OK, so the Zap-stop is wired across the output of the alternator, and it's
just a zener diode.

Now, how can a diode be damaged? One way is to exceed its reverse voltage
capability. Another way is to exceed its current capacity.

Put the Zap-stop as described, and apply the transient from the vehicle
side. Either a high-voltage positive or negative transient will result in
strong conduction through the zener. As long as the zener can sink it, the
voltage will be limited by the strong conduction.

But now, posit that the overvoltage transient is starting in the alternator
windings. The zap-stop, on the output side of the bridge, may go into strong
conduction. But that current will also have to flow through the bridge
diodes. So how does causing a massive current through the alternator diodes
provide protection?


Ed




"Glenn Ashmore" wrote in message
...
I believe we may be looking at it wrong. Go back to the original
problem. When the high amperage going to the battery is interupted, the
voltage starts to rise. What the Zap Stop does is provide an alternate
path for the current so that the flow is not interupted. Yes, the coils
are still outputting through the rectifier but if we can detect the
start of this rise fast enough and redirect the rectifier output to
ground, the current flow never stops so the voltage never gets high
enough to damage the rectifier.

BTW, in the other thread you were asking about a schematic. Here it is:

+ --------|------ -

It is just a zenier diode (or maybe several in parallel) in a nice box
wired against the normal current flow. When the voltage rises above the
breakdown rating of the diode, it conducts. As usual with most
"marine" devices, it is 5% material and 95% marketing but it does the job.

Ed Price wrote:

If the Zap-stop is connected to the alternator output, then it is
positioned
best to protect the alternator against transients originating in the
rest of
the vehicle's electrical system. (The worst source might be a
high-current
motor, like the AC blower fan.)

OTOH, earlier posts have said that the sudden removal of load causes a
high
voltage transient in the alternator's windings. This means that the
Zap-stop
protects the alternator diodes from a high voltage by requiring the
diodes
to pass a massive current transient into the Zap-stop and to ground.
That's
strange protection!

If you wanted to protect against an alternator winding over-voltage,
wouldn't you have to put the protection at the winding side of the
alternator diodes?

The last time I looked inside an alternator, it had three field windings
connected to a 6-diode full-wave bridge rectifier. Protecting the bridge
against voltage transients would require one Zap-stop type device across
each winding. An over-voltage event would then conduct through the
Zap-stop
devices, and not through the alternator diodes.

Could all of this discussion have been based on a misconception of what
happens when a heavy load current (into an inductive load like a motor
winding). The potentially damaging transient is caused by the counter
emf
from the load, and not from any "slow regulator" effect within the
alternator.

If the Zap-stop is mounted to the alternator output lug, then that's
apparently what the Zap-stop is configured to protect against.


Ed



--
Glenn Ashmore

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