How sensative are LED's

[suncrush]

I have some LED's with no datasheet, and am getting little help from the vendor.  I know the forward current is 3.2 V, but I don't know if the LED current is nominally 20 mA or 25 mA.

Does it really matter?  Is the LED sensitive enough to care?

[slacker]

A 20ma LED won't die if you give it 25ma, not for a short amount of time anyway, if you ran it like that permanently the life time might be reduced. For stompbox use you probably don't want to run the LED at anywhere near 20 or 25ma so it's not really a problem, with modern LEDs 1ma is often more than bright enough.

[suncrush]

All right, thanks.  I guess I should hook it up to a circuit and figure out how bright I want it.   

[UKToecutter]

some of the more 'modern' LED's are pretty bright at very low currents.
Try at 5mA first.

Do you know how to calculate the required resistor?

R=V/I where V=supply - Vf of LED
So for a 9V supply

R=9-3.2/0.005 (for 5mA) = 5.8/0.005 = 1160R
A 1.2K or a 1K would be a good test

[UKToecutter]

I apologise in advance If I'm teaching my grandmother to suck eggs......

[davent]

As slacker mentioned with some of the modern versions well under 1ma will be bright enough, i've some clear greens that i settled on 47k current limiting resistor. Eyeballing's the only way to determine a best resistor value for you and what you have,. Mark Hammer's led tester idea makes it it easy suss the random collection of led's i've amassed.
10k can be added in series to give a second range.


dave

[chromesphere]

Cool test rig Dave!  I have a cheapo chinese one I got for 5 bucks that does the job (once you rub off the silkscreen and correct the values!).  Believe it or not, the values on the silkscreen didn't correlate the resistor values inside the tester lol

Funny, I got into an argument / heated conversation with someone on youtube in my latest upload, essentially about whether you should do the math or eyeball. I didn't go into any of the math in the video as the video was intended for absolute beginners. Apparently that was pretty ignorant of me as it could cause a house fire.

Anyway, I tend to just eyeball (even sometimes after I do the math ).

[davent]

Thanks Paul, bits and pieces gathering dust put to good use. I'll do the math after the eyeball just out of curiousity to see how few ma i'm using or if i have to find the clr for a 300v drop based on my 9v eyeball test.
dave

[Mark Hammer]

Dave's rig is classier than my own.  But I agree that eyeballing is critical, since not every colour sticks out the same, even with the same specs.  Gotta control for the human retina.

[bluebunny]

^ +1

You (I) really do want to test your (my) LEDs in order to avoid this:

       

The guy I sold this one to hasn't been seen for a long while.  Now that his eyes are shot to pieces, he probably can't see enough to track me down...   

[deadastronaut]

^ i,ll find ya, when i find my glasses...


cool tool dave...

i use my resistor sub box with leds on breadboard...test in dark an light...luckily my

hall is dark...i did make a chart of my results for future reference, but god knows where thats gone....or i cant see it