Using a constant current driver - 13690

[CircuitsToPlowshares]

I was looking at making a small light panel for a photographer friend of mine. I picked up some 10mm 140kmcd leds and an driver http://dealextreme.com/details.dx/sku.13690 . The leds have an operating range of 3.4Vf to 3.8Vf with corresponding currents of 20mA and 40mA. Since the driver is a 320 mA constant current driver with output voltage between 16-26V, I plan on putting 8 strings of 6 leds in parallel.

I assume good design would be to use a resistor on each string but I am not sure what value because I don't have a fixed voltage. My assumption was to figure on the driver putting out some voltage above the string Vf (22.8). I had planned on just figuring on ~1v at 40mA so a value of somewhere around 22ohm or 26ohm. Does this sound reasonable kind of thought process.

Edit: For clarity I wanted to drive them at 40mA.

 

[nein166]

6 LED's in series at 3.8v is 22.8 - This string of LED's will draw 40ma

Now 8 of those in parallel will pull the 320ma out of the driver.

I would suggest adding 2 parrallel strings so there is a total of 10 strings of 7.
This way the leds will get 32ma each and though each will put out a bit less light there are more LED's also less heat is generated at each LED extending the life of all. Your better off running more LED's at lower power. 16 of 7 would definitely be brighter than 8 of 6, rules of efficiency. Or is it eficacy? A rise in current is not directly proportionate to a rise in output.

 

[CircuitsToPlowshares]

6 LED's in series at 3.8v is 22.8 - This string of LED's will draw 40ma

Now 8 of those in parallel will pull the 320ma out of the driver.

I would suggest adding 2 parrallel strings so there is a total of 10 strings of 7.
This way the leds will get 32ma each and though each will put out a bit less light there are more LED's also less heat is generated at each LED extending the life of all. Your better off running more LED's at lower power. 16 of 7 would definitely be brighter than 8 of 6, rules of efficiency. Or is it eficacy? A rise in current is not directly proportionate to a rise in output.

The total output of that driver is set for the 320mA which is why I picked the 6x8 since I had a 50 pack of them. I agree it would be more heat efficient to run them off of a lowered current but that is not an option without ordering more of them. My initial question still stands, is my use logic on choosing a resistor to prevent runaway correct? I am not sure the behaviour that the driver will implement to achieve the constant current when I stick the resistor in there.

 

[CircuitsToPlowshares]

I guess a month later I will check if anyone has any responses yet?