chuck wrote:
Paul Hovnanian P.E. wrote:
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).
Well, that's only half of the situation.
Your prop and any underwater metals
will also form galvanic cells with the
rest of the world, of different polarity
and potential difference. There is a
burden here of demonstrating that on
average these do not cancel, and a
greater burden of demonstrating that
their net effect is a 1.2 volt cell. But
the 1.2 volts measured was NOT an
open-circuit voltage measurement, but
one across a functioning GI.
The discussion has gone open loop. Here
is a recap.
1. An assertion was made that 0 VDC
across a GI means the diode(s) are shorted.
2. I suggested that 0 VDC also meant the
absence of galvanic currents through the
shore power ground wire; a good thing.
Either 1 or 2 could be true.
3. A counter-assertion was made that
there is always 1.2 VDC across a
properly functioning (NOT open-circuit)
GI due to the boat's zinc.
No. Less than 1.2V for this case. The potential between Zn and Cu in a
'standard cell' is about 1.1V. It is likely to be less than that in sea
water. Cu (copper) is one of the most electrically positive metals you
are likely to find in common use. So actual potentials are likely to be
much lower.
4. I suggested there was no path for the
boat's zinc/bronze galvanic current to
pass through the shore power grounding
wire and some other explanation was
called for.
- your boat - - the rest of the world -
+-------+------------shore pwr gnd-------+----------+-------+--------+
| | (w/o GI) | | | |
Zn Bronze more bronze steel aluminum
copper
-------------------------------- sea water
----------------------------
There are parallel paths through your zincs, your own prop (bronze) and
what might be
tons of metal in the outside world. While your zincs will protect your
prop, they will
also 'protect' the rest of the world. Since your zincs dissolve to
provide this protection,
they will just dissolve faster when connected to more metal. End result,
your zincs get
eaten up in days.
5. An assertion was made that the
galvanic couple in 4 made the boat
"live" and that led to the measured 1.2
VDC across the GI.
The galvanic couple might generate a few tenths of a volt between 'the
world' and your boat ground. If you see something near 1.2 volts, there
is something else going on. The GI diodes will maintain a 1.2 volt drop
for current levels from 10s of milliamps to many amps, so if you see
1.2V, suspect trouble.
6. I responded that the assertion failed
to identify the current path by which
this occurred. Further, I observed that
a measured forward voltage of 0.6 volts
per diode was equivalent to a forward
current on the order of 100 mA, and that
was far in excess of the currents found
in typical yacht-based galvanic couples.
0.6 V per diode is close to the knee in its I-V characteristic.
Depending on the exact diode, that might mean that it is conducting 10
mA. Or maybe 1 A. The various opens and shorts that might combine to put
this kind of current (plus amateur wiring jobs) are too numerous to list
here.
There shouldn't be any galvanic reactions that will exceed a double
diode (1.2V) drop.
7. A contrary assertion was made that at
0,6 volts, the forward current in a
diode is on the order of only microamps.
No. I said _below_ 0.6 volts.
And so here we are. I don't know your
basis for that assertion, Paul.
Disregarding what has come to be the
normal protocol for some in the group, I
actually measured a 1N1190A (I use them
in the GI's I build) a few moments ago.
The forward current at 675 millivolts is
100 milliamperes. Using a Keithley
electrometer, I measured a forward
current of about 10 microamps at a
voltage of 300 mV, consistent with the
675 mV/100 mA measurement.
Even the 1N914 signal diodes pass almost
one mA (about 700 microamps per the
datasheet) for a forward voltage drop of
600 mV.
And 100 uA at 500 mV and 12 uA at 400 mV. Its an exponential function
and current drops off very rapidly for forward voltages below 600 mV.
Here is where the discussion stands:
I. An observed anomalous current of ~100
mA DC is measured across a GI. (Actually
the measurement was 0.6 volts DC and
there is disagreement over every aspect
of that measurement.)
Did you actually measure that current? Given your above measurements, a
GI (with good diodes) should not conduct 100 mA with a 0.6 volt drop
(I'm assuming a double diode drop in each direction).
II. The current path (of 100 mA) between
the boat's zinc/bronze couple and the
shore power ground has not been
identified although much hand-waving has
transpired.
If the above measurements are valid (both I and V), I'd assume that one
of the GI diodes is bad (shorted). 100 mA at 0.6 V forward is near the
VI curve I'd expect for a single diode.
III. There is seemingly irreconcilable
disagreement about metrology, Ohm's law,
diode VI characteristics (e.g., the
switch analogy), and the properties of
galvanic currents.
Your patience with me is appreciated,
but there are other callings.
Chuck
PS: Paul, I inadvertently seem to have
sent an earlier draft of this directly
to you rather than to the group. No idea
how that happened, but my apologies.
That's OK. I'll respond here, so others may comment.
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Paul Hovnanian
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Marching to a different kettle of fish.