Well, I’m really surprised that I did not notice this before.
I created a board design and had it fabbed. It took me a while to realize that the silkscreen for an electrolytic was backwards of how it should be.
To make things more confusing for me, is that I have a board that was fabbed by, I believe, a proffessional design tool; and the square pads are marked “+”. This indicates to me that the positive lead should be pin 1.
However, this is exactly backwards from diodes, where pin 1 is the cathode.
Anyone have any history they can share about this issue?
I can’t relate the history, but I believe what you observed is the current consensus. I believe it was recently (last 10 years or so) formalized in IPC standards, but the closest I can (quickly) find to a citation from a published standard comes from http://www.pcunique.com/standard%20footprint%20construction.html
I don’t know why diodes should be the exception, except that the traditional “Blind-Date Code” method for semiconductor diode packages placed the emphasis on the negative (cathode) lead.
I am extremely surprised that I have not noticed this before now. So now I’m feeling sorta silly for having to ask. But, it is one of those things where if you don’t know what you don’t know then you simply don’t know.
Note that the positive (anode) terminal has “ordinary”, undistinguished characteristics - the normal lead length, undisturbed arc in the package housing, etc. In contrast, the negative terminal is notable for the clipped lead, or the truncated circle. On some early packages the negative lead had a wide “flag” crimped into the lead near the package body. So you identify the cathode by the flat side, the fat flag, or the short lead.
Now, blind dates are nearly always about appearances. You know almost nothing about the other person so you concentrate on impressing with looks. And, often as not, you accept a blind date because you don’t want to be seen alone. So the primary way to rate a blind date is by appearance. A very positive blind date would be tall, slim, and shapely. A negative blind date - like the negative terminal on the LED - is associated with short, fat, or flat.
I saw that pop up in my searches, but because the title concentrated on “Zero Orientations” I expected it was primarily about automated component placement. Well, it IS about that but the first half-dozen pages drive home the lesson that “Pin 1 is always the “Positive” pin”!
Thanks for making me feel better for not hearing this either! One would think that with as many parts I have let the smoke out of, one of them would have had me take notice.
I’ve yet to see a diode marked with ‘+’ or ‘-’ indicators. This idea that the anode is the ‘+’ pin is perhaps the reason for all the confusion. Would you refer to the emitter and collector of an NPN or PNP transistor as the ‘+’ or ‘-’ pin? How about a zener? Don’t you regularly have negative voltages on your anodes and positive voltages on your cathodes?
I agree with your observation and analysis. Equating the diode’s anode terminal to a positive potential with respect to the cathode assumes the device is in forward conduction. It’s a rather sophomoric assumption, but nevertheless quite common.
