how can i find led specs

[gcb]

I'm just starting, and still on the 'hello LED world' steps...

i have a bunch of leds, and no idea what are their voltage drops and other attributes... heck i can't even tell the anode from the cathode in most of them without testing.

Is there a standard i can consult? or are they completely non-standard?

i mean, can i infer the attributes by any markings? size? shape? Or if I don't have the manufacturer specs can i very well throw them off?

 

[CocaCola]

Is there a standard i can consult? or are they completely non-standard?

Non standard, but you can make educated guesses based upon output color and intensity...

Standards are generally safe at 2.0 - 2.5 volts and about 10 - 15mA

Super brights are generally safe at 3.0 - 3.5 volts and about 20 - 25mA

 

[BobK]

If you are talking about 3mm or 5mm round leaded LEDs, what CocaCola said. I would add the the voltage drop depends ,more on the color than the power. Red will have the lowest, 1.8 to 2.5V. Blues and whites will be more like 3.3 to 3.6V.

All of the higher power LEDs I have ever seen are surface mount.

Bob

 

[donkey]

this might help with the anode, cathode thing too
notice the anode is longer

 

[davenn]

what a pretty pic

Steve, maybe that should go in the LED's sticky

cheers
Dave

 

[KJ6EAD]

You can set up a 20mA constant current source and measure the Vf. If you've got a 20 - 30 Volt supply, you can run several LEDs together in a series string on a breadboard to measure them. Be sure you have a 20mA current then measure the individual voltage drops.

 

[(*steve*)]

On a practical basis, this is what I would do...

I would get a battery of 4.5V to 6V, a 1k resistor, and a multimeter (and of course the unidentified LEDs).

Connect the battery and the resistor in series and try the LED out (both ways, until it lights). Once it is lit, measure the voltage across it. This will be similar to Vf for the LED (close enough for calculating series resistors).

The 4.5V to 6V has been chosen because it is greater than the Vf of any single LED, but is also safe as a reverse voltage.

In use, most LEDs are fine at 20mA and without further information I would not use a higher current.

Once you know Vf, there are many sites listing details for calculating the appropriate series resistors for use with various voltage supplies.

 

[gcb]

Non standard, but you can make educated guesses based upon output color and intensity...

Standards are generally safe at 2.0 - 2.5 volts and about 10 - 15mA

Super brights are generally safe at 3.0 - 3.5 volts and about 20 - 25mA

+

If you are talking about 3mm or 5mm round leaded LEDs, what CocaCola said. I would add the the voltage drop depends ,more on the color than the power. Red will have the lowest, 1.8 to 2.5V. Blues and whites will be more like 3.3 to 3.6V.

All of the higher power LEDs I have ever seen are surface mount.

Bob

So it's safe to assume the vast majority if LEDS are one standard, that's there's no marking?

i gave up using the small ones and used the 10 segment one as it had markings. with the markings i could find specs. the chinese site mentions 1.8V-2.3V, 25mA. the leds are red (even though i bought green. go china!)

using calculations from this site (highly recomended!) http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/?topic=presentation

I attached 220Ohms and they lit very brightly. Which still goes against my comon sense that it should have been brighter without any resistors but soon i will end reading up that site. i hope.

this might help with the anode, cathode thing too
notice the anode is longer

I can't see that in any of the leds i have Or they are in weird enclosures or are too small.

You can set up a 20mA constant current source and measure the Vf. If you've got a 20 - 30 Volt supply, you can run several LEDs together in a series string on a breadboard to measure them. Be sure you have a 20mA current then measure the individual voltage drops.

Most of my boards, supplies, learning stuff is around 1.5V, 3V and 5V

now i will go do steve's method and report back!

 

[(*steve*)]

Without resistors, LEDs are typically very very bright, then they stop working.

Most LEDs are unlabelled because there's no easy way to label them. I have seen some that have metal bodies, and they were labelled.

But yeah, LEDs are unusual in that there are so many varieties with so many specs that you would think they would have a part number on them. The best you'll get in most cases is this information on the bulk packaging.

Even if you know the part number, they can be *really* hard to get the specs on. In most cases, people don't really care about the specs because they can use them in a certain manner that can be deduced from certain "headline" specs (power rating -- in the case of high voltage LEDs). This is particularly true when you're using them as indicators. It often doesn't matter exactly what the half angle is or the exact spectral information, or the brightness at differing currents.

The poor things get taken for granted

 

[gcb]

Without resistors, LEDs are typically very very bright, then they stop working.

that was my gut feeling. but the ones i have, applying 5v 1A no resistor, they have a really washed down light, and i can see it oscillating slightly.

with the 220Ohms, it has a very bright glow

 

[donkey]

what type of LED did you buy? the issue is these days LED's come in several assortments. the pic I posted was for the generic LED which alot of us use in test phase. (I like other ones like ultra brights for personal reasons.... mostly I like blinding people)
also did you get these LEDs from a pack or did you remove them from other projects or faulty equipment?
in both cases there is a possibility that the LEDs are different to each other.

 

[gcb]

what type of LED did you buy? the issue is these days LED's come in several assortments. the pic I posted was for the generic LED which alot of us use in test phase. (I like other ones like ultra brights for personal reasons.... mostly I like blinding people)
also did you get these LEDs from a pack or did you remove them from other projects or faulty equipment?
in both cases there is a possibility that the LEDs are different to each other.

most of them i got from dead eletronics. I got a few now that looks like your image, but i needed a loupe with 30x magnification to see it and they are not even SMD ones.

the only one i bought and coincidental the only one with markings was this one http://home.att.ne.jp/green/w-elekitmart/PartsDetail_LED-BarGraphR.html

 

[donkey]

what country you in? recycling parts is a good idea but in most countries you can pick up a pack of 100 LED's for cheap(depending on country that is)
http://www.jaycar.com.au/productView.asp?ID=ZD1690&form=CAT2&SUBCATID=976#1
for example, these are all new. you may get one or 2 that are faulty but most will work and you can guarantee the specs on them.

 

[gcb]

which would get me back to step one. As i bet the new ones wouldn't have spec marks either

 

[(*steve*)]

Then assume 20mA...

 

[KrisBlueNZ]

On a practical basis, this is what I would do...
I would get a battery of 4.5V to 6V, a 1k resistor, and a multimeter (and of course the unidentified LEDs).
Connect the battery and the resistor in series and try the LED out (both ways, until it lights). Once it is lit, measure the voltage across it. This will be similar to Vf for the LED (close enough for calculating series resistors).
The 4.5V to 6V has been chosen because it is greater than the Vf of any single LED, but is also safe as a reverse voltage.

As usual, Steve's advice is excellent.
I would use three AA or AAA cells in series (4.5~5.0V brand new) and a resistor round 1K.
The only thing I can add here is that the pretty drawing of the LED does not hold true for ALL LEDs. I have some low-current devices where the "cup" (the larger bit in the silhouette, which you can see is a kind of cup when viewed from above) is actually the anode. But it's true that most LEDs follow the "cup is cathode" rule.

 

[CocaCola]

I have some low-current devices where the "cup" (the larger bit in the silhouette, which you can see is a kind of cup when viewed from above) is actually the anode. But it's true that most LEDs follow the "cup is cathode" rule.

I have had both kinds as well, tosses you for a loop when you get accustomed to focusing on the insides vs the legs or flat...

what country you in? recycling parts is a good idea but in most countries you can pick up a pack of 100 LED's for cheap(depending on country that is)
http://www.jaycar.com.au/productView...SUBCATID=976#1
for example, these are all new. you may get one or 2 that are faulty but most will work and you can guarantee the specs on them.

WOW, $18.95 for 100? Since the US and AU dollar are about equal right now that is honestly crazy expensive for 100 red LEDs at least compared to what I can get them for in the US, for example under $2...

http://www.circuitspecialists.com/bag-red5mm.html
or
http://www.mpja.com/prodinfo.asp?number=15108+OP

Superbrights are even dirt cheap...

http://www.amazon.com/Red-5mm-LED-4000-Pcs/dp/B0066AZ0JY

I purchase LEDs by the 1000 direct from Asia as it works out even cheaper most of the time...

 

[CocaCola]

I have some low-current devices where the "cup" (the larger bit in the silhouette, which you can see is a kind of cup when viewed from above) is actually the anode. But it's true that most LEDs follow the "cup is cathode" rule.

I have had both kinds as well, tosses you for a loop when you get accustomed to focusing on the insides vs the legs or flat...

what country you in? recycling parts is a good idea but in most countries you can pick up a pack of 100 LED's for cheap(depending on country that is)
http://www.jaycar.com.au/productView...SUBCATID=976#1
for example, these are all new. you may get one or 2 that are faulty but most will work and you can guarantee the specs on them.

WOW, $18.95 for 100? Since the US and AU dollar are about equal right now that is honestly crazy expensive for 100 red LEDs at least compared to what I can get them for in the US, for example under $2...

http://www.circuitspecialists.com/bag-red5mm.html
or
http://www.mpja.com/prodinfo.asp?number=15108+OP

Superbrights are even dirt cheap...

http://www.amazon.com/Red-5mm-LED-4000-Pcs/dp/B0066AZ0JY

I purchase LEDs by the 1000 direct from Asia as it works out even cheaper most of the time, but I have purchased those 100 baggies of normal red/yellow/green from both those companies in the past and never had an issue with them not performing or having lots of deads... Can't always say the same for the Asian ones, it's sometimes hit or miss depending on the company I purchase from, but even worst case I would say no more than 2-3% dead...

 

[(*steve*)]

I have some low-current devices where the "cup" (the larger bit in the silhouette, which you can see is a kind of cup when viewed from above) is actually the anode. But it's true that most LEDs follow the "cup is cathode" rule.

Yes. When I added that image to my LED thread, I noted that it is "usually" the case.

I have seen this differ, most recently in the case of a device which "flickered" the LED -- a candle simulation LED.

These had an internal structure exactly the opposite of what you expect from a normal LED and to make matters worse, they fail after even a small reverse voltage is applied across them. I didn't attempt to determine how low, but 5V did it -- even with a series resistor that they don't need when operated normally!

 

[KrisBlueNZ]

I have seen this differ, most recently in the case of a device which "flickered" the LED -- a candle simulation LED.
These had an internal structure exactly the opposite of what you expect from a normal LED and to make matters worse, they fail after even a small reverse voltage is applied across them. I didn't attempt to determine how low, but 5V did it -- even with a series resistor that they don't need when operated normally!

How silly. They put a chip in there to flicker the LED and they don't even bother including a reverse-connected diode on it.