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A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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. 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. 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. 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. I would try to track down what is causing those readings. Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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