What is ethernet isolation tansformer

Hi there,

Why are they used.....the one which you can find in almost all ethernet
cards!!

Thanks

 

They remove common-mode signals. They also help with EMI supression.

You can get large voltages between mains earth in different parts of a
building (or opposite sides of a room even). Without the transformer, the
sensitive input circuitry would need to deal with the common-mode signal.

If you're playing around with ethernet like this guy
http://www.fpga4fun.com/10BASE-T.html you can get away without the
transformers for the purpose of a table-top FPGA experiment, but not
otherwise.

 

Your question was answered accurately by Andrew. Galvanic
isolation is also part of the computer's internal protection.
Any protection effective adjacent to the computer is already
inside the computer. That includes this galvanic isolation
provided by the transformer since typically destructive
transients are common-mode.

Just another reason why effective computer protection that
is distant from the machine and close to earth ground are to
keep destructive transients from overwhelming this internal
computer protection.

 

Doesn't the the transformer also provide a differential connection ?

Graham

 

Thanks Andrew and Tom.....BTW what is Galvanic isolation??

 

It simply means that there is no path for DC current, i.e. a metallic
connection.

 

Hi,

not really: it means that there is NO DIRECT PATH for any current
at all!

Direct path means any kind of conductive (or semiconductive)
material. By this definition, the magnetic coupling of a
transformer provides galvanic insulation.

With regards to saftey standards (like ISO 60065 or UL1650 ...) the
term galvanic insulation is mostly coupled to a clearly specified
value of withstand voltage, that the insulation must keep without
any wrong behaviour:

For a mains transformer (PRI to SEC) this is:

1500Vacrms or 2120Vdcpeak for basic insulation

3000Vacrms or 4240Fdcpeak for reinforced insulation

By this you can see, that also AC currents are not allowed to run
directly from one side to the other. These tests are made with one
probe connected to the (shorted) primary windings and the other
probe connected to the (shorted) secondary windings.

Regards
Michael