Andina Marie wrote:
That is a better analysis, Chuck. But two corrections.
1. Think of the zinc and the prop as joined together electrically so
they are at the same voltage as you indicate. Current is flowing
between them. The amount of current is limited by the resistance of
the water so you can consider the water as a resistor connected between
them.
I'm with you this far.
This resistor (the water) also is
connected to the dock, and
other boats in the vicinity so the voltage of your boat (at the
junction of zinc and prop) will be somewhere between the two.
Which end of the "resistor" is connected
to the dock? Why not both ends with full
cancellation?
If there
is 0.9 volts available it would be reasonable to assume this makes the
boat at about 0.45 volts to the water, and the dock etc. So this
voltage will always be present across a galvanic isolator and it is
extremely unlikely you would read zero volts with a GI in working
condition.
Sorry, Ann-Marie, but no cigar.
2. You are incorrect in saying
"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. "
That is wrong. No current flows through a diode until it reaches about
0.6 volts for silicone. It is like a switch that won't turn on until
it gets to 0.6 volts. Once it turns on the voltage across it
essentially stays at about 0.6 independent of current. You can't
determine the current flowing through it by observing the voltage. It
does not behave like a resistor. That is how a galvanic isolator
works. With two diodes in series (each direction) NO CURRENT flows
until the voltage gets above 1.2 volts.
Regards,
Ann-Marie Foster,
Well, that explains why our discussion
is not moving toward closure. Please
take a few moments and go the this website:
http://www.fairchildsemi.com/pf/1N/1N914A.html
Product Folder - Fairchild P/N 1N914A -
High Conductance Fast Diode
Download the datasheet and on page 2,
figures 3 and 4, you will see the
manufacturer's take on whether there is
a relationship between forward voltage
and current. You'll see that there is
absolutely nothing magical about 600 mV
in a V log I plot. It IS like a
(non-linear) resistor.
The relationship between current and
voltage for a pn junction is
well-established and has been for more
than half a century.
Finally, take a variable voltage source,
a 100 ohm resistor and a diode. Put them
in series and adjust the voltage so
there is, say, 300 millivolts across the
diode. You will be able to measure a
current through the diode of something
like one to ten microamps. Change the
voltage and watch the current change.
Then go back and read the posts.
Chuck
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