Quote:
Originally Posted by Maurice
Interesting. Can anyone explain to me why the zincs near (what I think is electricity) don't last as long...I just heard it happens but not sure why.
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Mo,
One cause of stray currents is from other boats nearby that are not properly protected. This is an excerpt from my Galvanic Isolator project on my website (
http://www.chessie.com/boat/projects.shtml) that explains it and what to do about it:
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...Soner or later, every boat owner gets introduced to the galvanic corrosion that happens when you put two dissimiliar metals that are electrically connected into saltwater. You quickly learn to install and maintain an appropriate number of zincs to protect your valuable underwater hardware, such as through-hulls and prop shafts.
What some don't know, however, is that it's possible to do everything right and still have galvanic corrosion! The problem happens when you're connected to shore power. Even if all your A/C devices, like your battery charger, are transformer isolated, there's still the safety ground. This is connected directly to your boat's system ground, to which all your underwater hardware is bonded. As a result your boat's ground, through the marina's wiring, is now connected to every other boats grounds and hardware! And if any of them aren't properly protected, they'll cause a galvanic current that'll start eating your zincs! This uses them up too quickly, and then least noble of your hardware starts going.
The obvious solution is to disconnect the safety ground. But this is a
serious safety hazard unless you are
absolutly sure that everything is transformer isolated, and remains that way. So, what we really want, is something in series with the safety ground that doesn't conduct most of the time, but will conduct the full service current in case of a short.
It turns out that there is such a device. It depends on the fact that virtually all galvanic circuits are driven by only a few tenths of a volt. A silicon diode drops about 0.6 volt. Below this, it doesnt conduct at all. Above this, it conducts heavily. Since 0.6 volt isn't really enough, we put two in series for 1.2 volt. And since diodes only conduct in one direction, we put another pair in parallel in the opposite direction.
This is basis of a galvanic isolator. Real-world devices are more complicated, including things like fail-safe, A/C leakage bypass, and monitoring. But at their heart, there are four diodes. ...