Akestor
Active member
I wonder if this setup could do the job! 2 rectifier diodes as in the pic...
DIY Galvanic Isolator?
- Thread starter Akestor
- Start date
rogerthebodger
Well-known member
My understanding is you need 2 diodes in series then 2 sets back-to-back so 4 diodes in total
VicS
Well-known member
No , for several reasons.
You need 2 diodes in series in each direction to be sure to block all galvanic currents ( as Roger says)
You need a capacitor as well to conduct AC leakage currents which might otherwise switch the diodes into conducting mode.
The current rating must be high enough to withstand the maximum fault current which might be expected
The diodes should be mounted on heat sinks
A galvanic isolator should "fail safe" i.e. not open circuit or you will loose the AC " earth " connection with obvious safety implications. ( I dont know how this is achieved )
If P Rainbow comes this way he will be able to explain the above in detail
Alex_Blackwood
Well-known member
As R the D says you need four diodes, two in series on each leg. As you show one leg biased one way and t'other the t'other
That way you block anyvoltage under approx. 1.4 v in either direction. My advice would be spend the money on a proprietary device which has fail safe built in and indication of operation. Faffing around with DIY for that sort of thing is "Nice to be able to do" but a waste of time, effort and money.
Akestor
Active member
I forgot to mention that each diode has a forward voltage of 1.2 volts. Should it be 4 diodes of 0.6 volts?
As for the capacitor, not sure how to wire it yet lol.
Thinking of installing shore power on the boat, and I would like to do it the proper way with earth protection, RCD, MCB, etc...
But reading that even quality galvanic isolators are sensitive and blow, don't like to spend on replacing galvanic isolators...
Another thought is maybe to go without a GI, and install the earth AC wire on the hull bulb anode which is not connected to anything, It is just "in sight" with the propeller.
Well I might as well stay with the DC and solar...but would love to be able to turn on a small heater in the winter, or charge the batteries on the ports where side boats create a shadow and panels half working...
As for the capacitor, not sure how to wire it yet lol.
Thinking of installing shore power on the boat, and I would like to do it the proper way with earth protection, RCD, MCB, etc...
But reading that even quality galvanic isolators are sensitive and blow, don't like to spend on replacing galvanic isolators...
Another thought is maybe to go without a GI, and install the earth AC wire on the hull bulb anode which is not connected to anything, It is just "in sight" with the propeller.
Well I might as well stay with the DC and solar...but would love to be able to turn on a small heater in the winter, or charge the batteries on the ports where side boats create a shadow and panels half working...
Dellquay13
Well-known member
You can get the four diodes needed in one of this type of rectifier. They are mains rated.
I have used them to make mains filters against DC offset for hifi purists, and that is basically a very similar cure for a similar problem.
Edit struggling to attach the picture…
I have used them to make mains filters against DC offset for hifi purists, and that is basically a very similar cure for a similar problem.
Edit struggling to attach the picture…
Dellquay13
Well-known member
Akestor
Active member
Saw online that someone made an isolator with those exactly. He has an article on it, although I am not sure of his connection later as it seems he does a parallel connection of the isolator and not in series.
DYI Galvanic Isolator
Dellquay13
Well-known member
Here’s a link to one of my previous threads about very simple 230V circuits on older boats and why you don’t always need a GI. I haven’t bothered with one, as I don’t have a hull anode on my 45 year old boat.
Simple 230v socket and an earth connection to 12v neg?
Paul Rainbow explains across a few posts why my basic setup was better without GI and an anode. It has been increased to now include a couple more sockets, but since there is still no earth link between my 230v and my 12v (only a twin insulated smart charger) I still don’t need a GI.
I don’t have an inverter, I only have 230v via shore power, but an inverter would need earthing through a GI and the hull.
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rogerthebodger
Well-known member
Changing my name now to Rogerthedodger but I have also been called Rogerthelodger
“What’s in a name? That which we call a rose by any other name would smell as sweet.
Too many call me a rear end but who cares I don't
Don't worry Alex I have been called worse
I fitted one of these when I was building my current steel boat
Buy Marine Galvanic Isolators - Free UK Shipping! - Safeshore Marine UK - Galvanic Isolators
Roberto
Well-known member
Fwiw, this is the one I copied from a commercial one, same rectifier diodes + radiators, some beer can foils as additional dissipators, it all fits inside a waterproof plug protection. Note all four tabs of the two bridges are used.
Isolatore galvanico
wonkywinch
Well-known member
Tranona
Well-known member
That is exactly how i did mine - used a button anode as the earth. Having an anode in sight of the propeller and not connected to anything is a waste of time. If it is to protect the propeller from galvanic action with the shaft then it needs to be bonded to the shaft. If you are not experiencing any dezincification then no need to connect the anode, just use it as the earth.
Agree with the others, don't mess with trying to make a GI. Thy are reliable bits of kit.
Akestor
Active member
That's a very informative thread thanks.
As I understand your setup. the AC earth wire is not going to the water at all correct? Does the shore earth wire terminate on your shore power inlet? Or continue to your socket inside the boat?
Is your rccb functional with this setup ( I read that it isn't unless you have the earth to water)? Or are you relying on the marina's rcd?
Also- what do you see with the plug tester?
Akestor
Active member
Good job. Do you think it does the job? Is there a way to know if it is working? Looks like you have used all the pins of each rectifier?
Akestor
Active member
So your AC shore earth is connected to the anode, and the DC neutral is isolated correct? Do you have a GI or do you think is not necessary? If DC neutral is isolated can we still have galvanic corrosion? Even better if the propeller and shaft are completely isolated from the DC neutral and the AC earth, they will only get galvanic corrosion because of bronze and stainless. In this case, I suppose prop and shaft will not be aware the boat is connected to shore power
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Dellquay13
Well-known member
My AC earth goes around all the sockets, but not to water. I rely on the earth back through the marina supply. My rccb breaker will work as normal, as if it were in a normal domestic system, both relying on the incoming supply for earth. (tbh it is a more reliable earth than my house which is solely protected by a metal spike, being rural and a long way from the substation.)
The marina electrics have a rigorous inspection schedule, and the incoming supply is also rcd protected on the pontoon bollard.
My simple plug socket tester shows all green lights for good connection to live, neutral and earth.
My only point of connection between my 230v and 12vdc is via the twin insulated smart charger which being a flat twin mains flex has no earth core.
The only anodes on my boat are the one on the shaft protecting the prop, and the internal pencil anode for the engine.
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lustyd
Well-known member
But does a dodger cover the companionway or shield the cockpit from side winds?
Tranona
Well-known member
The AC and DC are not in any way connected. The AC comes in via the the normal shorepower sockets through a GI I used this one Zinc Savers / Galvanic Isolators but there are other makes which retail around £65-70 in the UK to a consumer unit with 2 circuits (and space for one more) , one for sockets and the other for the battery charger. Earth to a button anode on the hull. The earth is not strictly necessary and probably thousands of boats are wired without it, but it became compulsory in the 2013 RCD so it makes sense to follow the new rules. I used a dedicated anode as at that time the boat did not have an anode for the propeller. However I have since fitted a feathering propeller and installed a hull anode to help extend the life of the anode protecting the prop.So your AC shore earth is connected to the anode, and the DC neutral is isolated correct? Do you have a GI or do you think is not necessary? If DC neutral is isolated can we still have galvanic corrosion? Even better if the propeller and shaft are completely isolated from the DC neutral and the AC earth, they will only get galvanic corrosion because of bronze and stainless. In this case, I suppose prop and shaft will not be aware the boat is connected to shore power
William_H
Well-known member
From comments by OP he is not needing full time AC power connection. Just available for occasional use. ie when he is on board. Many mains powered devices are double insulated ie plastic fan heater kettle etc. Even battery charger probably double insulated. This means that the 3 wire cord from shore will have an earth system safe for your operation. Even if you do connect the earth to the ship's earth negative. Should be no problem provided you disconnect cable from shore power when you leave the boat. ie 99% of time. GI is needed if mains power is connected 24/7. ol'will
