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Typically the guidance seen is that on LEDs the long pin is the anode and the short pin is the cathode. Often it's also said that if you look inside the LED, the large plate is the cathode while the small plate is the anode.

However, this Lite-On LED has the anode on the short pin according to the datasheet. The short pin is still connected to the large plate, however.

I've confirmed that you must wire the short pin to the positive terminal of the battery for the LED to light up. What gives? First of all, is there a reason the convention is broken? Also, how come the plates are also seemingly reversed? Is there any particular reason for this or is it just manufacturer discretion?

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  • \$\begingroup\$ You'd have to ask Lite-On those questions... \$\endgroup\$ – TDHofstetter Aug 16 '14 at 22:09
  • \$\begingroup\$ +1 - an intriguing question, especially if there is a concrete reason for it, with more rationale than 'we can'. \$\endgroup\$ – gbulmer Aug 16 '14 at 23:32
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    \$\begingroup\$ Um... referencing my deleted answer... is the point NOT to directly answer the asker's question in the clearest manner possible? The question was about why the manufacturer chose to sidestep convention, and the manufacturer is the only possible source of the ultimate answer. True? \$\endgroup\$ – TDHofstetter Aug 17 '14 at 0:48
  • \$\begingroup\$ @TDHofstetter: Yes, the point is to answer the question in the clearest manner possible. And a non-answer (i.e., go ask someone else) would definitely not fulfill that. Particularly when the accepted answer below gives a reason that explains how this type of diode has the plates flipped. \$\endgroup\$ – jcoleman Aug 17 '14 at 3:44
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I've noticed that the die is 'flipped' on AlGaAs SR LEDs (the bonding wire goes to the opposite side of the die) compared to other colors so I strongly suspect Lite-On wanted to standardize on a single lead frame type for all colors in the series, which necessitates reversing the SR ones.

You may have noticed that similar LEDs in the series such as the yellow LTL4252N have the longer lead as the anode:

http://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTL-4252N.pdf

I would find this very annoying- our standard footprint for LEDs and radial electrolytic caps has a big silk screen circle around the positive hole.

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    \$\begingroup\$ Excellent answer! I figured there had to be a specific reason--particularly given the fact that it wasn't just the lead length difference but also the plate sizes being flipped. \$\endgroup\$ – jcoleman Aug 17 '14 at 3:42

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