Home made Galvanic Isolator

[Bill Roosa]

Hi everybody. With all the power usage at the docks now-a-days the importance of a good grounding system cannot be overstressed. One important subsystem in you AC shore power circuit is a galvanic isolator. WM wants $490 for a 30 amp version. Well zincs are not that expensive so you could go that route but I wanted to have my zinc and use them too. To that end I did some research and found that all you really need is two diodes wired anode to cathode. The diodes need to be able to pass the maximum amount of current that your boat can draw (30 amps for most and the number I use for calculations here) and operate at 120 volts. After adding a X2 factor to the current and voltage I came up with a 60 amp X 240 volt diode as the minimum requirement. Breaking out the trusty Digikey catalog and going to the diode section I found one for 60 amps and 600 volts. Since I was wanting to make it "look factory" I ordered one with the "anode to stud" and one with the "cathode to stud" ($12.93 & $15.39). I then mounted them on a small aluminum "L" channel and wired them up and painted them. I put them right on the ground buss of the AC panel so they would be out of the elements.The link below is from WM and shows where to install the unit in the AC circuit.Boating does not have to be expensive but you have to be willing to become a jack of all trades.

 

[RAD]

Bill,

Nice job, but how bout some pictures?

 

[Patrick Lombard]

Number Please

What are the part numbers for those Digikey diodes?

 

[Don]

Could be dangerous

BillI admire your industriousness but there is a reason these are expensive. By the way, you can do a search and find good ones rated properly for around $100-$150.It's not the complexity of the circuit which drive the cost but the component ratings which are not cheap.Capacitors are required in both sides to prevent the diodes from an overload short which can create a live path to ground in the boat. What you describe can work but if anything goes wrong, a worse situation can result not worth the $$$ savings by doing it that way.If I have time later, I'll edit this post to include a link you can read containing a further explaination.Don

 

[Franklin]

Don't forget

Corrossion. Remember, there is a reason we use Tin Coated wires, not just plain wires, for all wiring. Are these parts you ordered rated for marine use? If not, the salty air may do a number of them.

 

[Ross]

There is an excellant material from

3M called liquid electrical tape. It come in a can and you just paint it on. It prevents corrosion. I don't understand the need for the capacitors in this circuit.

 

[Don]

isolator design

Two pairs of properly rated diodes would be required to be wired in parallel doubling the parts cost. Ross - caps increase the ability to conduct AC - or rather bleed it off. Good isolators contain caps which will conduct AC as a backup if/when the diodes fail. Most isolators would also have a built-in heat sink for obvious reasons.

 

[Ross]

I fail to see how this circuit can isolate

the ground system on the boat from stray currents in the water. Diodes as I understand the connections here will provide an unimpeded path for AC or DC. The Capacitors will of course block a DC source but if they are large enough will only shift the phase 90 degrees for an AC circuit.

 

[Ed Sutton]

Bill has an excellent idea

that will work just fine. The diodes need to handle fault current for the amount of time that it will take to clear the supply breaker. I would go a little heftier if possible. The diodes work because they have an inherent forward voltage drop of approximately 0.7 to 1 volt DC. That drop is far greater than the potentials that cause galvanic currents, therefore no current flow. Capacitors can be used but are not needed and, as mentioned in the West Marine link, if you think you need them then get a transformer. I have used this type of circuit in industrial applications as a power supply design engineer.There is one more point that needs to be addressed. There is a ground loop in the connection from the Marine VHF radio through the antenna shield to a mast mounted VHF antenna. This can be broken installing a DC inner/outer blocking capacitor such as a Polyphaser IS-IE50LN-C1 in the antenna coax or just by merely disconnecting the coax while in the marina.

 

[Ross]

Thanks, Ed. Like so many threads on

this forum, This forced me to go looking for explainations in detail. This link provides a good explanation.http://www.allaboutcircuits.com/vol_3/chpt_3/9.html

 

[Bill Roosa]

The specs

The two diodes are International Rectifier from page 898 in Digikey's catalog60 amp 600 volt 1.3 volts forward voltage PN 1N2138A-ND Cathode to stud60 amp 600 volt 1.3 volts forward voltage PM 1N2138RA-ND Anode to studBoth are DO-5 type which has a terminal that you wire to and a screw that is the other terminal. The screw is designed to be the mounting and if need be could incorporate a heat sink. In this case as we are dealing with the AC ground circuit there is generally no current being passed so a heat sink is not needed. Ed is correct in his description of how the circuit works. The forward voltage is the voltage that is needed to make the diode pass current in what is normally thought to be the "open" direction. So no current flows until the voltage in the ground circuit gets to 1.3 volts between the boat and the shore power line. By using both anode and cathode to stud versions you effectively have a circuit that will not pass current in either direction until 1.3 volts is present but will pass current in either direction when the voltage is above that. Which is exactly what you want for an isolator.There have been several comments about safety and just as any other system, you have to do maintenance on it to insure it is functioning properly. I handle this by checking the unit during the winter haul out and would check it if there was ever any short in the AC system. The diodes are not marine rated so a generous mitten full of silicon sealant is certainly in order once the unit is assembled. There are lots of "more heavy duty" diodes in the catalog if you need to have a bigger safety factor and the prices are not outrageous.The reason most factory isolators are expensive is they have built in testing devices. This amounts to a volt/ohm meter being installed in the unit and some indicator lamps. There is the fancy housing and correctly color coded wiring.... and the guys that make them think we should think it is really complicated and that only someone with an electrical engineering degree should be adding water to a wet cell battery. They have kids to send to collage and so have to make a profit. It is just business.

 

[JPC]

40 Amps 600V Diode, $3.50 each

Should you decided to make another Galvanic Isolator, you can get from here.I made one years ago but use much smaller diode(5 amps??) which happen to be available from my parts bin. Just make sure that the white power line wire has good connections so no load current would pass ground wire. No need to spend a fortune for such a low tech electronic piece.

 

[Bill Roosa]

NO NO NO NO and NO

Perhaps I am mis-reading your post JPCAn isolator does not go in the power circuit!!!! It goes on the ground wire of the circuit. It does not handle ships power but only handles electrical shorts and isolates the shore ground from your boat ground with a (in this case) 1.3 volt bias.DO NOT USE ISOLATORS IN THE POWER CIRCUIT!!!!See the original post for the WM link on how it is installed.

 

[Dan Johnson]

Bill...

I have found that unplugging the shore power connection at the dock when leaving the boat has minimized the galvanic corrosion on my shaft zinc compared to the PO's experience where he kept the shore power connected and ate most of a zinc each season. Two seasons on mine and still have at least 80%--will replace it this spring as we're going on a new dock system and I need to establish a baseline.