Screaming Circuits: What, what, what?


What, what, what?

Unknown cathodeQuick. Pretend this is the footprint for a diode. Which way does the cathode face?

Okay, if you can't figure out which side gets the cathode, how about the anode. Which side does it go on?

Hah! It's a trick question. You can't tell based on the limited amount of information available on this PCB footprint mock-up. Sadly, this image is all too frequently real. In theory, with a properly constructed centroid file (See yesterday's post), we really shouldn't need any marking on a PCB at all. And, if everything is properly done, and if no gremlins are swimming through the air, we won't need any marking.

However, when you get the thing back in your shop and hand it off to a technician to measure current and signal quality and slew rate and all of that junk, or if something does go wrong and it needs some troubleshooting, those missing or ambiguous markings can make a big difference.

I'd go ahead and put a little diode symbol there or maybe an "A" for anode. You can use a "C" for cathode too but just make sure it's clearly identified as a diode so it won't be mistaken for a rogue capacitor.

Duane Benson
No. It's not a contest. Just a question.

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Comments

I work @ a tiny OEM/ODM/CEM in the UK. Not once has any customer EVER supplied a centroid file. This means I always use the footprint and silkscreen to program my machine. Std convention usually applies; "+" = bar on diode and the ident will usually take the form DX so you don't confuse it with a cap. Some customers have taken to using 2 lines that open out at one end and other similar variations but its rarely rocket science.

This subject is also covered here :- http://circuitsassembly.com/blog/?p=499

Hi Chris;

A schottky barrier diode, a zener and a flyback diode can all be oriented with the cathode at "+".

Duane

Newbie confused here. I'd used a diode symbol for clarity, but wouldn't the default thinking here be to place the anode at '+' and the cathode at '-'? Why would it be any other way?

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