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A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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 ----== 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 =---- -- Paul Hovnanian ------------------------------------------------------------------ Klein bottle for rent -- inquire within |
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