Accursed Diode Marking

Am I a broken record? Pretty much — especially when it comes to confusing diode marking.

For example, everyone knows what the diode symbol looks like, and pretty much everyone knows which side is the anode and which is the cathode. Right? It’s just like in the following picture:

10 designersnb figure 2

Is that big enough?

Normally, the clearest way to indicate polarity on an LED is to put something like this diode symbol in silk screen next to, or between, the copper pads. In theory, that should remove ambiguity.

Ambiguity in marking is the enemy of polarized parts. Unfortunately, as I cover in this, and many other blog articles, LED manufacturers seem to conspire against us all when marking is concerned.

We recently ran across a case of built-in ambiguity. The PCB had, what looked like, a very clear marking. The image on the right is from the assembly drawing, which is just a blow-up of the board silk screen and documentation layer.

With that marking, I’d quickly come to the conclusion that the anode is on the right and the cathode is on the left. I’d even confidently state that it’s a sure thing and extremely unlikely to cause any problems. But …

Here’s where I’d be very wrong, and why it’s so important to always check the datasheet when dealing with diodes. Take a look at the following clip from the component’s datasheet. Scroll down to the bottom of the image for the punch line.
10 designersnb figure 3

Wow. I can’t even …,

The board designer was just following the datasheet. That’s a perfectly proper thing to do, except when the manufacturer flips a coin, as it appears to have happened here. In this case, dispense with the symbol altogether and use “A” for anode and/or “K” for cathode in the silk screen. (Use “K” because “C” looks too much like a reference designator for a capacitor.)

Duane Benson
In the land of the insane, only the sane are crazy

http://blog.screamingcircuits.com/