Russell,

As usual thanks for the detailed explanation. Much appreciated.

Shahid

-----Original Message-----
From: Russell McMahon 
Sent: Saturday, December 25, 2004 3:50 PM
Subject: Re: [EE] What is a Galvanic Isolator?


>I have a vague understanding of what it is but why is it called a  
>"Galvanic" isolator?

In its most general sence "Galvanic isolation" means that there is no
direct 
current path between the input and output*. Galvanic action is usually
taken 
to mean the setting up of an electrochemical action when a cell is
formed by 
two dissilar metals and an electrolyte. Often the context is related to 
"galvanic corrosion" when a metal object inadvertently becomes part of a

cell and is damaged by the electrochemical action.

A "galavanometer" is an ammeter - usually a very sensitive one. The name

comes from Luigi Galvani who discovered in about 1780 that a dissected 
frog's legs would twitch when a "cell" was formed by the metal scalpel
and 
dissection plate. His theory about why this happened, which he verified
with 
careful experiments,  was subsequently discredited but the basic
principle 
which he discovered forms the basis of a vast range of modern
applications. 
(They told me about him in school long ago but Google provides 
http://www.newadvent.org/cathen/06371c.htm which gives lots of
interesting 
detail.

> What I am looking at is a bunch of DC/DC converters
> for an industrial control system and they have them labeled as 
> "Galvanic Isolators". Why not just call them DC/DC power supplies? Are

> they more than just a DC/DC converter? The circuit is a simple 
> oscillator made from discrete components that fires a 1:1 pulse 
> transformer and the output is just rectified and unregulated. Nothing 
> fancy about it.

The fact that thye call them Galvanic isolators suggests that the main
task 
is electrical isolation of input from output. Whether they are used as
DC DC 
converters per se depends on the application. They may take a moderate
power 
input signal and produce an output whose main job is to indictae that
input 
is present. They may be powerful enough to drive a bulb or relay. In
such a 
case the isolation is the key thing and the power provided is necessary
but 
secondary.

Or they may provide power as their main function where isolation
matters.

* "no direct path" is relative. Isolation may be to a DC potential,
and/or 
to AC or to sudden voltage changes. Many different isolation techniques
are 
used. Many provide a degreee of AC coupling (albeit in the picofarad
range) 
that can cause problem sin some circumstances. Where substantial poweer 
transfer is required, high isolation against transient signals can be a 
major design task.






        RM

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