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  #1  
Old 10-07-2006, 03:39 PM
Jbolt Jbolt is offline
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Default DC LED Brightness control

I'm tring to help my 12 year old son with a school project. He wants to build a LED book light with variable brightness. We are finding this to be more difficult than we thought it would. From what I've been able to gather a PWM is necessary. Any help or guidance would be appreciated.

Thanks,

Jay
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Old 10-07-2006, 04:10 PM
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beenthere beenthere is offline
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Hi,

Led's are current-operated devices. There is some maximum value for current, but it can be controlled resistively. A PWM methodology is just overkill.

You need to determine if the led's are to be in series or parallel. White led's need about 2.2 volts applied to go into conduction, so more than a few would make the needed source voltage too high. Placing them all in parallel requires more current.

Say he is planning on using 6 led's. That's too much in series for a 12 volt source. Arranging in two parallel brances of 3 led's each would be better. That way, each branch would need at least 6.6 volts applied. For good brightness, figure on between 15 and 20 milliamps per branch.

Again, assuming 6 led's and 12 volts, subtract the led operating voltage from the applied voltage (12 - 6.6 = 5.4 volts). Above the operating voltage, led's have no resistance, so current has to be controlled with a resistance. For the total current of 40 mills (20 ma per branch), use a 140 ohn resistor. Make it a 1/2 watt resistor. The actual power in the resistor is right at 1/4 watt, so some overhead for safety is a good idea.

To vary the illumination, add a potentiometer in series with the 140 ohm fixed resistor. You might want to use about 1000 or 2500 ohms for a wide range of adjustment before things get too dim. Make sure the pot is rated at 1/2 watt, too.

Radip Shack might have all this stuff.
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Old 10-07-2006, 09:26 PM
mrmeval mrmeval is offline
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Rat Shack may have it but the people there no longer are competent to find it. They've shoved all of that stuff in drawers or eliminated it entirely now.

Digikey, Jameco are good, the school may have a source for parts as well.
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