LED confusion

[fasteddie455]

I am attempting to figure out a resistor to use with my LED (red super bright, fwd V=2, fwd current=20ma) my calcs say a 390 ohm resistor....in his build doc, drolo says he uses a 10K resistor with white superbright LED....what am I missing?  that is a big difference between 390ohm and 10K....what am I not understanding?
fasteddie455

[mjg]

It might just be a matter of personal preference.  While the LED may be able to take 390 ohms, it will be super bright.  The hint is in the name.    At that brightness, it's painful on a pedal, almost blinding I find.  A 10k resistor will make it still glow, but not burn out your retinas.

You can try it out by just touching the LED and resistor to each other and the power supply.  Test out a few values and see what you like.  The 390 is a minimum value, it won't hurt to go higher.  Just don't go lower.

[somnif]

LED brightness will vary with the CLR (because brightness is proportional to current, and ohm's law, etc).

Depending on the LED you use (diffuse vs water clear, normal vs superbright, red vs green vs blue, etc) you will want a different value. Sometimes this value can vary from batch to batch even.

I recommend getting a 50k linear pot. Wire one side to ground, the other to the cathode (-) of the LED. Twist the pot to max resistance. Hook The +9v to the LED anode, and twist the pot down until the brightness is where you want it. Check the pot with your multimeter, and use whatever resistor you have closest to that value. (Just be careful to not zero the pot entirely. Hence starting high and working down)

[Aleph Null]

The 390Ω value is probably the minimum required to prevent the LED from dying. The result may be Lasik surgery bright. Testing with a pot, or installing it with one is a great idea if you're not familiar with the LED.

[wgc]

Fwiw I typically use 33k with water clear leds, still very bright...