The tone of the debate was set by you. I was only responding.
The field coil is a gigantic inductor compared to the inductors used in
high speed electronics like computers and radio frequency equipment.
When the field current is removed the time it takes for the magnetic
field to collapse is related to the strength of the field and the number
and centerline spacing of the turns. Strong fields take a lot of time
to collapse through a lot of turns of relatively thick tightly wound
wire so we are talking about milliseconds rather than micro or nanoseconds.
Diodes are rated by the voltage and amperage they can take and how long
they can take the power. The larger the combination gets the more
expensive and larger the diode gets. We are facing the limits of size
and economics. The Zap Stop is sized to "do the job" without pricing
itself out of the market. In most cases it will rarely fail but the
potential is still there. Probably the worst case would be dropping the
load on a large cold alternator bulk charging at full power. All that
power that was going through a 4/0 cable now flows through a few inches
of #14 wire and a fraction of an inch of diode material. That can
generate a lot of heat very quickly. The newer Zap Stops have a fuse in
series with the diode. The idea being that in the time it takes for the
fuse to blow the field strength drops below the damaging level.
It would be nice however if they added a little circuitry and an LED to
indicate when the diode had been fried. Right now the only way to know
that your alternator is protected is to periodically test it with a
multimeter.
The diode that the Zap Stop uses probably only cost a dollar or two but
the case probably cost another couple of bucks and it might take 5
minutes worth of labor to put it together and stick it in a box. Total
manufacturing cost is probably under 5 bucks. BUT, a general rule of
thumb is that manufacturing cost of any item should be no more than 25%
of retail price. If you put the word "marine" in the description it can
drop to 20%.
Ed Price wrote:
Thanks for the more lucid explanation, despite the several gratuitous
insults. After reading your new information, I'll even go so far as to
retract my quip of a totally bogus explanation of energy within the
alternator; I now realize you were being more condescending than obscure. To
me, fast is microseconds or nanoseconds, and I guess you just live in a
slower world.
It further helps to hear that the Zap-stop only conducts a current similar
to the original load, and that for only a few hundred milliseconds. That's
probably why connecting the protection to the output of the alternator
doesn't kill the alternator diodes.
But that leads to another issue. You said the protection diode often fails!!
Question 1.: Why does the protection diode "often fail" if it's drawing the
same load as the alternator diodes?
Question 2.: You assert that the protection diode "will last long enough."
Is this some kind of smart silicon, in that it knows when it's OK to die?
Again, I'm not familiar with your world, but devices that I have seen in
which there is a failure under load usually fail under full load, or at
least not when most of the load has gone away. Whenever I cook off a diode,
it usually dies a lot faster than a couple of hundred milliseconds.
I would think that the protection diode would be sized so that it rarely
fails. After all, isn't a protection diode failure just another way of
describing load shedding? IIRC, you said that diodes that could handle
typical alternator currents could be had at Digikey for under a buck. Maybe
you should refer Xentrex, too.
Ed
--
Glenn Ashmore
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