Multiple LED Question

[jimmy54]

Hi fellas,

I plan to light up a muliti-effect enclosure with multiple LED's (at least 12).  The LED's I have are blue 3400mcd ones.  I've done a bit of reading about wiring them in series with a single current limiting resistor or in paralle with each LED having it's own resistor, but all that was in context of running them off a 9v battery.  I plan to use a power supply (9v, >1A) to power the effects and the LED's so I'm not sure if some of the concerns in what I've read are pertinent in my case.

Here's one of the sites I did some reading at:

http://www.llamma.com/xbox360/mods/How%20to%20use%20an%20LED.htm

http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng#series

http://wolfstone.halloweenhost.com/Lighting/litlpo_PoweringLEDs.html

Anyone got any suggestions for the best way to go about this?

[Seljer]

Handy little calcualtor thing for large arrays of LEDs
http://led.linear1.org/led.wiz

[jimmy54]

Thanks Seljer, that is a great calculator.  According to the calculator an array of 12 LED's with a 9v source voltage, 4.4v diode forward voltage, 20mA forward current should be arranged with 6 sets of 2 LED's.  Each pair of LED's are in series, and each pair are in parallel using six limiting resistors of 10ohms.  Apparently the total power dissipated by the array is 1080 mW and the array draws 120mA.  This should be OK in terms of my 9v, 1.4A power supply.

[Seljer]

You could pretty easily make those resistors larger without much trouble, as those ultrabright LEDs really don't need a lot of current to get bright (breadboard things out, just 2 LEDs and a resistor running of 9V to get the brightness you desire)

[jimmy54]

I'm actually planning on adding a pot in line with the LED's so I can adjust the brightness.  I'll do as you suggest and breadboard it to find the optimum brightness.  Cheers.

[cakeworks]

Wouldn't you just wire the pot instead of the resistor?

[darron]

Wouldn't you just wire the pot instead of the resistor?

no, you'd have to use a resistor at the minimum level to run the led as well as the pot. if you only used a pot then once you got the wiper to the output it would give zero resistance and fry the leds....

that smells.... hehe