chuck wrote:
Andina Marie wrote:
Chuck, you're confusing current and voltage.
I try to keep them separate, Ann-Marie,
but there's so much resistance. ;-)
Here is where I was going. The zinc and
the bronze prop are two dissimilar
metals and when they are immersed in an
electrolyte, a voltage can be measured
between them. No current flows through
the water between them until they are
electrically connected, usually by the
prop shaft. Then the current path is
through the water, returning through the
shaft.
There is no measurable voltage between
the prop and the zinc because the shaft
acts as a short circuit. There is an
electric field between the two metals in
the water, however, and if we knew how
to do it, we could measure a voltage at
the surfaces of the electrodes.
This galvanic couple sits in the water
producing a current that involves the
loss of Zn ions until the zinc is
depleted. It should produce no voltages
or currents anywhere else, including
across the galvanic isolator.
There should be no potential difference
between the shaft (which presumably is
bonded to the boat's DC and AC ground)
and the shore power ground due to the
zinc. Think about the boat's 12 VDC
system powering onboard lighting
circuits. There is no reason to believe
any of that 12 volts will show up as a
potential difference between the boat's
DC ground and the shore power ground,
barring some wiring anomaly.
Think of the shore power ground as a connection to the rest of the
world. A world with structures and ground rods that may be in contact
with sea water, but have no provisions for galvanic protection
themselves. If you have zincs on your boat, there will be a current (and
your zincs will dissolve). If you break that circuit, you will see the
voltage of that galvanic cell (the one between your zincs and the rest
of the world).
The zinc/bronze galvanic couple no more
makes the boat "alive" than the boat's
onboard 12 VDC system, which also forms
a closed circuit.
Regarding the operation of a
semiconductor diode, it is good to
remember that the voltage across a
forward-biased diode is related to the
current through it. If a voltage is
measured, then there is a current
through it. Alternatively, if there is a
current through it, a voltage can be
measured. The VI characteristic is
highly non-linear of course.
You can assume that the IV characteristic of a single silicon diode is
such that when forward biased but below 0.6 V, the current will be on
the order of microamps. The diode forms (effectively) an open circuit.
For two diodes, it's 1.2V.
A voltage of 1.2 volts measured across a
pair of series-connected silicon diodes
(like the 1N1190A) suggests a current on
the order of 100 mA or more! That is far
more current than you should ever
measure through a zinc/bronze galvanic
couple on a yacht even if your
measurement were directly between the
zinc and the prop.
True. But the voltage produced by a zinc-bronze or zinc-steel
electrochemical cell is low. The idea is that the resistance of the
current path on your own boat is low enough so that the zincs produce
the desired effect (they erode to protect your fittings). But the path
between your zincs and the fittings on every other boat in the marina is
high. It doesn't matter how many other boat fittings there are in the
water, the electrochemical reaction can only produce a voltage which
depends on the metals involved and the chemistry of sea water. As long
as the GI's diode drop exceeds voltage, no current (well, maybe
microamps) will flow. But, like a battery not connected to a load, the
'battery voltage' will be measurable across the open (the GI).
If you are measuring anything close to the GI's diode drop, it means
that it could have started to conduct. But since the zinc
electrochemistry isn't likely the source of this high a voltage, other
problems should be suspected. Bad wiring has the potential (no pun
intended) to put up to 12 Vdc or 115 Vac on a ground.
Here's a suggestion: next time you
measure a DC voltage across a GI, make a
note of the polarity. From the direction
of electron flow, you can determine
whether the current you observe is
protecting the zinc or depleting it. See
if the polarity is always the same, or
if it is random.
If you are seeing a voltage across the GI (less than the GI's blocking
voltage), it will be in one direction or another, depending on whether
your zincs are in better or worse shape than the rest of the marina's.
But below the blocking voltage (about 1.2 V) there should be no current.
I would try to track down what is
causing those readings.
Chuck
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Paul Hovnanian
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