Do you know the direction of the current in the zinc wire connection where you want the resistance?

 

The thruhulls need to be more negative in charge than the zinc.
That's why the confusion over polarity of the zinc.
Do you think the zinc and thruhulls need to be switched in Ron's diagram?

Steve
(still waiting for parts delivery and working as fast as I can in my business)
http://www.sjsewing.com

 

Put 2 shottkey (SP) diodes in series as a regulator, or build a linear regulator.

 

Parts arrived and I've breadboarded my first try.
I'll test it tomorrow and post results.
Fingers crossed.


Steve

 

It's still in breadboard stage, but it worked perfectly the first time.
I put the 25 turn mini potentiometer in the middle of its setting and came up with -0.540 volts on the thruhulls. A couple of turns, and the voltage is at -0.550 and rock solid.
No noise on AM, short wave at all.

Thanks to Ron H. and all who contributed to this project.
I hope I can return the favor someday.

Steve

 

It's still in breadboard stage, but it worked perfectly the first time.
I put the 25 turn mini potentiometer in the middle of its setting and came up with -0.540 volts on the thruhulls. A couple of turns, and the voltage is at -0.550 and rock solid.
No noise on AM, short wave at all.

Thanks to Ron H. and all who contributed to this project.
I hope I can return the favor someday.

Steve

Amazing! Does the MOSFET get warm?
How did you test this? Do you live on your boat?

 

The MOSFET gets 2 deg f. above ambient.
We don't live on our boat, but it's really handy, parked at a salt water canal dock behind our house.
That sure makes boat projects easy!

Steve

 

Wish I could figure out how to simulate my design. I seldom post something that I haven't simulated, unless it is very simple.

 

Ron,
In my case, a silver earring served as the reference electrode.
I soldered a wire to it, sealed the solder so only silver was exposed to the salt water.
I mounted that in a PVC bit which is immersed in salt water via the centerboard trunk.
That's the white wire in one of the pictures.
The zinc anode is mounted below the hull on a s.s. threaded rod attached inside the hull.
The bronze thruhulls are all connected together via a heavy copper wire.
They all meet up at the above mess.
I fed the 12 volts temporarily from a cigarette lighter socket.
The negative came from the thruhull bonding wire.
It's heavy, and the circuit draws next to nothing.

I'm building the final version in a small plastic box as we speak.


Steve

 

I am a little confused. In one post, you said your typical pot resistance was 250Ω. In another post, you said the typical current was about 35mA. Ohm's Law means there would be 8.75v across the pot. Did you slip a decimal point somewhere, or am I not envisioning how you are measuring this? Is the 35mA what you might get when the pot is zero ohms?

 

The original circuit was a small panel mount analog meter in series with a wirewound pot. The meter required about 20 milliamperes for full scale.
The current (at least at today's salinity level) from the zinc to the bonding system with a Fluke meter connected between the zinc and the bonding system and no other circuit in place gives about 60 ma.
I didn't measure the voltage between the silver and the bonding under those conditions, sorry I should have but I'll bet it's almost a volt.

That's the original setup where the bottom paint was being damaged, and the reason for your help with the circuit.
With your circuit in place and -0.550 volts dialed up, the current varies of course with the salinity changes, but seems to average around 15 ma or so.

The thing that makes for confusion is the changing salinity.
Being in the Pacific NorthWET we have rain which being less dense, floats on top of the salt water in the canals, the tide which flushes it out and replaces it with salt water.
This makes it tough to get concrete scientific data.

BTW, I did not connect the Ref and Cathode of X1 together as in your diagram. The Ref goes to the pot slider and the Cathode goes to the 4.7k and the end of the pot.

Steve

 

The original circuit was a small panel mount analog meter in series with a wirewound pot. The meter required about 20 milliamperes for full scale.
The current (at least at today's salinity level) from the zinc to the bonding system with a Fluke meter connected between the zinc and the bonding system and no other circuit in place gives about 60 ma.
I didn't measure the voltage between the silver and the bonding under those conditions, sorry I should have but I'll bet it's almost a volt.

That's the original setup where the bottom paint was being damaged, and the reason for your help with the circuit.
With your circuit in place and -0.550 volts dialed up, the current varies of course with the salinity changes, but seems to average around 15 ma or so.

The thing that makes for confusion is the changing salinity.
Being in the Pacific NorthWET we have rain which being less dense, floats on top of the salt water in the canals, the tide which flushes it out and replaces it with salt water.
This makes it tough to get concrete scientific data.

Thanks. Maybe that will help me set up my spice simulation.

BTW, I did not connect the Ref and Cathode of X1 together as in your diagram. The Ref goes to the pot slider and the Cathode goes to the 4.7k and the end of the pot.

Not sure why you made that change, but the results are the same either way.

 

Ron,
Actually, I didn't connect it by accident, but it worked so I left it that way.
I've just now finished construction and will hook it up.
I'll read both zinc current and thruhull bonding voltage and report back.

Steve

 

Ron,

Everything's installed and working.
About 20 milliamps at -0.560 volts on the thruhull bonding system wire.

I do get a sampling ZZZzt in the AM radio about every 45 seconds.
The sampling rate doesn't need to be that fast, so I'll take your advice on slowing the rate with a cap to ground from the + input on the op amp.
It would be fine with me if it had a sampling rate of 5 minutes or so.
I'll get there quickly enough with my handy dandy capacitance substitution box.

It sure is great to see a regulated voltage on my bonding system ! Thanks a million for your help.

Steve

 

Ron,

Everything's installed and working.
About 20 milliamps at -0.560 volts on the thruhull bonding system wire.

I do get a sampling ZZZzt in the AM radio about every 45 seconds.
The sampling rate doesn't need to be that fast, so I'll take your advice on slowing the rate with a cap to ground from the + input on the op amp.
It would be fine with me if it had a sampling rate of 5 minutes or so.
I'll get there quickly enough with my handy dandy capacitance substitution box.

It sure is great to see a regulated voltage on my bonding system ! Thanks a million for your help.

Steve

I'm waiting to see if it still does what you want when the salinity changes. My first try at simulation wound up with the circuit latching up, with the MOSFET on all the time.

 

I will set up my Fluke ScopeMeter today.
It has a recording feature so I can leave it running and take Max, Min, Avg, and Now voltages.
I'll let it go for a day or so and post results.

Steve

 

I set the voltage at -0.560 with tide going out.
I returned from a boat delivery about 8 hours later with the tide most of the way back in and the salinity was up considerably, so the voltage had dropped to about -0.520.
I cranked up the voltage to -0.560 and will monitor it for a few more tide cycles.

Steve

 

It's holding within roughly 0.01 volts over several days now.
I found part of the problem of the previous voltage swing which had nothing to do with the circuit.
It was a corroded connection at the rudder assembly which varied the zinc load by a considerable amount.
After repairing the connection and resetting the voltage at -0.560, it has never gone over -0.572 and is usually around -0.565.
That's a pretty close hysteresis which makes me one happy boater !

Steve

 

It's holding within roughly 0.01 volts over several days now.
I found part of the problem of the previous voltage swing which had nothing to do with the circuit.
It was a corroded connection at the rudder assembly which varied the zinc load by a considerable amount.
After repairing the connection and resetting the voltage at -0.560, it has never gone over -0.572 and is usually around -0.565.
That's a pretty close hysteresis which makes me one happy boater !

Steve

Please keep us posted if the voltage changes significantly.

 

Ron,

I'll do that.
This AM, the voltage was -0.568.

Steve