On Wed, 24 Aug 2011 08:21:17 -0400
Ethan Swint <eswint.r...@verizon.net> wrote:
> On 08/23/2011 08:47 PM, Matthew Lewis wrote:
> > I was double checking a pcb layout today and I discovered a rather
> > nasty gotcha. It seems that gschem and PCB don't agree on which end
> > of a diode should be pin 1. Gschem views pin 1 as the anode and PCB
> > considers pin 1 to be the cathode. It doesn't prevent you from
> > laying out a board correctly, but it does cause the silkscreen
> > polarity to be printed backwards (for the SOD devices at least).
>
> I've defined my own symbols and footprints to use 'A' and 'K' instead
> of 1 and 2.
That's a good idea. Anything you can do to error-proof yourself is
a Good Thing.
However, I refuse to use “anode” and “cathode” for diode symbols, since
these terms refer to electron flow and are _incorrect_ when the diode is
reverse-biased (most obvious for common Zener diode circuits).
I understand that it is electrical convention to name diode terminal
anode and cathode, but I reject it as a confusing and ambiguous naming
convention.
For my diode symbols and footprints, I choose to name the terminals
“P” and “N“ (for the p-type doped side and the n-type doped side).
This models the structure of the device in an unambiguous way. This
works perfectly for all types of diode (including LEDs), no matter how
the device is used in the circuit.
John Denker describes the most obvious problem with anode/cathode
for Zener diodes [1]:
You should never apply the terms anode or cathode to a Zener
diode, because the potential for confusion is too great.
Instead you should refer to the P-doped side and the N-doped
side, and you should insist that others do the same.
Note that reversing the labeling convention for Zener diode
arrays would not solve the problem in any fundamental sense,
because there are perfectly reasonable circuits in which –
part of the time – a Zener diode is forward biased, so that
it conducts just like any other diode. This is the same
situation we encounter in connection with rechargeable
batteries: if you attach permanent anode/cathode labels to
the structure, you will be wrong at least part of the time.
The terms “anode” and “cathode” properly apply to function,
not structure.
Rechargeable batteries are another place where the terms anode and
cathode can cause confusion due to the fact that current can flow
either direction between the battery terminals [2]. User PaulW had an
interesting insight in comment #8 regarding the fact that “anode” and
“cathode” are important from a battery chemistry standpoint, but
“positive” and “negative” terminals are more useful from an electric
perspective.
Regards,
Colin
REFERENCES
--------------------------------
[1] John Denker. “How to Define Anode and Cathode.”
<http://www.av8n.com/physics/anode-cathode.htm#i-zener>.
[2] candlepowerforums.com thread started by Clifton Arnold.
“anode is it positive or negetive” (sic).
<http://www.candlepowerforums.com/vb/showthread.php?41548-anode-is-it-positive-or-negetive&p=453381&viewfull=1#post453381>.
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