calculating strings of LEDs

Discussion in 'RapidLED' started by Dingo, Oct 26, 2010.

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  1. Dingo

    Dingo Giant Squid

    Aug 1, 2009
    New Freedom, PA
    I have a real quick question: You cant go under 4 LEDs per driver and your not supposed to go over 12 per driver... but what if you do? Will the forward voltage be reduced thus causing your LEDs to be underpowered, or will it not even drive more than 12?

    Do you know the equation?

    thank you in advance
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  3. FaceOfDeceit

    FaceOfDeceit Hockey Beard

    Jun 24, 2010
    Charlotte, NC
    I have 14 on one driver right now.
  4. Rapid LED

    Rapid LED 3reef Sponsor

    Sep 21, 2010
    You can go from 3-14 LEDs on the Meanwell LPC-35-700 (or LPC-60-1050 if you're using XP-G's). To figure this out, take the minimum voltage of the driver (in this case it's 9V, but if you're using a dimmable driver it's 24V) and divide by how much voltage the LED needs at that drive current (in this case 700mA). When we did tests the XR-E's came in at about 3.2-3.3V, though that may differ slightly from what the actual spec sheet says.

    In any case, if you divide 9 by 3.2 (or 3.3) you get ~2.8 LEDs, so rounded up means you can run as few as 3 LEDs in a string (you'll want to round up because you don't want to push too much voltage to the LED). To find out the maximum number you do the same thing, so 48V (max voltage of the driver) divided by 3.3V equals 14.5. In this case you'll need to round down so you get 14 LEDs as your max.

    Hypothetically if you were to throw in 16 or 17 LEDs they would still work, they just wouldn't all be getting as much voltage as they need so they'd be running much lower (dimmer) than they should be. Conversely, if you only ran 1 LED then that 1 LED would get 9V of power, meaning it would blow immediately.

    Keep in mind if you're only running 3 LEDs it is easier to blow the entire string if you do make a mistake (ie. by plugging in the driver before connecting the string to the driver), but if done properly it will run just fine.