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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.

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    \$\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
    Commented Apr 18, 2019 at 23:52
  • \$\begingroup\$ @SamGibson unfortunately this infrared led is outside of the range for a typical camera. \$\endgroup\$ Commented Apr 18, 2019 at 23:53
  • \$\begingroup\$ The wirebond clearly visible thru the lens is the Anode \$\endgroup\$ Commented Apr 18, 2019 at 23:54
  • \$\begingroup\$ @SunnyskyguyEE75 it's in a package so it appears black. Visible light doesn't penetrate. \$\endgroup\$ Commented Apr 18, 2019 at 23:55
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    \$\begingroup\$ There is more to this part than you are telling us. The reverse voltage of 0.5V indicates ESD diode protection. \$\endgroup\$ Commented Apr 18, 2019 at 23:56

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I experienced the same, new and surprising to me, phenomena today. In contrast to the other answer I would bet the polarity is pin 1 = anode. That is, the larger voltage drop is the forward bias. Here's my evidence:

  • OSRAM SFH 4170S A01 : forward 2.30V, reverse none.
  • OSRAM SFH 4171S : forward 2.30V, reverse none.
  • OSRAM SFH 4180S A01 : forward 2.08V, reverse none.
  • OSRAM SFH 4181S : forward 2.09V, reverse none.
  • OSRAM SFH 4182S : forward 2.08V, reverse none.
  • OSRAM SFH 4725AS A01 : forward 2.06V, reverse 0.77V.

Of course the measurements are sensitive to the test current the multimeter applies, which I don't know, but you get the idea.

The 4725AS is similar to the 41xx series, but is a little bigger and has two pads for the anode instead of one. From the datasheet:

SFH 4725AS footprint

By the way, the bond wire runs to the cathode in this case, unlike one of the comments suggested. The product photo shows this (from the product page, and in the datasheet), but it's also visible under a microscope:

enter image description here

Otherwise it has a similar forward V-I spec of 2.65V @ 1A (the others range from 2.95V to 3.25V at the same current). Surprisingly, they are all rated at 0.01µA @ 5V in the reverse direction.

Based on this evidence alone, I would suggest the larger measurement is the light emitting behaviour, and the small measurement is the photo diode behaviour. I don't know why some LEDs exhibit this behaviour and some don't.

Update 2024-02-06: a new datasheet has been released (version 1.8, same URL), with a very different reverse voltage specification (1.2V @ 20mA, abs. max 200mA). Crucially, it also adds a little note towards the end of the document:

The device is protected by ESD device which is connected in parallel to the Chip.

So it looks like in this case the reverse conductivity is not a diode effect, but an additional ESD protection device! That could also be the reason by the OP's experience (as proposed in earlier comments), though the rational behind the answer here remains intact.

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  • \$\begingroup\$ The specific part lists a reverse current of 200mA in "absolute maximum", with a note saying "but hey don't feed it in reverse". Which is probably unusual? A reverse voltage of just 1.2V seems unusual as diodes go as well. \$\endgroup\$
    – Lundin
    Commented Jan 30 at 10:42
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    \$\begingroup\$ Now I see what you're talking about @Lundin! Those figures weren't in the version of the datasheet I was reading. Yes, the 200mA and 1.2V are unusual, and as I've now clarified in the answer, explained by the fact that there's an ESD device in parallel! \$\endgroup\$ Commented Feb 6 at 2:49
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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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