Help Needed/Bond Ckt

Discussion in 'General Electronics Chat' started by senormechanico, May 5, 2013.

  1. senormechanico

    Thread Starter Member

    May 5, 2013
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    I'm new to this forum, retired electronic servicer.
    I retired in 1997, and have never worked on a design level with op amps.

    If anyone can give me a bit of help designing an automatic circuit to regulate zinc current on my boat, I'd really appreciate it.

    The boat is docked in salt water, bronze thruhulls are all connected by an 8 gauge wire, and are protected with a zinc anode.
    The only problem is zincs go away pretty fast unless I can regulate the zinc current to keep the thruhull voltage relative to a silver electrode (permanently installed) at -0.550 volts or slightly above.
    Unregulated, it runs nearly a volt, and besides going away fast, the zinc actually causes the copper bottom paint to "burn".
    It blisters around the thruhulls up to nearly a foot away.

    The boat is not connected to shore power, as I have solar panels and the boat is completely self contained with a MPPT controller, Lithium Phosphate batteries etc. so there are no electrolysis issues.
    The boat's single battery bank consists of four 200 aH LiFePo4 cells and the voltage never varies more than 14.2 at the high and 13.1 at the low.

    My idea is to compare the voltage between the silver electrode and the bonding system.
    The resultant difference would vary the series resistance between the zinc electrode and the bonding system.

    I would like to use a potentiometer or select a resistive value to get a permanent -0.550 to -0.0600 volts on the bonding system.

    Currently, I have a 250 ohm wirewound pot in series with the zinc, but with tidal changes, the current varies all over the place.
    Max current of the zinc is somewhere around 100ma, but usually around 35ma.

    Any diagrams or suggestions are gratefully accepted!

    Thanks in advance,

    Steve B.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Rather than trying to change a resistance, which is difficult to do electronically, it would be easier just to regulate the voltage. That could be done with a voltage reference IC, an op amp, a few resistors and capacitors, and a transistor.

    If that sounds OK, post a simple diagram of your connections and where you want to regulate the voltage. That will help in generating the proper circuit.
     
  3. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    Here are the players in this project.
    The thruhulls are bronze, connected together with a bonding wire.
    The zinc current needs to be controlled via the voltage differential between the silver electrode (.995 silver) and the bronze thruhulls.
    Here's a diagram.
    I hope it's legible enough.

    Thanks very much for your quick response!

    Steve
     
  4. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    I should add for clarity, the bonding system needs to be minus 0.550 volts relative to the silver electrode, as the electrode is only a reference.

    The zinc current must be the means of driving the bonding system negative.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Do you know the direction of the current in the zinc wire connection where you want the resistance?

    Do you mean the Thruhulls are at the negative battery potential?
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    The first thought I had was a constant current regulator, but I'm not clear as to what is being done here.
     
    #12 likes this.
  7. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    Zinc is positive with respect to the thruhulls.
    The thruhulls are common to the battery negative.
    I would prefer they weren't but it's necessary with the keel cooled refrigeration system.
    The keel cooler is a bronze affair with the refrigerant pumped through it.

    If the zinc is directly connected to the thruhulls, the voltage measured between the thruhulls and the silver electrode is almost one volt with the thruhulls negative.
    Measuring between a zinc hanging free in the saltwater and a bronze element, the zinc would be positive.

    Here's a voltage chart for various metals:

    http://www.corrosionist.com/Galvanic_Series_of_Metal_in_Seawater.htm
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    What kind of amperage and voltage range are you looking for? If I read your chart correctly it is about 1.8VDC max.
     
  9. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    Actually, a constant voltage is what's needed.
    Minus .550 is the minimum threshold for corrosion protection for the bronze bits of my underwater metal.
    More just eats the zinc anode faster and if high enough, damages the antifouling paint as well.
    The current needed by the bonding system to maintain that voltage varies quite a bit during tide changes, boat movement, rain diluting the salt content etc.

    Thanks for your interest!
    Steve
     
  10. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    The thruhulls will never need any more than about -0.7 to -0.8 volts.
    By the time it gets to -1.0, it's damaging the bottom paint.
    Current required by the bonding system is small, no more than 0.1 amp
     
    Last edited: May 5, 2013
  11. crutschow

    Expert

    Mar 14, 2008
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    OK, I'm still a little confused. Is the zinc connected to the thruhulls or is the zinc freehanging?:confused: What is the direction of the current flow from the zinc during normal operation?
     
  12. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    The zinc is mounted on the fiberglass hull with a stud going through the hull.
    There is a wire connected to the zinc which I have run to a central location for the intended circuit.
    Currently, I'm keeping the zinc current under control via a 250 ohm wirewound potentiometer.
    The voltage drop across the pot changes with changing salinity in the water.
    When the salinity goes up, the voltage drops on the thruhulls to less than what's needed for protection against electrolysis.
    When the pot is increased to make up for this, eventually the salinity changes the other way and now I have too much voltage on the thruhulls which damages the paint.

    That's why I hope to build a circuit using an op amp or similar to keep the impressed voltage on the thruhulls at -0.550 volts by varying the current generated by the zinc.

    Think of the circuit as a gnome in a box watching a voltmeter connected between the silver probe and the thruhull bonding system. He has a potentiometer in series with the zinc, and adjusts it so the meter returns to -0.550 volts on the bonding system whenever the voltage rises or falls away from that value.

    The zinc is positive with relation to the bonding system and erodes away in the water as it does its job.

    Steve
     
  13. crutschow

    Expert

    Mar 14, 2008
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    OK, I believe I have the basics.

    Let me mull it over for awhile.
     
  14. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    Thanks muchly.

    I hope I can return the favor to the board.

    Steve
     
  15. crutschow

    Expert

    Mar 14, 2008
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    OK, I'm having a problem understanding the polarities. :confused: The Galvanic series says the zinc will act as the anode (negative terminal) with respect to bronze which I would think means the zinc has a negative polarity, not positive as you state. Can you clarify that for me?
     
  16. #12

    Expert

    Nov 30, 2010
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    I hope you work this out. I know it's not magic, but there is so much to know! I'm watching so I can learn how this works.
     
  17. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    If you put a zinc and a piece of bronze in a glass of salt water with wires attached to each of them and connect them to a meter, you have a battery.
    The zinc will negatively charge the bronze as it gives up material to the salt water.

    http://en.wikipedia.org/wiki/Cathodic_protection

    http://en.wikipedia.org/wiki/Galvanic_anode

    That's the desired result on my boat, but I need to automatically regulate the current being generated by the zinc because if it is not regulated, the paint "burns" near every bronze fitting as copper in the paint is less noble than the bronze.
     
  18. #12

    Expert

    Nov 30, 2010
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    Yes, I know how to make a basic battery, but their current doesn't vary much because of tidal changes. How this applies to boats is exactly why it is different from two strips of metal in a glass of water.
     
  19. senormechanico

    Thread Starter Member

    May 5, 2013
    35
    2
    The boat is at my dock in a salt water canal.
    The tide change is on the average of 8 feet, and there is a certain amount of fresh water rain runoff coming in at the opposite end of our canal, so the salinity change is significant.
    Thus the need for an automatically adjustable circuit to keep zinc protection current at an optimum level.
    The zinc current will have to change significantly to keep the protective voltage around -0.550 minimum to -0.700 maximum.

    Steve
     
  20. timescope

    Member

    Dec 14, 2011
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    Where is the 250 ohms resistor connected ? Between the wire to [under water metal to be protected] and the zinc connection or in series with the zinc wire ?

    Timescope
     
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