I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode? (I'm also rather
proud of the photo. ;-) ) Neither of my ohmmeters even makes it conduct;
I'll probably have to set up a little tester, but I thought it was kinda
cool the way the leads hold the chip and so on.

Cheers!
Rich

"Rich Grise"

I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode?


** The longer lead is always the anode - impossible to tell that if the
leads have been cut.

So a test IS required for you to know.


(I'm also rather
proud of the photo. ;-) ) Neither of my ohmmeters even makes it conduct;
I'll probably have to set up a little tester, but I thought it was kinda
cool the way the leads hold the chip and so on.


** Mostly, the LED chip sits on the cathode lead - but this is not always
true.

See pic for example of the opposite.




....... Phil

On Wed, 11 Jul 2007 12:28:16 +1000, "Phil Allison"
wrote:


"Rich Grise"

I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode?


** The longer lead is always the anode - impossible to tell that if the
leads have been cut.

Not true. One lead is usually thicker in one dimension where it exits
the device body, if not all the way down the lead.

That is for stamped lead frame designs. Tin or gold wire designs differ,
but the as manufactured lead length indicator is the most common, if, as
you say, they have not been trimmed.

So a test IS required for you to know.

There is another way. The reflector cup is usually always tied to the
same side. Your example may bust that.



(I'm also rather
proud of the photo. ;-) ) Neither of my ohmmeters even makes it conduct;
I'll probably have to set up a little tester, but I thought it was kinda
cool the way the leads hold the chip and so on.


** Mostly, the LED chip sits on the cathode lead - but this is not always
true.

See pic for example of the opposite.




...... Phil

"Spurious Ratbag "


Not true. One lead is usually thicker in one dimension where it exits
the device body, if not all the way down the lead.


** ********.




...... Phil

"Rich Grise" wrote in message
news
I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode? (I'm also rather
proud of the photo. ;-) ) Neither of my ohmmeters even makes it conduct;
I'll probably have to set up a little tester, but I thought it was kinda
cool the way the leads hold the chip and so on.

Cheers!
Rich


Let me know if I'm right....

I believe the cathode is the upper lead in your photo.
The feature I'm recognizing is the 'anvil' to which the
die is attached.

Rich Grise wrote:

I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode?

Yes, the big one (that carries the die) is the cathode, but I see Phil found an
exception.

Graahm

"Eeyore" wrote in message
...


Rich Grise wrote:

I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode?

Yes, the big one (that carries the die) is the cathode, but I see Phil
found an
exception.

Graahm


The bulk of the LEDs I've worked with will have the anvil (the side the die
mounts to) as cathode while the cat whisker (in 1910 lingo g) is the
anode. I have seen exceptions to this - iirc, it was a GaAlAs led.

I still cringe when I remember getting my first monsanto led samples in 1970
and not realizing they were current fed devices. I connected it to a bench
supply that was capable of several Amps and slowly cranked up the voltage
adjust. Pleased at the light output, I turned to write some notes... just as
the glass lens of the led exploded and shot across the lab. So much for
assumptions. For the second of the two samples, I thouroghly read the data
sheet. Not nearly as much light but it didn't explode.
Oppie

On Wed, 11 Jul 2007 05:42:47 +0000, Lord Garth wrote:
"Rich Grise" wrote in message
news
I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode? (I'm also rather proud
of the photo. ;-) ) Neither of my ohmmeters even makes it conduct; I'll
probably have to set up a little tester, but I thought it was kinda cool
the way the leads hold the chip and so on.

Let me know if I'm right....

I believe the cathode is the upper lead in your photo. The feature I'm
recognizing is the 'anvil' to which the die is attached.

Yup! The "anvil" side is the cathode. And it's so easy to see, I don't
even need my magic marker. ;-)

Thanks!
Rich

"Oppie" wrote in message
...


The bulk of the LEDs I've worked with will have the anvil (the side the
die
mounts to) as cathode while the cat whisker (in 1910 lingo g) is the
anode. I have seen exceptions to this - iirc, it was a GaAlAs led.

I still cringe when I remember getting my first monsanto led samples in
1970
and not realizing they were current fed devices. I connected it to a bench
supply that was capable of several Amps and slowly cranked up the voltage
adjust. Pleased at the light output, I turned to write some notes... just
as
the glass lens of the led exploded and shot across the lab. So much for
assumptions. For the second of the two samples, I thouroghly read the data
sheet. Not nearly as much light but it didn't explode.


Ahh, the explosion! I recall the exploding shunt diode (1N4002) that
occured
before I was aware that the crap VOM I had reversed the lead polarity when
reading resistance. That was about 1969 or so, I was repairing a car radio
when it went * BANG *!

On Wed, 11 Jul 2007 01:34:57 GMT, Rich Grise wrote:

I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode? (I'm also rather
proud of the photo. ;-) ) Neither of my ohmmeters even makes it conduct;
I'll probably have to set up a little tester, but I thought it was kinda
cool the way the leads hold the chip and so on.

The other guys have already given you the clues that work for almost
all of the leaded LEDs out there. For the exceptions (and to help
matching color/brightness) this little gizmo is cheap enough to be
handy. (Jameco is just one source)
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=355805

Rich Grise wrote:

Yup! The "anvil" side is the cathode. And it's so easy to see, I don't
even need my magic marker. ;-)

As has already been mentioned, this is by far the most likely, though not
always the case. I've been caught out on some "odd" leds I thought were
otherwise "normal".

Would be worth testing one in the batch and assuming the same for the rest.

--
Linux Registered User # 302622
http://counter.li.org

Eeyore wrote:

Rich Grise wrote:

I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode?

Yes, the big one (that carries the die) is the cathode, but I see Phil found an
exception.

Usually is the key word,That's usually my experience too,but I've found
a few LED's that were "backwards",so beware.

On Thu, 12 Jul 2007 08:53:30 +1000, John Tserkezis wrote:
Rich Grise wrote:

Yup! The "anvil" side is the cathode. And it's so easy to see, I don't
even need my magic marker. ;-)

As has already been mentioned, this is by far the most likely, though
not
always the case. I've been caught out on some "odd" leds I thought were
otherwise "normal".

Would be worth testing one in the batch and assuming the same for the
rest.

Yeah - that's what I just did and am reporting back to the group on my
success. :-)

Thanks!
Rich

Rich Grise wrote:

Yeah - that's what I just did and am reporting back to the group on my
success. :-)

What was worse in my experience, the notch cut at the skirt of the LED, AND
the length of the legs indicated otherwise.

Not only that, I've seen this TWICE. Different decades, different
suppliers, different sources.

Because of this, I now instinctively try the LED both ways before even
looking at it.

"Industry standard" markers my butt.
--
Linux Registered User # 302622
http://counter.li.org

"Rich Webb" wrote in message
...
On Wed, 11 Jul 2007 01:34:57 GMT, Rich Grise wrote:

I've just salvaged some LEDs - here's one:
http://mysite.verizon.net/richgrise/...-green-LED.JPG

and I'm wondering if anybody knows how to tell from the shape of the
leadframe parts, which is the cathode and anode? (I'm also rather
proud of the photo. ;-) ) Neither of my ohmmeters even makes it conduct;
I'll probably have to set up a little tester, but I thought it was kinda
cool the way the leads hold the chip and so on.

The other guys have already given you the clues that work for almost
all of the leaded LEDs out there. For the exceptions (and to help
matching color/brightness) this little gizmo is cheap enough to be
handy. (Jameco is just one source)
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=355805


Another simple polarity tester is a 9V PP3 with your body as the
current-limit resistor. You may need to lick your fingers, but there's no
risk of a reverse voltage damaging the LED.