Bill and Steve,

The "K" for "Cathode" dates back to the initial release of ANSI Y32.2, where
in the discussion on identifying the terminals of semiconductors, where they
had already discussed "B", "C", and "E", for identifying the Base,
Collector, and Emmiter of a transistor respectively, then continued, and
when discussing a diode, they ised "A" for the Anode, and so as to not cause
confusion, they used "K" as the designator for Cathode. Ironically, the spec
never specifically stated that one should use "K" for the Cathode of a
diode, and it clearly stated that it was only using it to avoid confusion in
the discussion. The actual "Official" pictoral symbol for the diode did not
in fact have any identifiers.

Throughout the years there has always been one group of people, who based on
their understanding of Y32.2 have insisted on using "K" for the Cathode of a
diode, and there has alos been another group of people who have insisted on
using "C" for the Cathode.

Either way, "K" or "C" has always indicated the Cathode of the diode.

For those that might not know, the Cathode end of the diode is always the
end of the diode that has the band or stripe.

Again, for those that might not know, on a schematic, the "triangle" or
"arrowhead" end of the symbol is always the Anode, and the "bar" or straight
"line" is the Cathode. This is fairly easy to remember if you just remember
that the large "A" (for Anode) looks like the "triangle" or "arrowhead", and
the "bar" or "line" looks like the "band" on the actual part.

Regarding the comments respecting the "tradition" in the olden days, there
never was any use of "pin numbers" to identify either diodes or capacitors
prior to the advent of "cad systems". Polorized capacitors were ALWAYS
identified by a "+" (plus) symbol, absolutely without exception (although
sometimes you might also find both ends of the capacitor identified), and
diodes were ALWAYS identified by a line indicating the "band", again
absolutely without exception (unless the entire diode symbol including both
the "band" and the "triangle" was used).

Once again, prior to "cad systems", there were no pin numbers used on diodes
or capacitors. period.

JaMi

* * * * *

----- Original Message -----
From: "Brooks,Bill" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, June 23, 2003 10:44 AM
Subject: Re: [TN] Diode Pin Assignment


> Hi Steve,
> I agree with your assessment that the 'K' or Cathode side of the diode is
> typically the indicator for the polarization of the part. There are no
> references that I have seen in my 30+ years of PCB design that indicated
> that it should be otherwise. I did find the industry change on radial
> electrolytic CAPs to be a bit disturbing back in the 1980's they started
> marking the NEG side of the capacitor with a stripe that indicated the
minus
> symbol. Before that, we always indicated the Positive terminal of the Cap
on
> our silk screen legends. The connection between pin one and the
polarization
> of the part is traditional, and has been an industry wide practice since
the
> 1960's.. so I am told. I was in grade school back then so I guess I will
> have to take their word for it. We often indicated the polarity or pin one
> by using a square or rectangular pad shape in that component lead instead
of
> a cut pad or oval pad. Our indications for polarity came from observing
the
> manufacturer's standard practice. Transistors typically indicated the lead
> that was connected to the emitter junction as opposed to indicated the
base
> or collector leads. We followed their choice for indication on the legend.
> IC's are also customarily oriented by the location of pin one with a DOT
or
> an indentation on the correct end of the molded plastic package...
>
> In Darryl Lindsey's book "The Design and Drafting of Printed Circuits"
> published by Bishop Graphics back in 1979, Chapter 3 refers to discrete
> component polarization and he says that Diodes MUST be identified by the
> cathode end.  I started laying out boards in 1970 and it has always been
the
> 'standard' practice to identify the cathode, not the anode, of discrete
> diodes.
>
> All Diode manufacturers indicate the cathode on the actual physical part
> with some sort of band or marking close to the lead or surface mount
> termination. They do not indicate the anode side of the diode.
>
> Therefore I would suggest that perhaps your customer is trying to set a
new
> precedent in the component marking standards, hoping that the rest of the
> world will follow their lead.... (don't count on it) or, they just made a
> mistake in creating the symbol and should correct their database.
>
> I hope that helps a little.
>
> Bill Brooks, CID
>
> -----Original Message-----
> From: Steve Vargas [mailto:[log in to unmask]]
> Sent: Friday, June 20, 2003 7:18 AM
> To: [log in to unmask]
> Subject: [TN] Diode Pin Assignment
>
> All:
>
>     As a SMT subcontractor, we see CAD files from many different sources.
> An item of frustration for us is how some customers assign a Pin 1
> designator to diodes. Primarily, we see Pin 1 being assigned to Cathode
and
> Pin 2 to Anode. For those customers who use Pin 1 to anode, we manually
> rotate the part within our CAM system to stay consistent with what our
> operators typically see on their visual aids. This is both cumbersome and
> inefficient.
>    I don't believe (or should I?) that there is an industry standard that
> addresses this, but is there some type of defacto standard that exists
> which I can use as evidence to some of our customers that they should
> consider using the 'Pin 1 to cathode concept'? Thanks.
>
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