that GaN on a silicon substrate may hold some promise for a phosphor-free white LED. The ultimate goal would be to produce these on a silicon wafer using conventional technology. Not there yet, but there's a lot of work being done.

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