Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 27-07-2009, 16:15   #76
֍֎֍֎֍֎֍֎֍֎

Cruisers Forum Supporter

Join Date: Apr 2006
Posts: 13,029
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.
__________________

__________________
hellosailor is offline   Reply With Quote
Old 27-07-2009, 17:15   #77
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
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.
__________________

__________________
s/v Jedi is offline   Reply With Quote
Old 28-07-2009, 00:43   #78
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,864
Images: 4
Quote:
Originally Posted by hellosailor View Post
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.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline   Reply With Quote
Old 04-04-2010, 02:54   #79
Registered User

Join Date: Mar 2010
Boat: Catalina 320
Posts: 13
Back to Basics

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.
__________________
I plan on living for ever....so far so good.
Camel is offline   Reply With Quote
Old 04-04-2010, 09:52   #80
Registered User

Join Date: Feb 2009
Posts: 1,321
Quote:
Originally Posted by Camel View Post
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.
__________________
chala is offline   Reply With Quote
Old 04-04-2010, 10:11   #81
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
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 ;-)
__________________
s/v Jedi is offline   Reply With Quote
Old 04-04-2010, 10:53   #82
֍֎֍֎֍֎֍֎֍֎

Cruisers Forum Supporter

Join Date: Apr 2006
Posts: 13,029
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.
__________________
hellosailor is offline   Reply With Quote
Old 09-04-2010, 04:56   #83
Registered User

Join Date: Mar 2010
Boat: Catalina 320
Posts: 13
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.
__________________

__________________
I plan on living for ever....so far so good.
Camel is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Alternator and Starter Isolation southernman Electrical: Batteries, Generators & Solar 21 16-02-2012 03:39
Isolation transformer Pa La O La Construction, Maintenance & Refit 11 12-08-2008 14:14



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 06:09.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.