|
Zinc is dissappearing FAST!
Hi folks,
I just relaunched my boat after painting the bottom, installing new zincs and instruments. That was in March of this year. Around the 1st of July, I scrubbed the bottom in preparation for a race, and noticed the zinc on the strut was nearly gone, despite only having been in the water 3 months. Someone tells me this is because I need an isolator. What is an isolator, and how does it work? Big picture and simple answers preferred. thanks, Luc |
Zinc is dissappearing FAST!
luc wrote:
Hi folks, I just relaunched my boat after painting the bottom, installing new zincs and instruments. That was in March of this year. Around the 1st of July, I scrubbed the bottom in preparation for a race, and noticed the zinc on the strut was nearly gone, despite only having been in the water 3 months. Someone tells me this is because I need an isolator. What is an isolator, and how does it work? Big picture and simple answers preferred. thanks, Luc The galvanic isolator goes in your AC ground line (the green wire) and prevents your zinc, which is presumably connected to your boat's AC ground wire via the boat grounding systems, from trying to protect your neighbors' boats, the dock and any other immersed water in the vicinity. The shore power green wire goes to the isolator as it's first connection in the boat. Alternatively, disconnect your shore power. bob |
Zinc is dissappearing FAST!
RW Salnick wrote:
luc wrote: Hi folks, I just relaunched my boat after painting the bottom, installing new zincs and instruments. That was in March of this year. Around the 1st of July, I scrubbed the bottom in preparation for a race, and noticed the zinc on the strut was nearly gone, despite only having been in the water 3 months. Someone tells me this is because I need an isolator. What is an isolator, and how does it work? Big picture and simple answers preferred. thanks, Luc The galvanic isolator goes in your AC ground line (the green wire) and prevents your zinc, which is presumably connected to your boat's AC ground wire via the boat grounding systems, from trying to protect your neighbors' boats, the dock and any other immersed water in the vicinity. The shore power green wire goes to the isolator as it's first connection in the boat. Alternatively, disconnect your shore power. bob Bob is correct, but keep in mind that other causes could be responsible for the rapid zinc depletion. They're much less common, however. If you hire a competent marine electrician, he can quickly verify that the problem would be fixed by installing a galvanic isolator. 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 =---- |
Zinc is dissappearing FAST!
"luc" wrote in news:1153249242.027049.36020
@i42g2000cwa.googlegroups.com: Hi folks, I just relaunched my boat after painting the bottom, installing new zincs and instruments. That was in March of this year. Around the 1st of July, I scrubbed the bottom in preparation for a race, and noticed the zinc on the strut was nearly gone, despite only having been in the water 3 months. Someone tells me this is because I need an isolator. What is an isolator, and how does it work? Big picture and simple answers preferred. thanks, Luc http://www.yandina.com/electrolysis.htm http://www.jefa.com/install/electro.htm http://marinesurvey.com/yacht/corrosion_1.htm http://www.sailnet.com/collections/a...eid=hughes0004 http://www.protroll.com/blkbox3.html |
Zinc is dissappearing FAST!
|
Zinc is dissappearing FAST!
Mike wrote:
My boat also goes through zincs in 100 days +/-. I have a GI installed. How can I tell if my boat is the problem or if it's neighbors or the marina (without disconnecting from shore power for 90 days :-) )? Mike Hello Mike, First thing is buy a copy of "The 12 Volt Doctor's Practical Handbook" at your local marine store. With a simple DMM you can do some testing to answer your question. You can even test neighboring boats if you can clamp a test lead onto a shroud or grounded piece of metal. Some folks will have strong feelings about this "trespassing" but you need not board the vessel and will not mar it or subject it to damage in any way by the testing. If you know the owner, you may even find him willing to participate. Anyway, you need to disconnect the shore power only for the duration of the test, which should be less than 5 minutes. Read the (very short) book and post any questions you have. Having said all that, I must say that 100 days is too short a period for zinc life. There are many possibilities, starting with whether your zincs are too small. Did you notice a difference in zinc life when you installed the GI? Can you triple the zinc size easily? Have you considered a supplemental zinc hanging over the side? Good to have one on board anyway. Good luck. 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 =---- |
Zinc is dissappearing FAST!
On 19 Jul 2006 05:51:08 -0700, "Andina Marie"
wrote: By far the majority of accelerated zinc loss cause is the shore power connection. The first step is to install a galvanic isolator. Defender has them on sale for about $95 at http://www.defender.com/product.jsp?path=-1|328|303336&id=605562 ================= Is it possible to install the isolator external to the boat using the right combination of connectors and junction boxes? The shore power circuits on my boat are almost impossible to get at in any reasonable way. |
Zinc is dissappearing FAST!
Wayne.B wrote:
On 19 Jul 2006 05:51:08 -0700, "Andina Marie" wrote: By far the majority of accelerated zinc loss cause is the shore power connection. The first step is to install a galvanic isolator. Defender has them on sale for about $95 at http://www.defender.com/product.jsp?path=-1|328|303336&id=605562 ================= Is it possible to install the isolator external to the boat using the right combination of connectors and junction boxes? The shore power circuits on my boat are almost impossible to get at in any reasonable way. The only problem I can imagine is dealing with the heavy connectors and cables. Otherwise, it should work fine. You could use a conventional power cord from the dock to your boat and locate the GI box on the boat rather than on the dock. 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 =---- |
Zinc is dissappearing FAST!
|
Zinc is dissappearing FAST!
Wayne.B wrote in
: Is it possible to install the isolator external to the boat using the right combination of connectors and junction boxes? T Sure. Buy the 30A or 50A isolation transformer he http://www.charlesindustries.com/main/ma_iso_bost.html Mount it in a ventilated NMEA enclosure next to the dock power pedestal so it can cool itself. Run a short cable from the transformer to plug into the dock pedestal, then, with the appropriate AC twistlock outlet for the size of the transformer mounted on the outside of the NMEA box, plug the boat into the box....KEEPING THE BOAT GROUND DISCONNECTED FROM THE DOCK GROUND, PLEASE! Both sides of the isolation transformer are isolated from ground. There is no grounded neutral like shore power uses. The only way you can be shocked is when you are directly connected ACROSS L1 and L2 on the transformer secondary. That's why they call it "Isolated" in the first place. Unconnected from dock ground and the grounded nonsense of the marina and rest of the halfass-wired boats, your galvanic current ashore is zero, leaving only galvanic currents caused by your boat eating the zincs. |
Zinc is dissappearing FAST!
I understand how NO ground connection is better than a typical setup with an
isolator, but if the isolator is functioning, isn't the effect very close to the same? Let me ask it another way. If I have about 0.2 VDC across the galvanic isolator, would I see about the same between the shore ground and neutral/ground on the boat side of an isolation transformer? "Larry" wrote in message ... "Mike" wrote in news:b8idnTRka-tz- : (without disconnecting from shore power for 90 days :-) )? Install an isolation transformer with an ISOLATED GROUND. That allows the AC to flow across the transformer windings but not the galvanic current, which is DC.....unless some idiot hooks you boat ground to the dock ground rendering the isolation useless. |
Zinc is dissappearing FAST!
Mike wrote:
I understand how NO ground connection is better than a typical setup with an isolator, but if the isolator is functioning, isn't the effect very close to the same? Let me ask it another way. If I have about 0.2 VDC across the galvanic isolator, would I see about the same between the shore ground and neutral/ground on the boat side of an isolation transformer? Larry may have answered your question indirectly. The short answer is yes. If you follow the recommended practice of using one of the transformer secondary wires as a combination neutral/ground, then you would measure the same 0.2 volts from there to the shore power ground. 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 =---- |
Zinc is dissappearing FAST!
On Wed, 19 Jul 2006 18:28:19 -0400, Larry wrote:
Sure. Buy the 30A or 50A isolation transformer he http://www.charlesindustries.com/main/ma_iso_bost.html I understand the benefits of an isolation transformer but I was looking for lighter, cheaper, etc. |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Take a fairly decent digital voltmeter and measure the voltage across
the Galvanic Isolator with the boat hooked up and normal loads turned on. Check the readings when refrigerators and AC are running and drawing current. First measure the DC voltage (reversing the leads if necessary to get the right polarity if the meter doesn't correct automatically). This voltage should measure somewhere between about 0.l volts and 1.2 volts. If it measures exactly zero volts (less than 0.1) this indicates the isolator is shorted out and needs replacing. If it measures above 1.2 volts this indicates the isolator is open circuit and needs replacing or your ground circuit is broken somewhere. Second, measure the AC voltage across the isolator. This should read 0.25 or less. If it is higher than this voltage then AC current flowing through the isolator will be decreasing the efficiency as much as 50% depending on the voltage. AC voltage can come from:- 1. on-board AC equipment that is wired incorrectly and using the ground return instead of the neutral. 2. on-board leakage that tends to build up on older boats due to salt deposits on electrical items. 3. (the most likely one) a neighbors boat is using the ground for the return instead of the neutral and this introduces AC volts back into the ground connection of all the other boats on that ground. Cu- 1. One by one turn off all the AC items and see if turning one of them off makes the AC voltage on the isolator go away. If you find a culprit, then check the wiring at the point of connection and in the device itself to make sure it is wired correctly. 2. There is no easy way to correct accumulated leakage. You have to bypass the AC leakage around the isolator with a galvanic capacitor. Some Galvanic Isolators have a capacitor built in but they are typically just a woefully inadequate size so they can advertise it. You will note they never quote the current carrying capacity of the capacitor in their specifications. We make a Galvanic Capacitor rated for 5 amps AC. No others come even close to this capacity. You wire the capacitor in parallel with the isolator - see the link below. 3. Turn off the main breaker at the dock. If the isolator AC voltage remains across the isolator then it is coming from another boat through the ground lead. In cases like this I've had someone watch the meter about mid-day when no one is on board and then turn your neighbor's dock breaker off for a couple of minutes and then back on. (Wait at least 2 minutes so you are not re-starting compressors under load). If that made the AC voltage go away, THEY are the culprit with the mis-wired boat and you need to report it to the marina. I've had criticism for recommending this in the past but the bottom line is the same thing happens in a thunderstorm blackout and that is an act of God so the boat should be able to survive. If you can't get rid of the AC voltage across the isolator you need a Galvanic Capacitor. http://www.yandina.com/GIsolCap.html Finally, if the DC voltage is within limits and the AC voltage is less than 0.25 volts then it is time to consider an instrument to detect galvanic stray current issues. Rather than purchase one and attempt to do the measurements yourself I would suggest getting someone who has a meter and extensive experience in tracking galvanic currents because the meter just gives you a stack of voltage readings. Analysis of these readings can take years of experience unless there is something very obvious. Regards, Ann-Marie Foster, |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Some observations:
Be extremely careful in measuring the voltage across a galvanic isolator. If the isolator should be "open", there may be a dangerous (possibly fatal) shock hazard between the two isolator terminals. This is all the more critical since you usually suspect an electrical problem when you have to resort to checking your isolator. A DC voltage reading across the isolator of zero or near-zero volts is a highly desirable situation! It means that galvanic (and other DC) currents passing through the shore power ground system are negligible, and that is always desirable. A reading of zero volts does not automatically mean the isolator diodes are shorted! Similarly for AC voltage readings of zero or near-zero volts. A reading of zero volts AC means there is no evidence of appliance leakage on your boat and no reason to suspect problems with AC wiring on neighboring boats. 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 =---- |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Larry wrote:
115 is nothing compared to watching him "finger test" 3-phase 408VAC on a motor panel trying to figure out what it's doing. NEVER TURN YOUR BACK ON HIM! Your ear will suffer as he touches your earlobe with one hand in the panel with that ****-eating grin on his face....(c; I think I've already met him, Larry. The ear buzzing brings back a lot of memories. Don't see much of that anymore. Must be the dry callouses on the fingers that help keep the body resistance at a high and steady level. I'd be drenched in perspiration just thinking about it, despite which, I've acquired a badge or two over the years. ;-) 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 =---- |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
chuck wrote:
Some observations: Be extremely careful in measuring the voltage across a galvanic isolator. If the isolator should be "open", there may be a dangerous (possibly fatal) shock hazard between the two isolator terminals. This is all the more critical since you usually suspect an electrical problem when you have to resort to checking your isolator. Before setting the DMM to DC, set it on a higher (start at 200V) AC setting and measure across the isolator. If it reads zero, switch it lower until you can see a 2V AC reading. If you see a high AC voltage, the isolator is open AND you have a dangerous fault onboard. If it reads a couple of volts AC, the isolator is OK, but you still have a fault (or leakage). If it reads an AC voltage no higher than the diode drop, you're OK. The switch to DC and check for galvanic currents. A DC voltage reading across the isolator of zero or near-zero volts is a highly desirable situation! It means that galvanic (and other DC) currents passing through the shore power ground system are negligible, and that is always desirable. A reading of zero volts does not automatically mean the isolator diodes are shorted! Similarly for AC voltage readings of zero or near-zero volts. A reading of zero volts AC means there is no evidence of appliance leakage on your boat and no reason to suspect problems with AC wiring on neighboring boats. Its always a good practice to check the AC voltage of anything first. 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 ------------------------------------------------------------------ Marching to a different kettle of fish. |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Excellent advice, Paul!
----== 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 =---- |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Chuck, in over 20 years working on boats I've never encountered a boat
in the water on shore power that had zero volts across the Galvanic Isolator. Just the zincs on your underwater metal will introduce about 0.5 volts DC. Getting a voltage reading does not in itself indicate electrolysis unless it is over about 1.1 volts since that is the purpose of the GI to block DC current from sources up to about 1.2 volts. So a reading of zero is a fairly good indication that either the GI is shorted internally or more likely there is another connection somewhere on the boat that is by-passing the GI and rendering it inoperative. One source to watch is the television cable if you have one. The shield on this will probably be grounded at the dock end and depending on how it couples to your TV or cable box it can easily be grounded at the boat end too. This shorts out or bypasses the GI and voids the electrolysis protection. However you are correct that an AC reading of zero is a good indication that there is no AC leakage. Regards, Ann-Marie Foster, chuck wrote: Some observations: SNIP A DC voltage reading across the isolator of zero or near-zero volts is a highly desirable situation! It means that galvanic (and other DC) currents passing through the shore power ground system are negligible, and that is always desirable. A reading of zero volts does not automatically mean the isolator diodes are shorted! Similarly for AC voltage readings of zero or near-zero volts. A reading of zero volts AC means there is no evidence of appliance leakage on your boat and no reason to suspect problems with AC wiring on neighboring boats. Chuck |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Thanks for the comments, Ann-Marie.
Andina Marie wrote: Chuck, in over 20 years working on boats I've never encountered a boat in the water on shore power that had zero volts across the Galvanic Isolator. Just the zincs on your underwater metal will introduce about 0.5 volts DC. I don't see how the zincs on your boat can cause a current to flow through your GI (and thus the shore power ground). In the common case of a bronze prop and a zinc on the shaft, for example, the galvanic current passes through the shaft. It does not pass through the GI. A neighboring boat using the shore power ground to complete a galvanic circuit with your zinc would definitely cause a current to flow in the GI. Even if the neighboring boat was protected with its own zinc, there could still be a small galvanic couple created with your zinc, especially in sal****er. Not nearly as likely in fresh water. Other than that, I would have a difficult time explaining the voltages you measure. Chuck |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Chuck, you're confusing current and voltage.
First consider the DC voltage measurement. You don't have to have any current flowing in order to have a voltage across the Galvanic Isolator. Think of the isolator as an on/off switch. If the voltage across the switch is less than 1.2 volts the switch stays off and a meter across the switch will read the voltage with no current flowing. The zincs are used because they create a voltage in the right direction to protect your underwater metal. They also make your boat "alive" at a very low voltage to the water and it is this (plus other influences) that you measure across the Galvanic Isolator. It is theoretically possible, but extremely unlikely, that other DC voltages in the vicinity could completely cancel out the zinc but like I said above, I've never seen one in 20 years of working on boats. So if you read exactly zero volts across the Galvanic Isolator it is a pretty good indication that the "switch" is shorted out and it is not functioning. Of course if you measure greater DC than 1.2 volts that indicates the GI is open circuit. Now consider the AC voltage measurement. AC voltage across the GI can come from two sources. The first is AC return or leakage from your own boat going or attempting to go back to the dock. The second is AC voltages on the ground wire on the dock attempting to go through your boat to the water. This latter can be caused if a boat on the same dock circuit is mis-wired and returning neutral current through the ground wire. The ground wire is typically not designed to carry "working" currents so there is a voltage drop along it back to the distribution point. This voltage can often be in the range of 1 or 2 volts but occasionally I've seen higher and it appears on the ground terminals on the dock and the dock side of your GI. AC voltage on its own does not create electrolysis however if it is excessive it can cause blisters on metal boats where chlorine forms under the anti-fouling paint. But that AC voltage is arriving across the Galvanic Isolator. In one half cycle the AC voltage is subtracting from the DC voltage but in the other half cycle it is ADDING to it. So if your AC has a peak of 0.5 volts your DC protection is reduced from 1.2 volts down to 0.7 volts for a percentage of the time that increases with the AC voltage to a worst case of 50%. Putting a large capacitor across the Galvanic Isolator shorts out the AC voltage while making no change to the DC and this prevents the AC from piggybacking the DC through with it. Regards, Ann-Marie Foster, chuck wrote: Thanks for the comments, Ann-Marie. Andina Marie wrote: Chuck, in over 20 years working on boats I've never encountered a boat in the water on shore power that had zero volts across the Galvanic Isolator. Just the zincs on your underwater metal will introduce about 0.5 volts DC. I don't see how the zincs on your boat can cause a current to flow through your GI (and thus the shore power ground). In the common case of a bronze prop and a zinc on the shaft, for example, the galvanic current passes through the shaft. It does not pass through the GI. A neighboring boat using the shore power ground to complete a galvanic circuit with your zinc would definitely cause a current to flow in the GI. Even if the neighboring boat was protected with its own zinc, there could still be a small galvanic couple created with your zinc, especially in sal****er. Not nearly as likely in fresh water. Other than that, I would have a difficult time explaining the voltages you measure. Chuck |
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 =---- |
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 |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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. 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. 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. 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. 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. 7. A contrary assertion was made that at 0,6 volts, the forward current in a diode is on the order of only microamps. 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. 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.) 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. 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. ----== 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 =---- |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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. 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, |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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 ----== 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 =---- |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
chuck wrote in news:1153748524_106745
@sp6iad.superfeed.net: 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. At this level, a lot of what the Keithley is measuring is the diode's own generated voltage. Hook the Keithley to a silicon diode sitting on the bench, not attached to anything. Depending on how hot it is (and how close to any radioactivity it is), there's always a junction voltage from the thermionic emission of the junction, itself. Got a hot radioactive source around? Keep the Keithley across the diode and move the diode up against any beta or gamma sources and watch it fly... (c; (Depleted uranium bullets are great for this experiment. They're free in Afghanistan and Iraq...) Which Keithley is it? I used to repair and cal them at the Metrology Laboratory of the Quality Assurance Office, Charleston Naval Shipyard (Code 132)...may she rest in peace. |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Larry wrote:
At this level, a lot of what the Keithley is measuring is the diode's own generated voltage. Hook the Keithley to a silicon diode sitting on the bench, not attached to anything. Depending on how hot it is (and how close to any radioactivity it is), there's always a junction voltage from the thermionic emission of the junction, itself. Indeed! Actually the junction voltage was ~50 millivolts so I "tuned it out" as they say. Got a hot radioactive source around? Keep the Keithley across the diode and move the diode up against any beta or gamma sources and watch it fly... (c; (Depleted uranium bullets are great for this experiment. They're free in Afghanistan and Iraq...) I could try a smoke detector or Coleman lantern mantle with thorium, I guess. I'll pass on the DU though. Which Keithley is it? I used to repair and cal them at the Metrology Laboratory of the Quality Assurance Office, Charleston Naval Shipyard (Code 132)...may she rest in peace. It's a 610B with electrometer tubes. Works amazingly well for its age. It does have a personality as you know. 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 =---- |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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. ----== 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 ------------------------------------------------------------------ Marching to a different kettle of fish. |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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. ;-) -- Paul Hovnanian ------------------------------------------------------------------ Opinions stated herein are the sole property of the author. Standard disclaimers apply. All rights reserved. No user serviceable components inside. Contents under pressure; do not incinerate. Always wear adequate eye protection. Do not mold, findle or sputilate. |
A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
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 ------------------------------------------------------------------ Power corrupts. And atomic power corrupts atomically. |
| All times are GMT +1. The time now is 08:27 AM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004 - 2014 BoatBanter.com