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enter image description here

In the figure above, LED A is a regular 5mm LED and the visual identification of its terminals polarity is well known.

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As you can see, on LED B the anode is identified by the longer leg, but the other rules for visual identification are inverted.

I've confirmed the situation above with a multimeter.

I have no datasheet or reference for LED B.

I never saw an LED with these issues before. Is LED B the result of a manufacturing error?

Thanks in advance.

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  • \$\begingroup\$ Is it possible someone has trimmed sample B's leads after it left the factory? Anyway it's easy enough to test with a multimeter. \$\endgroup\$ – The Photon Sep 16 at 19:38
  • \$\begingroup\$ In this case the leds are new and have all the same issue. \$\endgroup\$ – AmssmA Sep 16 at 19:45
  • \$\begingroup\$ Are A and B the same color? What results do you get when you test with a multimeter? \$\endgroup\$ – The Photon Sep 16 at 19:48
  • \$\begingroup\$ No they have different colors LED A e bright white and LED B e red. I don't have the leds now wtih me so i can not tell you the Vf values \$\endgroup\$ – AmssmA Sep 16 at 19:52
  • \$\begingroup\$ I've yet to encounter an LED that had the anvil be positive, but I imagine they probably do exist. \$\endgroup\$ – Hearth Sep 16 at 20:06
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Super-Red (AlGaAs) LED dice are mounted the opposite way of most others, so they appear backwards from the silhouette of the leadframe.

The leadframe pin length should tell you the difference but there's been a case of one identified in another question here that's reversed even for that (presumably so they didn't have to buy two leadframe types or trim them differently- it's described in the datasheet so it's not an error).

So, the bottom line is "no" you can't trust anything that's not specified in the datasheet (and even then, stuff happens, I've seen a 1N400x diode with reversed markings, presumably dropped and replaced on the line- post test, pre-marking by a worker in Taipei where the factory was at the time).

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  • \$\begingroup\$ @ChrisStratton Maybe made all the same way? Some of the LED encapsulation is very manual and some less so, IME. Anyway, if it's supplied without a datasheet who knows. You buy 10K LEDs at the market in Shenzhen it's not necessarily the same as the next batch. Practically the Wild West out there. \$\endgroup\$ – Spehro Pefhany Sep 16 at 20:16
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Unfortunately manufacturers and LED's are all different, and there is not a good way to determine what the polarity is, I've seen datasheets that are wrong at times.

The best thing to do would be to use a digital multi meter with a diode tester, then reverse the polarity

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