Maximum current rating for blue led?

D

Daniel Morrow

Guest
I want to buy some blue leds from digikey (haven't heard back from them yet
about this) and their catalog doesn't list the maximum current rating for
any of the leds they carry, the catalog only lists the maximum voltage
rating. How can I find out the maximum current rating without trial and
error for each individual led and without a risky potentiometer/multitester
method? I need to know the maximum current rating of these blue leds so I
can use the proper resistor with the leds so that the leds do not get
damaged. Thanks a mil!
 
"Daniel Morrow" <videoman@ccountry.net> wrote in message
news:11bh70u7garboe7@corp.supernews.com...
I want to buy some blue leds from digikey (haven't heard back from
them yet
about this) and their catalog doesn't list the maximum current rating
for
any of the leds they carry, the catalog only lists the maximum voltage
rating. How can I find out the maximum current rating without trial
and
error for each individual led and without a risky
potentiometer/multitester
method? I need to know the maximum current rating of these blue leds
so I
can use the proper resistor with the leds so that the leds do not get
damaged. Thanks a mil!
Click to expand...
Don't expect too much from Digi-key.

In general the 5mm LED package can dissipate 100mW, so for a 4V maximum,
that's 25mA. But a lot depends on how well the heat is conducted away
fhru the leads. That being said, the usual white or blue LED is usually
rated for 20mA max. But it's common to see them run at 30mA, which at
3.3V drop gives 100mW dissipation. See www.nichia.com for specs on some
commonly used LEDs. See Don K's LED pages for a lot more info.
http://members.misty.com/don/ledx.html
 
"Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com> wrote
in message news:11bhst9o9djvqdd@corp.supernews.com...
"Daniel Morrow" <videoman@ccountry.net> wrote in message
news:11bh70u7garboe7@corp.supernews.com...
I want to buy some blue leds from digikey (haven't heard back from
them yet
about this) and their catalog doesn't list the maximum current rating
for
any of the leds they carry, the catalog only lists the maximum voltage
rating. How can I find out the maximum current rating without trial
and
error for each individual led and without a risky
potentiometer/multitester
method? I need to know the maximum current rating of these blue leds
so I
can use the proper resistor with the leds so that the leds do not get
damaged. Thanks a mil!

Don't expect too much from Digi-key.

In general the 5mm LED package can dissipate 100mW, so for a 4V maximum,
that's 25mA. But a lot depends on how well the heat is conducted away
fhru the leads. That being said, the usual white or blue LED is usually
rated for 20mA max. But it's common to see them run at 30mA, which at
3.3V drop gives 100mW dissipation. See www.nichia.com for specs on some
commonly used LEDs. See Don K's LED pages for a lot more info.
http://members.misty.com/don/ledx.html
Click to expand...
Thanks a mil watson, thanks!
>
 
Daniel Morrow wrote:
I want to buy some blue leds from digikey (haven't heard back from them yet
about this) and their catalog doesn't list the maximum current rating for
any of the leds they carry, the catalog only lists the maximum voltage
rating. How can I find out the maximum current rating without trial and
error for each individual led and without a risky potentiometer/multitester
method? I need to know the maximum current rating of these blue leds so I
can use the proper resistor with the leds so that the leds do not get
damaged. Thanks a mil!


Maybe you can help me. I have seen your question, or a variation
Click to expand...
quite often. Personally, I am too chicken to run a LED at or
near its maximum current, except experimentally. What benefit
do you see to running a blue LED near its max? Or is it a question
of finding the max, so that you can run it well below that?

So far (again, aside from experimenting) I have seen no
advantage to running LEDs "hard" (for lack of a better term).
I figure 10 mA (or less) as the design figure, unless there
is a specific reason to go over that. That's not a reccomendation,
it's just to give you the background behind my question.

Ed
 
On 22 Jun 2005 05:22:59 -0700, the renowned "Tim Shoppa"
<shoppa@trailing-edge.com> wrote:

and their catalog doesn't list the maximum current
rating for any of the leds they carry, the catalog
only lists the maximum voltage rating.

You are not interpreting the listings correctly.

Every LED in the Digikey catalog has a listed "IF (mA)". This is the
maximum continuous current.

The forward voltage drop (VF) is not the "maximum voltage", it's the
typical voltage at the given I_F. V_F is usually not a guaranteed
parameter, solid state physics makes determines it.

Most (but not all) components in the Digi-Key catalog have a PDF data
file on their website. Very useful.

Tim.
Click to expand...
Anyway, there's not just one number here. It depends on the nature of
the current (is it DC?) and the ambient temperature, and how close to
the maximum you'd like to run the junction. Blue (and white) LEDs tend
to have the highest voltage drop of all common colors, so they get the
hottest at a given current.

Eg. http://rocky.digikey.com/WebLib/Fairchild/Web%20Data/MV8B01.pdf

If (abs max.) is 30mA at 25°C.

But in Fig. 5 you can see that at 75°C you need to reduce that to
20mA. If you work at 50-70% of the maximum, you're at about 10-14mA.
That's probably about right for most ordinary 5mm blue LEDs. If you
put resistors or other parts nearby that raise the ambient you'll want
to reduce the current further. They probably assume the leads are
short and soldered into a PCB with certain characteristics. If its
hanging out in mid-air on long legs you'll have to further reduce it
or reduce the maximum ambient.
 
Or is it a question
of finding the max, so that you can run it well below that?
Click to expand...
Exactly what I am trying to do Ed. This project of mine is hobbyist-grade, I
am not trying to make a military-grade weapon or anything like that - just
to let you know. I do think it is awesome you guys are so precise - don't
get me wrong about what I said before. Looking forward to trying out the
previous information from you guys. I never plan on running any led "hard",
in fact I need the maximum forward current specs so that I can make sure I
never damage or destroy the leds I use. Interestingly I received an answer
back from digikey and the rep. said that indeed the specs I am looking for
are not on the web or in their catalog and he sent me a data sheet with his
e-mail answer specifying the maximum forward current for the led I inquired
about. I much prefer having all of the specs online in case I change my mind
about what led to get and it keeps the bandwidth free for other questions
instead of clogging up the internet with requests for specs, plus I prefer
to not wait for a response. Later!
 
"ehsjr" <ehsjr@bellatlantic.net> wrote in message
news:bUjue.13443$tG.11373@trnddc05...
Daniel Morrow wrote:
I want to buy some blue leds from digikey (haven't heard back from
them yet
about this) and their catalog doesn't list the maximum current
rating for
any of the leds they carry, the catalog only lists the maximum
voltage
rating. How can I find out the maximum current rating without trial
and
error for each individual led and without a risky
potentiometer/multitester
method? I need to know the maximum current rating of these blue leds
so I
can use the proper resistor with the leds so that the leds do not
get
damaged. Thanks a mil!


Maybe you can help me. I have seen your question, or a variation
quite often. Personally, I am too chicken to run a LED at or
near its maximum current, except experimentally. What benefit
do you see to running a blue LED near its max? Or is it a question
of finding the max, so that you can run it well below that?

So far (again, aside from experimenting) I have seen no
advantage to running LEDs "hard" (for lack of a better term).
I figure 10 mA (or less) as the design figure, unless there
is a specific reason to go over that. That's not a reccomendation,
it's just to give you the background behind my question.

Ed
Click to expand...
I came across a receipt from Rat shack back in 2001 when I bought my
first white LEDs. They were four bucks apiece, and I found some others
at a local electronics store for even more. Out of the flashlights I
was building I was trying to get an amount of light that was competitive
with an incandescent lamp, but without spending a fortune. So pushing a
few LEDs, maybe 3 to 6, to their maximum was the only way to go. When I
bought the first hundred Nichias for over $200, I was under a bit less
pressure, but still it's not easy to see most of the cost of a
flashlight tied up in just the white LEDs.

Nwowadays the cost is a dollar each so it's not a ptoblem using twice as
many, but still there's issues of space and complexity.
 
and their catalog doesn't list the maximum current
rating for any of the leds they carry, the catalog
only lists the maximum voltage rating.
Click to expand...
You are not interpreting the listings correctly.

Every LED in the Digikey catalog has a listed "IF (mA)". This is the
maximum continuous current.

The forward voltage drop (VF) is not the "maximum voltage", it's the
typical voltage at the given I_F. V_F is usually not a guaranteed
parameter, solid state physics makes determines it.

Most (but not all) components in the Digi-Key catalog have a PDF data
file on their website. Very useful.

Tim.
 
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