Galvanic Isolator

[JEG]

In the December edition of pbo there is an item on the above [p 53]. The diagram appears to show two diodes operating in opposite directions; the final sentence refers to two in series being sufficient. The diagram & the text appear to me to contradict each other; would someone kindly enlighten me?

 

[HaraldS]

Haven't seen the PBO issue, but can tell you about galvanic isolators.
A typical one would look like this:

----+--->|--->|----+----
| |
+---|<---|<----+

that's two in series antiparallel to another two in series.
Two diodes in series create a voltage threshold of about 1.4V before they conduct in one direction, in the other direction they will always block. IN the other direction the other pair will conduct from 1.4V onward.
The threshhold is high enough to block galvanic voltages generated by dissimilar metals. One diode with 0.7V wouldn't be sufficient for all cases.
AC of higher voltage however will be conducted, one half wave through the upper pair, the other half wave through the lower.
The isolator is isolating the ground wire. A ground fault AC current can flow and thus protect a person. The diodes will have to be able to take a serious load to be safe. Always highly recommendable is to combine this with a ground fault breaker, which will trip at smaller fault currents and thus reduce the maximum load on the diodes.
The approach isn't as good as an isolation transformer, because a small AC fault current, below the ground fault threshold, will enable galvanic DC to 'ride'on it.


<A target="_blank" HREF=http://www.taniwani.de>http://www.taniwani.de</A>

 

[pvb]

Good explanation...

...and a cute diagram! There was a thread on galvanic isolators on the CWBB recently, which included a link to Colin Foster's website, with a <A target="_blank" HREF=http://www.yandina.com/galvanicIsolator.htm>feature on DIY galvanic isolators</A>. Might be worth a look.

 

[JEG]

Thanks for confirming that four are needed. [I did email ipc on 13.01 but no reply!]

 

[alan]

If you look at the galvanic table for metals in seawater you will find that there is about 800 mV difference between the most anodic and the most cathodic metals that are normally used in boat/pontoon construction. Hence one diode with a nominal 0.7v drop would not be enough to block any galvanic current, but two in series will do the trick.

However, the reference article suggests using low power diodes and relying on the RCD to "save you" if these diodes should be fried during an earth fault situation on the 240V circuit. Personally I would buy diodes that are able to support a continuous current of 30A (better still 50A) and know that they will not "pop" if there is an earth fault. There is an American company that sells a novel device that checks the earth continuity of the galvanic isolator by firing a pulse up the neutral wire from the boat and checking for a return on the earth wire at the boat (via the power generating station).
I did a quick search for suitable high current diodes at Radio Spares (about 2 years ago) and came to the conclusion that the cost of buying the bits would be more than buying a ready made unit. So, I have a unit installed which I bought from Sterling.

Alan.
Nettuno, Italy.



Alan Cloke