Larry wrote:

[snip]
When a student asked me for a silicone diode, I always had a handy tube
of RTV Silicone Sealant to hand him for his request.

I think the guys have a different sort of silicone on their minds at
this age. ;-)

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Paul Hovnanian
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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. 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. 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.

The water will be at the potential between the two half cell voltages. I
looked up some standard cell voltages (sea water voltages will be
proportionately lower):

Zinc -0.76 v
Copper +0.34 v

The boat would theoretically be at -0.21 V with respect to the water.

The other effect here is the small resistance in the connection between
the prop and the zinc. Typically, the zincs are bonded to the boats DC
ground through fixed connections. The prop is connected to the DC ground
through bearings and the engine block with some grease and moving parts.
This path will have a higher resistance, so the 'voltage divider'
created will put the boat's ground closer to the zinc voltage than the
prop voltage (more negative). This is why zincs on a boat, without a GI
will readily dissolve to protect any immersed metals on the dock or
adjacent boats.

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.

Not 'no current'. Just very low currents. At 0.3 volts, its in the
microamp (or 10s of microamp) range. Its also very temperature
sensitive, so your assertion that one cannot determine diode current by
measuring voltage is correct (without gobs of sensitive lab equipment).

Regards,

Ann-Marie Foster,


--
Paul Hovnanian
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Power corrupts. And atomic power corrupts atomically.