"Glenn Ashmore" wrote in message
...

Ed Price wrote:

Well, pardon me for asking you to address my question. In case you
didn't
remember, since it always seems to slip your attention during your
posts, I
wanted to know why the suppression is applied to the LOAD side of the
alternator diodes. Perhaps you two could stop slapping each others butts
long enough to try to answer that question.

True, there are none so ignorant as those who refuse to learn. Now that
we're even on stupid witticisms, can you try for a technical answer?

OK, I am going to try one more time. What I am about to say was
verified yesterday afternoon by Randy Johnson, formerly of Cruising
Equipment and developer of the Zap-Stop and confirmed by the tech
support people at Balmar and Leece-Neville.

When an alternator is producing a significant percentage of its capacity
to a load, be it a battery, motor or other device and that load is
suddenly removed the output voltage of the alternator will rise. While
the rectifier diodes can handle higher than normal amperage for short
periods they cannot tolerate voltages significantly over their rating
for even an instant. Therefore if this voltage rise is not checked
there is a very good possibility that the alternator rectifier diodes
will be damaged.

The physical law of conservation of energy says that the amount of
energy output by a device must equal the amount supplied to it. The
power output of an alternator is determined by the amount of energy
being supplied by the engine. The amount supplied is a function of RPM
and torque. The torque is governed by the intensity of the magnetic
field. As the engine speed cannot normally be adjusted quickly the
regulator is used to control the magnetic field. This control is fast
but it is not instantaneous. As the field collapses, a back EMF is
inducted in the field coil slowing the process. Therefore there is a
lag in reducing the total power being produced. FOR AN INSTANT TOTAL
POWER REMAINS THE SAME. Power is volts times amps. As there is no
demand for the amps basic math says that the voltage must rise.

The current practice for preventing damage to the regulator diodes is to
place a sacrificial diode between the alternator output and ground to
provide an alternate path for the energy. Both of the alternator
manufacturers I talked to strongly recommend the installation of one of
these diodes whenever there is a possibility that a heavy load might be
suddenly dropped.

This diode will not conduct until the voltage exceeds a certain preset
limit determined by its construction. It has only one function: To
provide a way for the alternator to shed the surplus current so that the
voltage will not rise to a damaging level. The rectifier sees only a
slight drop in current demand and a slight rise in voltage. It does not
know anything about whether it is supplying the original load or the
protecting diode.

The protecting diode however has to bite the bullet and very often gives
its life in the process but it will last long enough to handle the surge
for the fraction of a second required to get the power output below a
damaging level.

That is about as simple as I can get it. If that is not satisfactory,
go talk to JAX. He is more on your level.

--
Glenn Ashmore


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