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I have this IR LED

enter image description here

IR LEDs emit when forward biased right? And we all know that the longer leg is positive.

When I forward-bias this LED it acts as an open circuit. And it emits when reverse biased!

Would you please help me sorting this out? If you want further explanation please let me know.

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  • \$\begingroup\$ Datasheet please. \$\endgroup\$
    – Bort
    Commented Jan 27, 2018 at 16:47
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    \$\begingroup\$ Clearly the longer leg is not positive. \$\endgroup\$
    – Matt Young
    Commented Jan 27, 2018 at 16:47
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    \$\begingroup\$ Long leg is not always positive. I have a set of red leds which have the anode on the short side. \$\endgroup\$
    – Passerby
    Commented Jan 27, 2018 at 16:48
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    \$\begingroup\$ @Passerby I have also had LEDs that are totally backwards. \$\endgroup\$
    – vofa
    Commented Jan 27, 2018 at 17:05
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    \$\begingroup\$ Alternative solution: re-cut the leads so that the short is - and the long is +. \$\endgroup\$
    – Fredled
    Commented Jan 27, 2018 at 18:06

2 Answers 2

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Quite simply it is backwards from how standard LEDs are configured.

Usually the long leg is the anode and the leg that connects to the bigger metal object inside the plastic case is the cathode, but that is not always the case.

For example:

You'll have to trust me that the leg with the resistor soldered to it was shorter than the other one, but the LED only illuminates when the longer lead is connected to battery negative.

enter image description here

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  • \$\begingroup\$ No; the issue with this picture is that the anode (+) is on the reflective cup instead of the wirebond (see answer above by Tony Stewart) contradicting Tony's explanation. \$\endgroup\$
    – Fredled
    Commented Jan 27, 2018 at 18:01
  • \$\begingroup\$ Yes this is what I am talking about. Much appreciated. \$\endgroup\$
    – Macit
    Commented Jan 27, 2018 at 18:04
  • \$\begingroup\$ What is the type of this LED \$\endgroup\$
    – Macit
    Commented Jan 27, 2018 at 18:04
  • \$\begingroup\$ @Majid_L - No idea, it came in a random assortment of loose parts. \$\endgroup\$
    – vofa
    Commented Jan 27, 2018 at 18:05
  • \$\begingroup\$ @Fredled - I don't understand your objection or I am missing something. The actual anode being on the cup is exactly the cause of OP's problem. Its physical terminations are backwards of the prevailing convention, leading to an incorrect assumption in how it should be connected. \$\endgroup\$
    – vofa
    Commented Jan 27, 2018 at 18:09
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Modern LEDs have a transparent substrate so much of the optical energy reflected from the parabolic reflector and then the distance from the 5mm lens determines the angle of the beam.

The cathode is always the substrate, so this is handy to know when both leads are cut short and may have no flat edge on case. ( also cathode mark)

So remember this.

The gold wirebond goes to the center of the Anode (+)
The reflector cup is always the Cathode (-) enter image description here

I also added some other interesting factoids to my photo. More importantly REMEMBER THIS;

Murphy's Law

- If anything can be backwards , It will be.

Always double check the datasheet and if not avail. ( bad choice) when in doubt use the diode test.

  • weird example enter image description here
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  • \$\begingroup\$ Thank you for your answer I am sure the info you have given is right but in my case I still get the LED emit when reverse biased. \$\endgroup\$
    – Macit
    Commented Jan 27, 2018 at 17:20
  • \$\begingroup\$ Then explain this: i.sstatic.net/AIHus.jpg \$\endgroup\$
    – vofa
    Commented Jan 27, 2018 at 17:33
  • \$\begingroup\$ Looks like a home made LED not a commercial one with leadframes \$\endgroup\$
    – D.A.S.
    Commented Jan 27, 2018 at 19:18
  • \$\begingroup\$ @TonyStewart.EEsince'75 I assure you it was acquired commercially. \$\endgroup\$
    – vofa
    Commented Jan 27, 2018 at 19:32
  • \$\begingroup\$ OK I believe you @vofa but does not look like any of the parts from >100 LED factories I once knew in TW. Now there are thousands in China selling online. Buy the wafer, chip dicer, molds and wirebonder, then bond and make like cookies. Did it have a datasheet? \$\endgroup\$
    – D.A.S.
    Commented Jan 27, 2018 at 19:40

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