Re: To fuse equipment or not to?

From: Fred (Fred_at_nospam.com)
Date: 03/27/05 

Date: Sun, 27 Mar 2005 18:04:09 +0100

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:s1MPlbb8kvQCFwEl@jmwa.demon.co.uk...
>I read in sci.electronics.design that John Larkin
><jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote (in
><0dq54198d00h6l4h1m0ogjvf6hbbpfk664@4ax.com>) about 'To fuse equipment or
>not to?', on Thu, 24 Mar 2005:
>
>>Around here some transformers (like for warts or doorbells) are "class 2"
>>impedance limited. If you short them, they just get sort of hot.
>>Regulation is ghastly, of course.
>
> It doesn't have to be. In a normal transformer, with just two windings,
> you allocate half the winding area to the primary and half to the
> secondary, so that the resistive loss is equal (well, nearly) in both.
>
> For an impedance-limited design, you have to allocate much less area to
> the primary so that its resistance is high enough to limit the
> short-circuit current to a safe value. You then let the secondary take up
> all the remaining winding area, so that its resistive loss is much smaller
> than in a normal transformer.
>
> You get approximately the same regulation as for a normal transformer, but
> of course if it's a very small transformer, using laminations with
> conventional geometry, it won't be all that good anyway. These small
> transformers really need lams with a BIG window in proportion to their
> size, because the insulation on the wire, and the bobbin if there is one,
> take up more winding area in proportion, at the expense of the copper,
> than they do in a larger transformer.
> --
> Regards, John Woodgate, OOO - Own Opinions Only.
> There are two sides to every question, except
> 'What is a Moebius strip?'
> http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I'm not convinced by this argument. You might as well distribute resistive
losses equally between the two windings to distribute power loss rather that
just in one winding. You'd also need more copper with putting all the
resistive loss in one winding.

Another method of introducing impedance is inductive loose coupled windings
but this makes the transformer more difficult design and hence more
expensive. The resistive - lossy method is cheaper using less copper and
hence becomes the chosen solution. It's a doddle with small transformers..

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