Re: Transformer question; safety
- From: "Larry Brasfield" <donotspam_larry_brasfield@xxxxxxxxxxx>
- Date: Tue, 5 Apr 2005 10:49:54 -0700
"ChrisG" <cgonz@xxxxxxxxxxx> wrote in message news:4252c2f9$1_2@xxxxxxxxxxxx
> I'm trying to get a better intuitive understanding of transformers.
>
> One text I have explains that an increase of current in the secondary
> causes an increase in back emf (of the secondary) which acts to reduce
> the back emf of the primary, which increases the current in the primary.
That is a good way to understand it. Another way
is to solve a simple circuit containing at least two
inductors with a high coupling coefficient. Just a
voltage source across the primary and a resistor
across the secondary is enough for that purpose.
> Since power supplies frequently fuse the primary side, it seems that
> the secondary side can cause excessive currents in the primary. But
> what is the limit to which the primary current can be increased? In
> other words, what happens when the secondary is shorted?
The fuse should blow for the reasons given in
that text you have. The limit is affected by the
circuit resistances or impedances in series with
the primary and secondary.
> I have seen comments regarding isolation transformers - that they
> limit the current that can be delivered, and thus are safer. If output
> power is close to input power where does the energy go? Heat?
> Impedence from the coils?
An isolation transformer is safer because it
reduces the chances of a person completing
a circuit carrying the 5-10 mA normally
required for a dangerous shock (in healty
people). It does not limit the current to a
degree effecting safety.
> I have also seen things saying that a small voltage secondary is less
> dangerous (than high voltage), but isn't that low voltage secondary
> capable of delivering high current, which is usually described as more
> dangerous than voltage?
Voltage sufficient to electrocute with no more
help that a person completing a circuit is usually
considered more dangerous than current which
requires a good conductor (not a person) to flow
and whose main danger is a burn from the short
created by that conductor.
--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.
.
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