Re: Figuring out LED specs

From: Mark Fergerson (nunya_at_biz.ness)
Date: 01/20/05 

Date: Thu, 20 Jan 2005 09:32:29 -0700

Anthony Fremont wrote:
> "Mark Fergerson" <nunya@biz.ness> wrote in message
> news:YQKHd.6100$0B.5475@fed1read02...
>
>> Mike Chambers wrote:
>>
>>> I have a bunch of loose LEDs. Some of them are completely clear.
>>> Is there anyway that I can figure out the specs for the LEDs? I
>>> no longer have the packaging?
>>>
>>> i.e. how many volts they use, and their current?
>>>
>>> I am afraid if I hook them up to check them out, I will burn them
>>> out.
>
>> First you have to sort by color. Put the leads directly across a 3V
>> lithium watch battery (no resistor) which will safely light red
>> LEDs,

> You've been lucky. Without current limitation (perhaps your battery
> was weak),

   Dumb Luck has always played a large part in my life; I always use
batteries from "dead" or otherwise defective watches (I suppose I might
have mentioned that), so maybe you're right.

> pretty much any LED will burn out soon after you exceed the Vf and
> get it turned on. Granted that a quick hit probably won't do it
> (I've done dangerous things too ;-) I do find that many LED's are
> quite durable to this kind of thing, but there are plenty that will
> just go poof in the blink of an eye.

   Haven't had any die from testing so far.

>> and won't kill them if you reverse-connect them. If it stays dark,
>> stack
>> two lithium cells for 6V; same lack of reverse-voltage concern.
>> Been sorting LEDs this way for some time. As for ultimate current
>> limits per device, well, suck 'em and see.

> AFAIK, 6V is likely to destroy just about any LED without some kind
> of current limiting. Most LED's will light dimly if you supply as
> little as 1 or 2mA.

   Again, Dumb Luck on my part. I'll go with the other suggestion about a
current-limited supply and voltmeter if the OP can manage it, but I felt
comfortable recommending watch batteries for quick 'n' dirty sorting
since I inferred the OP was more interested in not throwing the unknowns
away than assembling "proper" test gear. Might be wrong again...

   Mark L. Fergerson

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