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Re: [APD] Help With My New Tank!! -- or - Bravely into the Shadows

Now I know this is a kinda crazy question; but since
you mentioned it...
How many white LEDs would be required to provide
enough light for my tank?  I know 3 LEDs provide one
watt of output but that is about all...
There is no "n number of LEDs produces x amount of watts" -- different LEDs have different electrical specs... For example, a typical white LED may have a forward voltage drop (Vf -- they are DIODES after all) of about 3-4 volts or so. White LEDs use higher voltages than the approx 2.1 volts or so of the more common red and green LEDs. Taking specs for a Lumex LED from page 1333 of the current DigiKey catalog, the Vf is 3.6 volts, and the forward current (If) is 30 mA. 3.6v * 0.03 A = 0.108 watts, and some of that will go off as heat. That means you need about ten of those for a watt of light, and even in quantities of 500, they are still $ 1.20 each. Not cheap.

These LEDs are really cheap (50 for 15bucks)  - I
thought it might be interesting to try making a hood
with a grid of these.  I have a degree in compsci and
my friend has one in compeng.  I'm sure we can rig
something up :)

Not that easy. This from an EE :-) To drive a large number of LEDs *properly* you really need a constant current drive. Maxim (http://www.maxim-ic.com) does now make a suitable driver chip, but it can only run up to 10 or so LEDs since it is intended to drive the backlights for things like cellular phones. You can just connect strings of LEDs together to run on a certain voltage, but it's not ideal.

Also, the cheap white LEDs that you see surplus are usually surplus due to color variations that prevent them from being sold through normal channels. This means that the LEDs will have varying color light output which might look a bit odd. IME, the surplus ones usually produce a white area of light with a blue ring around it (when used like a flashlight projecting a circular pattern). I think the problem is due to phosphor placement misalignment on the LED die during manufacturing, but that's just a guess on my part.

LEDs make great moonlights though -- color isn't critical and you don't need too many to do it.

The other option I am considering is using a few of
the 13watt cf tubes from ah.

The larger 55 and 96 watt tubes will be better suited to lighting your larger tank unless you are just using the 13 watters for accents.


Waveform Technology
UNIX Systems Administrator

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