"Glenn Ashmore" wrote in message
...


Ed Price wrote:


So I looked at the Xantrex site, trying to get some idea of a
Zap-stop's performance. I couldn't find a circuit diagram for how the
Zap-stop is hooked up to the alternator. Xantrex says something
about connection across the rectifiers, but that isn't clear enough
for me. Also, Xantrex doesn't say anything about how fast the
Zap-stop begins to conduct when presented with a voltage excursion.

Does Xantrex have some hard data on their product, or do they just
expect you to trust them?

The Zap Stop connects between the alternator output and ground. It is
just a high voltage diode with a reverse breakdown voltage a bit higher
than the operating voltage of the alternator. When the voltage exceeds
this level the diode starts to conduct instantly shorting it to ground.

The alternator diodes are designed for high amperage at relatively low
voltages. The Zap Stop diode is designed for high voltages but will
only handle high amperage for the few milliseconds it takes for the
regulator to regain control. I don't think it could stand up to
something like a loose sense wire on a high amperage alternator where
the voltage and amp output stays high for any length of time.

--
Glenn Ashmore


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