Isolation Transformers

[hellosailor]

Gord-
"I don't know how a switch-mode power supply could provide true isolation without a core & coil isolating transformer."
IIRC many switching supplies DO use a transformer, often a toroid and coil rather than an old fashioned one. Consider an MPPT controller. It is a switched-mode supply, but it is applying DC pulses to a toroidal core/coil that is still being an effective isolation transformer. At least, that's the impression I have of them.

With a real transformer, your "isolation" ultimately comes down to just some varnish on the transformer windings. In a pure switching supply, the silicon junctions in the switches are the isolating element instead. The world has gone to switching supplies for many things, simply because high power transformers are just too damned expensive. (Space, mass, cost.) And too many of them buzz like crazy after they age out.

Switching noise can be tuned (or detuned) out of the frequencies that would cause RFI problems for any particular user, if the manufacturer provides for that. There are some ham radio supplies that literally have a frequency offset adjustment for that purpose. Other manufacturers just need to be contacted to ask what their design priorities were, i.e. if they ignore RFI or design for marine SSB users.

[s/v Jedi]

HelloSailor,

Every switching power supply or inverter has an transformer. The difference is that they operate it on much higher frequencies than the 50 or 60 Hz we are used to, like 20 kHz. At these high frequencies, transformers work much better, are much more efficient, so a much smaller transformer can be used for the same amount of power transfer.

For an isolation transformer, the "isolation" part is purely the protective ground isolation. The hot(s) and/or neutral are isolated between input & output with all transformers because the medium for transfer of power is a magnetic field.
An isolation transformer has two housings: the outside casing which connects to the boat ground and an internal one which encapsulates the primary winding which is only connected to the ground wire from shore. There is no connection between these two housings thus the ground is isolated.

Silicon is never an isolator, it's a semi-conductor and thus unsuitable for isolation.

cheers,
Nick.

[Paul Elliott]

Originally Posted by hellosailor

In a pure switching supply, the silicon junctions in the switches are the isolating element instead.

In some cases perhaps, but many switching supplies have full transformer isolation for both the power and the control sections. You should look at the specs, but the isolation ratings can be excellent.

It is also possible for a switching supply to share a common ground between the input and the output -- this is often the case in a DC-DC converter. It depends on the application and the design.

[Camel]

I know this thread is dated, but I see some very educated people have responded to this threat and I hope another will respond to my questions on this subject.

I have a steel hull boat. I don't live aboard. I use the boat for weekends trips only.

I have a a portable fridge, depth gauge, radios etc, and two batteries, one to start the inboard diesel and one AGM used as the house battery.

It has shore power with an earth leakage safety switch, but no IT. (PS I have never had the shore power connected....partly due to the issues highlighted on this forum)

It has a solar panel with a charge regulator? (if that is what it is called) to charge the batteries.

So, my question is...(drum roll) if I don't use shore power at all, I don't have to worry about galvanic corrosion and I don't need a IT? Is this correct?

PS I am definitely not considering a GI, so let's not go there.

I'm not highly educated in electrickery, so please keep it simple.

[chala]

Originally Posted by Camel

I don't have to worry about galvanic corrosion and I don't need a IT? Is this correct?

Yes so long you maintain your sacrificial zinc s. Search this forum there is some more informative threads on galvanic corrosion.

[s/v Jedi]

You are fine and the only thing to check and clean every 6 months is the zinc anodes.

ciao!
Nick.

Edit: chala is too fast for me ;-)

[hellosailor]

Since you're not plugged into shore power you don't need an isolation trasnformer.

But that's got nothing to do with the larger question fo whether you have, or will have, galvanic problems. Stray current from your own 12v system, or simply from dissimilar metals on your boat, is all it takes to chew a hole and let the sea in.

Shore power just helps make it happen faster. And if the guy at the next berth is putting stray current in the water--it can still chew your prop off.

You still need to look at the issue of bonding and galvanic protection, sooner rather than later.

[Camel]

It has just been anti fouled and new zincs put on. As for bonding, I'm not entirely sure how to check it, but i think the easiest way will be to keep regular checks on the zincs.

Thanks for clearing up the need for IT issue.