Re: [APD] LED Lighting

[Wright Huntley <whuntley at verizon_net>]

Andy and Scott,

> Message: 2
> Date: Mon, 29 Dec 2003 09:17:15 -0800 (PST)
> From: "S. Hieber" <shieber at yahoo_com>
> Subject: Re: [APD] LED Lighting
> To: aquatic plants digest <aquatic-plants at actwin_com>
>
> There's nothing to stop one from using the new super bright
> LEDs for lighting a tank.  There are a varitety of color
> temps to satisfy diff tastes. The problem has been the cost
> of the LEDs and the assembling a suitable array of the
> devices to light a tank.

It is a great idea, and gives us an ability that has been difficult or 
impractical with other lighting -- immerse the lamps! This will bring a 
whole new era of illumination design to planted aquaria. I have just 
been coasting along, waiting for prices to drop a bit more.

> It's not impossible to make such an array, just a pita. And
> if one doesn't get the voltage right, the LEDs burn out
> very quickly.

Voltage isn't a good thing to set. The diodes will develop a certain 
voltage for a given current, and current regulation is more certain to 
not overdrive them in most cases. The Wattage and light output track 
current very well, but the voltage is low and is not changing very much 
over a wide range of currents.

[It is exactly analogous to using a needle valve to control the flow 
rate of CO2 and not a pressure regulator (that allows any old flow 
rate), like an Eheim check valve.]
> Try running them at a bit below the rated level. If you use
> diff colors (red, blue, green), then put the diff colors on
> different circuits back to the power supply -- the reason
> is that diff colors heat up and draw current at diff rates
> and the LEDs will tend to last longer if you yu keep them
> separate.

Again, each kind has its own *current* needs, so for crying out loud 
don't put each on a Voltage-regulated circuit! Drive them with the 
collector of a transistor, and regulate the current through them, not 
the Voltage across them, which varies with doping, temp, age and a lot 
of other non-essential parameters.

By building the leds into a bar that mounts just below the water line 
along the front of the tank, illumination of plants and fish will be 
easy to control, and many irridescent fish will light up unbelievably. 
Note that the directional design of the LEDs means no reflector is 
probably needed or desired. A diffuser in front of the LEDs can be used 
if more spread is needed, but it will cost efficiency, fiercely, I suspect.

Narrow-beam LEDs are made worse when the beam hits the water, as all 
rays are bent toward the center of the beam, somewhat. The surface acts 
a little like a focussing lens. Immersed, they will actually probably 
spread much wider as the index difference between lens and water is much 
less than the lens-air difference. Surface refraction and reflection are 
no longer the problem they are with hoods in air, and the illumination 
may be really efficient. Heat can be conducted away from the LEDs by a 
simple aluminum heat sink that extends up out of the water, if that is 
needed.

LEDs typically operate at only a few volts, so the insulation needed is 
just a thin paint of silicone potting compound o/e. [I would still 
provide a good ground on the assembly just in case something fails in 
the power source. That and a GFI could save your fish, or your life, 
even! :-)]

Wright


--
Wright Huntley -- 760 872-3995 -- Rt. 001 Box K36, Bishop CA 93514




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