![]() ![]() It could be said that the square pad is the Schelling point.Įlectrolytic 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. 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." With things that are numbered it's number 1: IC pin 1, cable conductor number 1. Which one is special may be context sensitive. Making diode junctions into arrows, as in a BJT symbol, came later.Īs 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. That correctly describes the cathodes of semiconductor diodes, electron tubes, and discharging batteries.Ī 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. 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. 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.īut that may not be really helpful to people who didn't come to it through chemistry. A mnemonic for remembering "cations" and "anions" (which are the negative ones) is that the "t" in "cation" is like a plus sign. ![]() ![]() In an electrochemical cell, it's the cathode that deals with the cations, which are named after it and are the positive ions. 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. I think of the cathode as positive, even if the diode is forward-biased and that puts the cathode at a more negative voltage. ![]()
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