Screaming Circuits: How NOT to mark a diode

How NOT to mark a diode

A while back, I wrote about ambiguity in the markings on electrolytic capacitors. In doing that, I cobbled together a little image to illustrate how surface mount electrolytics are marked. Take a look at the image below:


Note how I have illustrations showing how tantalum and metal can electrolytic capacitors are marked. Further note, that I have the capacitor schematic symbol there too. Finally, note that all three are oriented in the same direction. I have the plus side on the left and the negative side on the right.

Now for comparison, I have two nearly but not quite identical 0805 SMT LEDs in the following photo. Look at the photo of the two LEDs below. I did not alter this image in any way. The mark on the LED image could be interpreted either way. The bump could be seen as pointing toward the cathode, since it is the cathode mark. Or, The line could be on the side of the cathode. That would make sense because the line on the schematic symbol represents the cathode.

There's one final thing to look at - wait for the punchline:

Backwards markings

The punchline is that the  cathode is on the left on both of these LEDs in the photo. I have empirically determined that to be the case, both by putting them on a board and by subjecting them to a diode checker. Punchline number two is that both are correct according to their respective datasheets. The following excerpts from their respective data sheets shows the problem.Reverse marked LEDs

And, drum-roll please ... The third punchline is that both of these parts are from the same manufacturer!

If your board uses SMT LEDs, send the datasheet with your assembly order. Include it as a PDF in your files set. It would also behoove you to double check your CAD library footprint against your specific part number datasheet. IPC says the cathode is pin-one and pin-one zero degree orientation is with pin-one to the left.

Duane Benson

Forward, the LED pick and place
Was there a machine dismayed?
Not tho' the engineers knew
Someone had blundered
Cathodes to right of them
Cathodes to left of them
Cathodes behind them
And I cannot reason why


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I agree, that is horrible! I have come across these diodes before and always assumed that the line on the bottom side indicated the cathode...luckily I have not been caught out with the opposite marking LEDs yet.

Are those Everlight LEDs? I have seen that they use different dies in the same package, but the different dies has to be mounted in different up/down orientations. I guess it is a sort of cost saving. But it ends up with confusion and 100eds of LEDs to rotate...

Wow that is an ugly situation, sounds like everyone is making up the standards as they move along.

Adam - There is also a polarity indicating dot visible from the top. On one of the LEDs, it's noted to be a cathode indicator and on the other, it's an anode indicator, just like the marking on the bottom. That's no help.

The die is visible, but only barely so. On very close inspection, it agrees with the markings, but it's not something you could easily pick out.

Wow, that is both hilarious and sad at the same time. I assume there is no marking on the top at all for visual inspection. Can you tell by looking at the die, or does it have an opaque lens?

Martin - That's a great solution.

And that's why I prefer SOT23 LEDs now (having had very similarly "entertaining" experiences :)

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