Will this power adapter work on LEDs? - The Planted Tank Forum
 
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post #1 of 4 (permalink) Old 10-14-2011, 09:19 PM Thread Starter
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Will this power adapter work on LEDs?

I have tons of these things lying around and they just get tossed. Could I use it to power a few 3w LEDs? I was thinking I could drive 4 cheap bridgelux at 500ma off of each one. Would it work? If so, I can actually RAOK them I'm guessing I have at least 20 of them.

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post #2 of 4 (permalink) Old 10-15-2011, 02:57 AM
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Um, what power adapters lol?

Most likely you are referring to DC power adapters that are something like 12v or 24v (or any voltage) that are used for powering and charging various electronic devices like cell phones, laptops, or toy trainsets etc.


Those, alone, will not reliably power LED's such that you can use them over an aquarium of any size.


They are not "constant current" (CC) but rather "constant voltage" (CV) sources. High power LEDs really need to be put on a constant current driver for reliable operation.


There are times that you can run some LEDs on a CV power adapter, but you have to get VERY lucky in the actual voltage output. I've run some of the 20-watt chinese LEDs on 12v DC power adapters (CV) because those LEDs have a wide voltage range (12-14v) and most 12v DC adapters put out actually around 13-14 volts.

But even then --- in terms of brightness---- I'm at the mercy of whatever the voltage from the adapter actually is. Even a minute variation in the driver's voltage output will cause the LED to draw DRASTICALLY varying amounts of current.


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post #3 of 4 (permalink) Old 10-15-2011, 03:20 AM Thread Starter
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Aaaahh! I swear this picture was in there!


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post #4 of 4 (permalink) Old 10-15-2011, 03:49 AM
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Each LED has a forward voltage drop that varies with the current through the LED, going up a little as the current goes up. So, you can use that power adapter if you put a resistor of the right resistance and with enough power capacity in series with the string of LEDs. You could run 3 of the 3.5 volt forward voltage LEDs in series, with a resistor so the voltage drop on that resistor is the voltage output of the power adapter minus 3 x the voltage drop on one LED all divided by the current you want to run. This is not a reliable way to run LEDs anywhere near their rated current, but it isn't too bad when you are running them at 1/3 or so of their rated current.

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