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Thread: looking for a LED

  1. #1
    Enlightened
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    Default looking for a LED

    Hey all im a lil stupid on LEDS......Im making my own dive light and I want a LED to go inside the battery canister to light up incase my main light bulb pops. (Top of canister is clear) My goal is only to get say 20 lumens out of it or so.My battery will be a 12v 4ah....so I guess I need a 12v LED. White would be optimal any help would be great!

  2. #2

    Default Re: looking for a LED

    Quote Originally Posted by rkboyer911 View Post
    Hey all im a lil stupid on LEDS......Im making my own dive light and I want a LED to go inside the battery canister to light up incase my main light bulb pops. (Top of canister is clear) My goal is only to get say 20 lumens out of it or so.My battery will be a 12v 4ah....so I guess I need a 12v LED. White would be optimal any help would be great!
    An inexpensive XRE P4 CREE LED on a star board is probably your best bet.

    like this one at DX:

    http://www.dealextreme.com/details.dx/sku.1302

    The light pattern from this LED's built in lens, with no reflector or other optics, is a nice wide even pure white flood pattern.

    LED's like this are current driven, not voltage driven, so there is no such thing as a "12 volt" led (unless it has it's own internal driver of some sort).

    There are efficient switching drivers, but for your simple application, you can just drive the LED right from your 12-14 volt battery voltage using an easy to find 100 ohm 2 watt series resistor to limit the current to about 100mA, like this:

    12vBattery(-)---(-)LED(+)---100ohmRESISTOR---(+)12vBattery

    [Note the LED is polarity sensitive, but has the gold (+) and (-) solder pads clearly marked]

    With the current limited to about 100ma through the LED, it will dissipate about 350mW of power and easily put out more than 25 Lumens of intense pure white light.

    With the low 350mW power level on the LED, the aluminum star board that it is mounted on should be a pretty good heat sink all by itself, but the above LED can put out three to four times the light level if mounted to a larger aluminum plate and driven with more current.

    For twice the light with about the same battery drain, you can wire two of these LED's in series with a 65 ohm resistor, to create a more efficient series circuit like this:

    12vBattery(-)---(-)LED(+)---(-)LED(+)---65ohmRESISTOR---(+)12vBattery

    You could go to three LED's in series but then you would probably want to go with a constant current solid-state circuit instead of a simple resistor.

    The resistor values in the above circuits are not critical and you can change them 30% or more without causing problems for the LED. If you have a problem finding 2W resistors, you can just combine some readily available lower power types in series on a piece of perf-board. For example ten 1/4 watt 10 ohm resistors in series to create a 100 ohm 2.5 watt resistor, or six 1/4 watt 10 ohm resistors in series to get a 60 ohm 1.5 watt resistor (which would be fine for the 2 LED version above).
    Last edited by Luminescent; 08-18-2007 at 04:29 PM.

  3. #3
    Flashaholic* LukeA's Avatar
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    Default Re: looking for a LED

    You could always run 4 emitters in series.
    A little madness never hurt anybody.

  4. #4
    *Flashaholic* Gunner12's Avatar
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    Default Re: looking for a LED

    You could also buy a LED dive-light(Babrolight is a company most would recommend)

    As the saying goes, "Two is one and one is none"

    You could buy a Downboy(400mA board, 70+ lumens at the Cree P4 emitter, seems to be more then you want) or SOB(400 mA).

  5. #5

    Default Re: looking for a LED

    Quote Originally Posted by LukeA View Post
    You could always run 4 emitters in series.
    Four emitters in series in direct drive from a 4 amp hour high capacity battery?

    Sorry, really bad idea!

    This is why I said LED's ARE CURRENT DRIVEN.

    In a direct drive voltage driven circuit the current is never stable and one of two things will most likely happen:

    1. If Vf forward drop of all four LED's is above about 3.2 volts the light level will be low and inconsistent on 12 volts.

    2. If the Vf drop of all four emitters is lower (around 3.1 or less) the setup will run a few seconds while the LED's heat up and their Vf goes even lower then go into catastrophic thermal runaway and fry the LED's.

    In a dive light you want reliability, so fancy switching regulators are probably also not worth the added complexity if the light level needed from the LED's is low (in this application the LED just provides backup illumination for the battery compartment).

    The simple circuit with two LED's in series with a resistor would be more than 50% efficient when driven at 12 volts and should be extremely reliable.


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