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Unread 03/30/2009, 07:08 PM   #577
kcress
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Join Date: Dec 2007
Location: Central California Coast.
Posts: 5,383
Hi ReefEnabler,

If you want to reduce the current thru a particular LED in a chain you put an appropriate resistor in parallel with the LED of interest.

To do this you need to figure out what the resistance needs to be and the resistor's required power rating.

If we take a cyan triple chip as an example:
Looking at the data sheet we see the forward voltage is 6.84V at 700mA.

Now if you only want to run 550mA thru this puppy you need to shunt 150mA around it somehow, (700-550 = 150). Hence the shunt resistor.

Using the voltage: 6.84V / 150mA = 45.6 ohms.

So if there is a 45.6ohm resistor in parallel with the LED it will be robbing 150mA of the 700mA that is being provided. This means there is 150mA less to run thru the LED and it will get only 550mA thru it.

There is a caveat here which is why I said the actual measured values would be better - but we will persevere. If the LED's forward voltage is 6.84V at 700mA it will be less than this at 550mA. How much less? It could be a lot less.. Especially if, out of the box, your LED is already a low VF value out of the batch it was made in.

What does this really mean? Well it means that after the math above the the actual shunt current will end up being something a little less than what's calculated. This means you will reduce the current thru your LED a little less than expected. But since this is all a bit seat-of-the-pants, less than 700mA, is already going to help the LED's life dramatically.

Ok, now on to the power calculation.

It's easy:
Power(watts) = V x V / Resistance.

P = 6.84V x 6.84V / 45.6 ohms
P = 1 watt

Prudent design requires you use a resistor that is at least two times this and better would be 3 times. Two times would probably still lift your finger prints off instantly. I would probably go 3X

So you want a 3 watt or larger resistor. Now that we know what size we are hunting we can find one. No one makes a 45.6 ohm resistor so you pick a lesser value due the 'caveat'.

43 ohms is a standard value.

I would use one of these two they are both 5W which is much more common and will run cooler yet:

http://search.digikey.com/scripts/Dk...&name=43W-5-ND
http://search.digikey.com/scripts/Dk...me=805F40RE-ND

Hook it up in parallel with the aforementioned LED.


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