Diode PCB Pattern. Square pad A or K?

From circuitbending to homebrew stompboxes & synths, keep the DIY spirit alive!

Moderators: Kent, Joe., luketeaford, lisa

Post Reply
KSS
Super Deluxe Wiggler
Posts: 3685
Joined: Mon Jan 25, 2016 7:28 am

Diode PCB Pattern. Square pad A or K?

Post by KSS » Sun Oct 18, 2020 4:42 am

Thread title says it all.

User avatar
MikeDB
Common Wiggler
Posts: 182
Joined: Fri Feb 08, 2019 8:28 am
Location: UK

Re: Diode PCB Pattern. Square pad A or K?

Post by MikeDB » Sun Oct 18, 2020 7:44 am

For through hole Cathode

For SMT both are square but various identification methods, arrow towards cathode, wider surround band at anode end or a slighly wider anode pad.
Personally I hate cats ! Set a VCO to about 25kHz with a fairly fast +/- 5kHz frequency modulation into a piezo tweeter and cats will hate you too !

User avatar
mskala
Super Deluxe Wiggler
Posts: 2668
Joined: Thu Mar 27, 2014 7:33 am
Location: Toronto
Contact:

Re: Diode PCB Pattern. Square pad A or K?

Post by mskala » Sun Oct 18, 2020 8:44 am

K, because that's where the identifying band on the component package is. But I wouldn't depend on only the pad shape as the way of indicating the orientation.

KSS
Super Deluxe Wiggler
Posts: 3685
Joined: Mon Jan 25, 2016 7:28 am

Re: Diode PCB Pattern. Square pad A or K?

Post by KSS » Mon Oct 19, 2020 6:54 am

Thank you.

Both replies agree with what I've always believed and done.
Was called into question by seeing an increasing number of PCBs using square pad on the Anode. Explanation given was based on plus of electrolytics being square pads, so plus of diode should be the same. Didn't know if I'd somehow missed the memo updating standard practice. ;)

User avatar
MikeDB
Common Wiggler
Posts: 182
Joined: Fri Feb 08, 2019 8:28 am
Location: UK

Re: Diode PCB Pattern. Square pad A or K?

Post by MikeDB » Mon Oct 19, 2020 8:01 am

KSS wrote:
Mon Oct 19, 2020 6:54 am
Thank you.

Both replies agree with what I've always believed and done.
Was called into question by seeing an increasing number of PCBs using square pad on the Anode. Explanation given was based on plus of electrolytics being square pads, so plus of diode should be the same. Didn't know if I'd somehow missed the memo updating standard practice. ;)
Interesting. In the 1970s before silk layers became common, there was sometimes a + sign in the copper at the cathode end as well as the square pad to indicate the positive voltage came out of that pin.
Personally I hate cats ! Set a VCO to about 25kHz with a fairly fast +/- 5kHz frequency modulation into a piezo tweeter and cats will hate you too !

User avatar
mskala
Super Deluxe Wiggler
Posts: 2668
Joined: Thu Mar 27, 2014 7:33 am
Location: Toronto
Contact:

Re: Diode PCB Pattern. Square pad A or K?

Post by mskala » Mon Oct 19, 2020 9:04 am

I think of the cathode as positive, even if the diode is forward-biased and that puts the cathode at a more negative voltage. It took a bit of introspection just now to figure out why I think of it that way, but I guess it's because I studied chemistry before electronics. In an electrochemical cell, it's the cathode that deals with the cations, which are named after it and are the positive ions. A mnemonic for remembering "cations" and "anions" (which are the negative ones) is that the "t" in "cation" is like a plus sign. And then a mnemonic for the diode symbol is that the cathode end of the diode symbol is also like a cross or plus sign.

But that may not be really helpful to people who didn't come to it through chemistry. Wikipedia's article on "cathode" has a fair bit of interesting material on the history of where the term comes from, and that article claims that when it comes to electrical components, the cathode is the place where the conventional current flows out of the device in some possibly-arguable state of "normal" or "forward" operation. That correctly describes the cathodes of semiconductor diodes, electron tubes, and discharging batteries.

A couple of other interesting points: the positive terminal of an electrolytic capacitor is chemically the anode because it's where the oxidation occurs when forming the dielectric (through anodization); and the diode symbol didn't actually originate as an arrow or a cross indicating plus, but as a picture of a crystal point-contact diode with a triangle poking a flat piece of semiconductor. Making diode junctions into arrows, as in a BJT symbol, came later.

As for the square pad in a PCB symbol, I don't think the rule is (or should be) "square equals cathode" but rather "square equals the distinguished connection." We have a general rule throughout electronics that one connection is the special one and that's the one that is marked so that we can identify them all. Which one is special may be context sensitive. With things that are numbered it's number 1: IC pin 1, cable conductor number 1. With diodes it's the cathode. With capacitors it's the positive. This general rule answers the question of "why is the red stripe negative in a Eurorack cable?": it's not a red stripe (not conceptually and sometimes not even physically), it's a stripe indicating one conductor as special and that's always conductor number 1, taking precedence over weaker rules like "red equals positive" and "positive equals special."

It could be said that the square pad is the Schelling point.

Electrolytic caps are a little strange in that although they normally have a long lead for positive, and I would put the square pad on the positive side of the footprint, they normally also have a prominent marking stripe on the negative side. I don't know but it might possibly be because that's the electrolytic cathode.

The standard packaging for jelly-bean LEDs seems to use the flat side of the package for the cathode, but the long lead on the opposite side for the anode. I'm not sure that I have always been consistent in which pad I made square for LEDs on boards I've designed (but I've at least noted it in the documentation!); I think on a new board I would probably want to make the LED cathode pad square both for consistency with other diodes and to line it up with the flat side of the package (which I would mark on the silkscreen). An argument could be made for going the other direction, though, and making the square pad correspond to the long lead because that's the easiest thing to check while soldering and it's consistent with capacitors. For the bipolar LEDs I often use, I'd probably want to make the schematic symbol match what the package flat side and long lead represent by pretending it's a single LED, even though it's understood that the package actually contains two opposed diodes and each pin is the anode of one and cathode of the other.

User avatar
MikeDB
Common Wiggler
Posts: 182
Joined: Fri Feb 08, 2019 8:28 am
Location: UK

Re: Diode PCB Pattern. Square pad A or K?

Post by MikeDB » Mon Oct 19, 2020 9:18 am

@mskala got me thinking some more. Bridge rectifiers have the cathode marked + on the package as that is where the positive voltage comes out of. Imagine the chaos if it was anything else.
Personally I hate cats ! Set a VCO to about 25kHz with a fairly fast +/- 5kHz frequency modulation into a piezo tweeter and cats will hate you too !

Post Reply

Return to “Music Tech DIY”