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[APD] Re: Idle Thoughts on LED Illumination

Quick e-bay survey shows bright whites at $0.23 ea in 1000 quantity plus shipping, while red, green and blues (high intensity) run at or below $0.20 ea *including* shipping at only 50 quantities from Hong Kong.

Surplus LEDs are often surplus due to variations in their color. Might be something to keep in mind...

Only apparent advantage of whites is that you can pack more in a smaller space than three colors. Disadvantage is the lack of ability to control the exact "whiteness" (aka color temp.). [May be OK if you really like to look at 8600K (bluish) lamps.

White LED = blue LED + phosphor, hence the usually very blue-white appearance of the light. It used to be really noticeable in the older white LEDs, but the newer ones are better about keeping the color even across the die at least.

The suppliers all expect you to waste power by converting 12Vdc to 20mA with a resistor. Since 9V will be dropped in the resistor and only 3 in the diode, that seems to be wasting about 3/4 of your input power. [Starting with 3X as much heat as you really need!] You possibly cannot operate with diodes in series to get around that, as their Voltage drop at a given current isn't predictable enough to keep current within safe bounds.

It should be safe to series-connect a number of LEDs together as long as they are all from the same batch (buy 20 from one supplier and you're probably safe). A milliamp or two difference between LEDs isn't going to be a problem. I have myself connected groups of LEDs in series to allow them to be directly driven by a 12 volt supply, but each series-connected string has to consist of all the same type of LED. You couldn't safely string red and blue LEDs in series without manually matching them -- and you'd have to use the ultra-high-power red LEDs to do this to get the currents close.

The safer solution may be to use a constant-current supply (aka battery charger) if the diodes are pretty well matched. You can then use series/parallel combinations without risking overdriving them.

Maxim makes an LED driver chip that is a few dollars and does just that. I think it also has a PWM function to allow brightness to be controlled.

Otherwise, I'd start by trying to put groups of three in series of RGB and parallel those up to see if the constant current through each set is close enough to not burn some out. A standard car battery charger might be perfect, as you can often set them at 3A or 6A as needed.

Except that a string of LEDs is going to need maybe 20-50 milliamps, so you'd need a *lot* of LEDs to draw several full amperes like that.

Enough idle thoughts for now. Asbestos britches going <ON> in case I got some of the above wrong. ;-)


As long as you didn't let any smoke out of any parts there's nothing to yell at ;-)


Waveform Technology
UNIX Systems Administrator

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