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Old 01-03-2013, 03:05 PM   #11
tdcarter's Avatar
Joined: Aug 2012
Oddometer: 79
Originally Posted by craftycoder View Post
I don't know what a buck puck is but I assure you the only thing you need to run LEDs other than this box is a current limiting resistor setup so that the maximum current allowed through the circuit is <= the max through the LEDs.
Sorry.. a buckpuck is a name brand of a LED constant current driver. Name just kind of means generic constant current to the DIY led community now. A buckpuck is when your supply voltage is higher the the forward voltage of the leds, a buckboost is a current limiter and voltage booster used when your supply voltage is lower then the leds required forward voltage.

A quick run-down of what I was referring too..
So a resistor will limit current, but only a specified amount at a specified voltage. Has to be calculated each/every time. Increase voltage and the current flow can increase too. A buckpuck (aka constant current supply) limits the supply but lets voltage float, upto an amount that is slightly less then the supply side. With a constant current supply there is no need for the resistor to limit the Led. As long as your string of leds have a forward voltage less then the supply, you can control their brightness by limiting current. If forward voltage is greater then supply, you can still limit current but the led's will not achieve max brightness.

If you pop open a high quality led light you won't find any current limiting resistors in line with the led's themselves. Using a resistor to save an led from runaway is an inexpensive way to do it when you know the supply voltage and have limited amounts of low light leds. But when you start talking high power led's, varying voltage supplies, resistors would not be a very efficent way to go. Now you'd have to worry about the heat the resistors generate and your losing supply power to that resistor, killing your batteries faster then necessary. Plus as the batteries drop voltage when they age the led's will dim since available current will drop due to the resistor being a fixed value. Again, resistor, cheap and easy for small led, constantly wastes energy, not effective for high power.

Onto the variable part.

Using pwm to flash the led's on/off faster then we can see is one way to dim them. There is where I have seen some pwm (dimmers) cause problems. The buckpuck or buckboost (ie the led driver) does not like the frequency that the dimmer is using. Not real common occurrence, but it happens. Low powered led's that are using resistors for current limit don't really care about the current being pulsed. But if you can just flat out limit the current, and by varying the current you adjust the brightness, it's even better and ultimately more efficient.

Clear as Mississippi mud? and more then you wanted to hear?
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