LED Current Limiting Q's

[aziltz]

I build a BYOC kit and swapped out the LED for a sweet Blue one I had.  How can I know or test if i need to change the limiting resistor to it?  I don't have any information on the LED type I have.  Thanks.

[hubble]

depends on the LED, depends on how bright you want it. 
generally a 4.7k resistor works well.  that would get you around 2mA

[aziltz]

depends on the LED, depends on how bright you want it. 
generally a 4.7k resistor works well.  that would get you around 2mA

the LED affects current draw quite a bit no?  I know there's no wrong answer here.  Should I worry myself with this?  As it stands now, its Bright and I dig it...

[hubble]

well too much current will blow your led.  those clear blue ones are pretty bright even with low current.  what is your current liming R right now?

[aziltz]

well too much current will blow your led.  those clear blue ones are pretty bright even with low current.  what is your current liming R right now?

4.7k, according to the schematic.  its a kit so its completely stock.  i dont have it out right now i'm waiting for parts before i box it up.

[hubble]

thats fine, that gives 2mW through the led, which is pretty low. i think most led's max current is something like ~20mA

[Heemis]

LED current limiting resistors can be pretty subjective.  The main thing is that you limit the current enough to keep the LED from being blown, which is usually accomplished by a relatively small resistor.  You'll usually find that with high brightness LEDs you can often use a much larger resistor and still get a pretty bright LED.  The bonus is that you save yourself some current consumption, which results in longer battery life!  I typically try to use about a 22k resistor if I can.  One way to test it is to take a battery, and some alligator clips and just test out different resistors in series with the LED, and determine which brightness is best for you.

Also, here's a handy calculator:  http://www.muzique.com/schem/led.htm

[R.G.]

Also, here's a handy calculator...

... the use of which will help you to ... never... learn anything about LEDs and current limiting.

[BAARON]

... the use of which will help you to ... never... learn anything about LEDs and current limiting.

Heehee, nice.

The other down side of using too small a current limiting resistor in conjunction with a high brightness LED is that it can sometimes be too bright.  The white LED on my Ross compressor is so bright that it'll leave spots in your eyes if you look directly at it, and you can see through your finger if you touch the LED.  It's kinda fun, but it's also hazardous for people who don't know what to expect (like one of my previous band students who picked it up one day while it was plugged in, said "what's this?" and turned it on a foot from her face).

[Heemis]

Alright alright, you're right RG... an addendum: LEDs have a voltage drop, around 2v in normal LEDs and 4v in White, blue, some others.  If you are supplying 9v to the circuit, and the LED drops 2v, you have 7v.  Using ohm's law, the current through the LED will be V/R.  For example, 7/4.7K ~ 1.5mA.

Check the data sheet on your LED to find the forward voltage and recommended current. Using current higher than the recommended current will shorten the life of your LED, too high above it and you will burn it out.

[MohiZ]

Hmm... I've been using 470R resistors in all my pedals so far with success. Although that should put the LEDs pretty close to their max. current. Maybe I'll try putting in larger resistors.

[JKowalski]

Or you can simply use the easy method of real time testing!

Use a breadboard, put a 400 ohm resistor in series with a 10K potentiometer (variable resistor) and your LED. See what gives you a decent brightness, and then once you find a value double check the current draw at that value with max currents and then you've got your resistor value! Plus you can visually see what it will end up as at different values beforehand.

Or you can just use a 10k trimpot.

[aziltz]

thanks for the suggestions.  i'll just stick with 4.7k for this build