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LED Driver Question


josh_scci

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I'm probably overlooking something simple but my question is, why do we need these big led drivers for our diy lights when you can grab an led fixture from a store and there is nothing there but a small 12v power supply and a housing that weighs nothing but houses and powers 45 leds. I know there is a circuit board inside with resistors and such, but what I cant figure out is why we cant build our own circuit boards and just wire in the power supply. Again I am sure I am missing something here, but from my limited electrical knowledge all that is required is a power supply and then a resistor to knock the voltage down to the proper level. Help me out what am I missing here.

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A resistor is the most inefficient way to get the correct current as I understand it. When you see an LED fixture at a store or whatever chances are it's using simple LEDs, not the high power LEDs typically used in aquarium settings. Not that they work differently, but I think there is a difference in scale.

 

LEDs are current driven, and then it depends on how many watts the power supply produces and the voltage draw of the individual leds how many it can power. In the case of a typical 5mm LED I think the voltage draw is like 3v but the current needed is like 20mA. In a 3W led it's like 3-4v and 700-1000+ mA. I think the extra current required just requires a more sophisticated driver to deliver the power reliably and in a manner that won't cause excess heat and/or damage the LEDs.

 

Anyways I'm not an engineer either, but I've dabbled in building circuitry with simple 5mm LEDs (using a cell phone charger and some resistors) but I've also messed with making a circuit producing about 200mA which had a lot more to it. I suppose you could use the simple method with a DC power adapter and some resistors, but my guess is that the power wouldn't be very reliable or efficient, and could possibly burn out the LEDs if something were to go wrong or the power adapter had too large a variance in power output.

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If you had a simple 12V 9W power supply you could drive a three 3W led fixture. Enough to light a 3 gallon tank lets say. Since the fixtures we use are a lot more powerful they need a larger power supply. The issue with most DIY setups is that they replicate the 120V -> 36V conversion circuitry multiple times. Commercial fixtures use one large power supply and house the driver portion inside. In the Radion for example there is just one power supply that outputs 36V at probably 5-7A and the individual divers are inside the fixture itself. Limiting LED current with resistors is very poor design. You are wasting energy to basically heat a resistor. LED drivers use circuitry that adjusts current and voltage precisely for each specific LED string. You could drive the LEDs with simple power supply, but they would not last long.

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This is my understanding. You have two ways to power the LED's. You can do the "bruit force" method, which is to just apply the voltage directly to the LED, or you can power them through a driver. What the driver does is it pulses the LED. This is how LEDs are designed to work. The driver turns on and off the LED's so many times a second that they appear to be on continually. I also believe "some one can chime in hear if I'm wrong" but I believe you can't dim them with out the use of a driver. They also say you will get a much longer life out of the LED with the use of a driver.

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