Re: diode question

On Sat, 26 Aug 2006 10:22:48 -0700, Don Bowey <dbowey@xxxxxxxxxxx>
wrote:

On 8/25/06 8:36 PM, in article 78gve25frq2hendneqn8vjleo56af0p3n4@xxxxxxx,
"Alan B" <nannerbac@xxxxxxxxxxxxxxxxxxxxxx> wrote:

On 25 Aug 2006 13:46:54 -0700, in message
<1156538814.560422.82090@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, "PeteS"
<PeterSmith1954@xxxxxxxxxxxxxx> scribed:

Don Bowey wrote:
On 8/25/06 1:14 PM, in article
1156536850.748767.14800@xxxxxxxxxxxxxxxxxxxxxxxxxxx, "randomname"
<randomname12345@xxxxxxxxx> wrote:

What am I missing here? Current flows from negative to positive, yet
the schematic for a diode points from positive to negative...

-sam


It doesn't point; it identifies the anode and cathode.

Don

Sure it points; it points in the direction of conventional current
flow. For the OP - we often (very often) refer to currents in the
conventional sense

I believe that what Don means is that the "pointing" of the diode symbol is
purely coincidental. I don't know this, but I can make a guess that the
graphical interpretation is meant to be of a spreading stream of electrons
from the cathode to the anode, in a crude representation of the action in a
diode tube.


You're right. Very often people will make their own "rule" for the
graphical representation to help them remember what's going on. The only
one that always works, because it is correct, is: The Anode is the Arrow.

Newbies often have problems with diodes, because when they build their first
project where it matters, a power supply, they expect to see a positive
voltage come out of the anode, which is backward. It's so simple, I
apologize for saying it, but.... A diode will conduct when its anode is
positive with respect to the cathode. If you build a negative PS the AC
from the transformer connects to the cathode and you collect the negative
pulses from the anode. If you build a positive PS the AC from the xfmr
connects to the anode and you collect the positive pulses from the cathode.

In the early days of solid state diodes, they were all point-contact. To us
hobbyists of the time, a "crystal diode" was a piece of germanium crystal
encased in lead, but with some of the germanium exposed. Then there was the
cat-whisker, a small spring-wire, that was used to contact the germanium
(making the point contact). The "arrow" of your sketch is the catwhisker
and it's pointing to the germanium. The original 1N34 point contact diode
had the arrow and bar printed on its white enclosure.

That's actually how I built my diode, too, though I didn't use
germanium. I got a piece of galena crystal and poured molten lead
around it and used the sharp tip of a needle on a swivel arm to make
contact for my AM radio I built. I had tried to use a selenium
rectifier, but that was near useless.

Jon
.

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