I have an infrared LED as part of an IR transceiver, and I want to determine its polarity. As the device is unmarked, finding its datasheet is out of the question. And, as it is infrared, seeing it light up is also out of the question.

Using my multimeter on its diode setting, I connect the positive multimeter lead to one of the device pins, which we will call pin 1. I then connect the negative multimeter lead to the other pin (pin 2). Doing this, I measure a voltage drop of ~1.3v. I then swap the leads and measure a drop of ~0.5v. Unfortunately I do not know what to do from here, as both directions give me a reading.

Based on these readings, is it possible to determine its polarity? And if so, what IS the polarity?

Thank you for your time.

  • \$\begingroup\$ "as it is infrared, seeing it light up is also out of the question" FYI a typical cellphone camera will detect IR LED light when it is emitted (try this with your phone's camera app using an IR TV remote control - notice the glow on the phone screen looking directly at the remote control's IR LED, when you hold down a button; it's easier to see in a relatively dark room). Hope that helps. \$\endgroup\$ – SamGibson Apr 18 '19 at 23:52
  • \$\begingroup\$ @SamGibson unfortunately this infrared led is outside of the range for a typical camera. \$\endgroup\$ – The Elemental of Creation Apr 18 '19 at 23:53
  • \$\begingroup\$ The wirebond clearly visible thru the lens is the Anode \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 18 '19 at 23:54
  • \$\begingroup\$ @SunnyskyguyEE75 it's in a package so it appears black. Visible light doesn't penetrate. \$\endgroup\$ – The Elemental of Creation Apr 18 '19 at 23:55
  • 2
    \$\begingroup\$ There is more to this part than you are telling us. The reverse voltage of 0.5V indicates ESD diode protection. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 18 '19 at 23:56

The direction with the lower voltage drop is likely the forward direction. The reason you get a voltage drop in the reverse direction is likely because when reverse bias, your IR emitter LED becomes a photo diode that's sensitive to same wavelength of IR. If you can get another strong IR source, shine it while it's reverse biased and see if it changes.

This is similar to this questions: IR emitter in reverse bias?

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