Backward LED Leads

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haklesup
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Backward LED Leads

Post by haklesup » Tue Mar 02, 2010 10:43 pm

Just about every LED (in a T-1 package) I have ever seen has one long lead and one shorter; and the long lead has always been the anode (positive). Recently we had a customer spec a lamp that was backward, but it's not a flawed device, that's how it was intended.

http://optoelectronics.liteon.com/en-us ... =LTL-4261N

Has anyone seen this configuration before. If so why would a manufacturer want to make a few parts that are reverse of most of the rest of their product line?

http://optoelectronics.liteon.com/en-us ... 78F339C2C0

Needless to say this ultimately led to an assembly error and our faces were LED red with embarrassment.

I still need to find out how we came about those parts

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MrAl
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Re: Backward LED Leads

Post by MrAl » Wed Mar 03, 2010 5:29 am

Hi there,


I dont know if that longer lead thing is a standard written down anywhere or not, but what i always
looked for (and created silk screen pc board layout drawings for) was not the longer lead but the
'flat' on the negative side of the LED package. That 'flat' was only on the side where the negative
lead was and the other side was perfectly round like the rest of the package. This allowed the
pc board parts layout pattern to have a round circle with part of it a straight line that indicated
the flat that would be located near the hole that took the negative lead. It would look sort of
like the top view of a TO92 package transistor (not a half round but something like that with more
round part than straight line part).

warning: poor azz drawing follows...

Code: Select all

            
                  *  *
              *           *
           *                *
          |                   *
          |                    *
negative  |  O             O   * positive
          |                    *
          |                   *
           *                *
             *            *
                  *  *
I see those parts dont seem to follow that convention either though, so they must be making them
on another planet where manufacturers make parts and ignore what is typical here on earth :smile:

I remember something else like this happening many years ago. The person who did the silk screen
drawing for a commonly used pc board went by the drawing for a particular negative voltage regulator
IC package when placing a positive voltage regulator package which had the input and output pins
swapped. Test technicians were blowing out regulators for years before the problem was finally
fixed with the board drawings update.
LEDs vs Bulbs, LEDs are winning.

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Externet
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Re: Backward LED Leads

Post by Externet » Wed Mar 03, 2010 1:12 pm

Hello haklesup.
Yes, seen that several times. Since then, cannot rely on any flat side or lead lenght 'standard' ; I just apply power to make sure on what polarity they turn on and touch black permanent marker on the cathode.

Flat cathode --->http://www.instructables.com/files/deri ... .SMALL.jpg
Flat cathode --->http://www.semicon.panasonic.co.jp/ds4/SHD00562BEK.pdf

Flat anode --->http://media.digikey.com/pdf/Data%20She ... L(UQS).pdf
Flat anode --->http://media.digikey.com/pdf/Data%20She ... AL(UR).pdf

Want it worse? Flat side on the side --> Look at figure 1 :

http://dkc3.digikey.com/PDF/US2010/P2776.pdf

Figures 2 and 4 are different flat side polarity.

Miguel
- Abolish the deciBel ! -

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haklesup
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Re: Backward LED Leads

Post by haklesup » Wed Mar 03, 2010 4:43 pm

So I took the time to look at many datasheets for LEDs (round, rectangular, oval, various colors). The vast majority of them are what I am going to call Conventional with the long lead being anode and the flat side being cathode. However, there is a selection of non conventional parts with the leads and flat reversed or the falt removed entirely.

I can only assume that these parts are intended to remidy board layout flaws when they get to assembly. Consider the goofup where a designer reverses the silk screen symbol around an LED. The assembly people will go nuts trying to remember to insert that one part backward, this can and does lead to errors that require rework. Furthermore, it would be tricky to inspect those boards and remember about this device. If however the OEM would purchase and kit the non conventional LEDs then the device will match the silkscreen. Only the purchaser needs to worry about it after the engineer decides to go that way.

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Re: Backward LED Leads

Post by jverive » Thu Mar 04, 2010 7:43 pm

Applying power that is current limited is the only "foolproof" method (including the use of a curve tracer.) Several years ago our manufacturing floor was debugging assemblies that had reversed LEDs, but after determining the proper polarity and reversing the parts, many of them didn't work. I asked to see their "polarity verifier" and discovered it to be just a 9V battery in a case with two probes. They were frying the little devices! i knew the problem right away because of the smell in the area; a person rarely forgets the pungent aroma of fried LEDs!

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Externet
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Re: Backward LED Leads

Post by Externet » Thu Mar 04, 2010 8:40 pm

- Abolish the deciBel ! -

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MrAl
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Re: Backward LED Leads

Post by MrAl » Fri Mar 05, 2010 4:22 am

Hi,

Yes, you can not test LEDs with a 9v battery and series resistor because if the LED is backwards the
LED either blows out immediately or gets damaged and then may blow out sometime later.
Many LEDs have a reverse voltage rating of 5v, and when connected backwards they dont draw
any current so the series resistor does not drop any voltage and so the LED sees the full 9v
reverse voltage which could damage it.

Best bet is to use three AA or AAA cells in series with a series resistor to test LEDs. That puts
a max of about 4.8v across the LED if it gets connected backwards, yet is still enough voltage
to forward bias most types of LEDs so you know when you get it right.
LEDs vs Bulbs, LEDs are winning.

Dean Huster
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Re: Backward LED Leads

Post by Dean Huster » Sat Mar 06, 2010 5:38 pm

Lessee ... the tab on a TO-5 or TO-18 transistor package indicated the emitter. It got a little confusing when a fourth lead was added, but it was always the emitter for the tab. I've seen the base and collector swapped after that.

The TO-92 tended to have a somewhat standard lead configuration except for when it didn't. Same with TO-3, TO-220 ....

Well, OK. How's this: I've never seen an exception to the dot, indentation, stripe or whatever indicating "pin 1" of a DIP IC.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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