DIY Galvanic isolator

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GAJ

GAJ

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Has anybody made one of these things?

Surely it cannot be too difficult, boffins please reply with relevant circuit diagrams etc.
Keep it simple though, I am just a humble joiner!
 
Lots of circuits available on the internet. Basically a diode to join mains earth to the boat earth and battery negative. BUT lots of arguments for and against, depends on your type of boat (grp, steel, etc) and your use of shore power. Lots of posts about this previously as well.
 
Probably better to buy one...

You'll be lucky to find circuit details on the internet, although basically it's just 2 pairs of series-connected diodes, with the two pairs connected in parallel in opposing directions (that does make some sort of sense, trust me!). The diodes need to cope with mains voltage and a current of around 30A minimum. It's probably safer to buy a ready-made isolator, eg for £59 inc delivery from JG Technologies.
 
I did...

I did, but I didn't find details of how to make one. Do us all a favour, and post one or two links to the "lots of circuits available on the internet".
 
Re: I did...

A galvanic isolator is just 2 diodes connected in paralall but one reversed relating to the other. It is connected between the ships ground and the mains earth. (where previosuly there was a wire connection) A diode has the ability to conduct current only in one direction, If you used only one diode then should you get a large current from the mains to earth (ships ground) then only the positive half of the cycles would go to ground the other half would not. So you use 2 diodes so both halves of the cycle go to ground. Just likke a piece of wire. Not quite.
A diode has a peculiar characteriustic that it will not conduct current until the voltage has reached a certain level. For the common silicon diode that is .7 volt. it is this characteristic that gives isolation. Anyvotlage above .7 will get through as required for the safety aspects of your mains earth system however any small voltage will not get through.
it is the small voltages generated by galvanic (corrosion) action that are stopped. These voltages (currents) can be generated by other boats or metal parts near your boat which would normally be connected via the earth wire of the mains system to you sacrificial anodes. (This area becomes very vague with so many variables.)
So yes you can purchase 2 high current silicon diodes from an electronics supplier for very little cash. pvb suggests that the commercial isolators contain 2 pairs of diodes in series. This gives an isolation voltage of 1.4 volts which may be better although I am not at all sure. Most galvanic voltages are less than .5 volt.
The important part to the isolator is the physical construction so that it will provide a current path when needed for your safety which may be never.
So you will never know if it is actually working unless you test it with a multimeter meter.
Testing with a meter is a bit unsure because of the design of meters. A good digital multimeter may have a scale with a dioded symbol which directly reads volt drop if the diode. It should read .5 to .8 volt. (in both lead connection directions) If you use a multimeter with only ohm scale then you get a ohm reading of hundreds to several thousand ohms depending on the scale and meter type. This uncertainty is because the volt drop regardless of current confuses the meter designed to cope with resistance directly related to voltage and current.
So give it a go but remember a failure of the isolator can expose you to dangerous electrocution hazard if you have an electrical fault.
regards olewill
 
Re: I did...

Will, what you say is just about spot on but I think you do need to use diodes in pairs, connected in series, to give rather more than the protection that single diodes will give against "galvanic" emfs. There is no harm if the level of blocking is rased to 1.4 volts and at that level you are sure to block all of what you need to.

It is essential that the diodes used can withstand the maximum current they are likely to be subjected to which will be transiently much greater than the fuse rating in the supply. Double? I'm not sure. That means if one is making a galvanic isolator from components available from some where like Maplin electronics you now have to start building up banks of diode pairs in parallel. The highest rated diodes listed by Maplins are 6amp so if you have a 13 amp fuse you need at least two sets in parallel, preferably four to give an adeqaute safety margin.

That means each of your original single diodes has grown to eight making a total of sixteen for the whole unit. but at only 50p each it is still cheaper than a commercially bought unit.

One way that I have seen described is to use bridge rectifiers. If you ignore the AC input connections, or link them perhaps, and connect to just the +ve and -ve terminals you have two pairs of series connected diodes in parallel with each other. So a single bridge rectifier rated at 25 amps becomes good for a 50 amps peak current, ample I would say for a 13 amp circuit, and gives 1.4 volts worth of protection against galvanic emfs. Only two of these at £1.50 are needed for the complete unit. Maplins stock them up to 35 amp.

The bridge rectifiers are also a convenient package to mount and wire up.

I'm not sure what to recommend in the way of voltage ratings. They should not be subjected to any significant reverse voltage but since 280v max 400v PIV ones are available one might just as well go for those, unless anybody knows differently.

In conclusion then if I were thinking of making a galvanic isolator I would use two KBPC2504 bridge rectifiers from Maplins (or KBPC3504 for an even higher current rating), connect the AC input terminals on each one together. Connect the +ve on each to the -ve on the other and using these two connections put in series with the incoming earth connection, the whole thing being built into a little box with suitable terminal connectors.

I have two reservations however. Firstly I wonder if it is wise or, even acceptable, to improvise in this way on something which, as part of the earthing system, is vital to your overall electrical safety. Secondly I believe there is a more sophisticated piece of equipment available commercially which is the only permitted solution in the US.
 
Re: I did...

Thank you all for your input, I think I'll buy a commercially made and tested unit, it is not worth taking a chance with 240V!
 
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