There will be a jumper SOMEWHERE to change the primary winding
configuration. I have even seen a fuse holder where there was
a tiny PCB next to the fuse, with contacts that you flipped around, 
depending on voltage.

Some switch mode supplies autoconfigure so that the only
thing you need to change is the fuse (and sometimes not
even the fuse).

R

Edward Cooper wrote:

> Hi all
> 
> I'm trying to figure this out, I've seen a few products now where they 
> place two fuse holders on the board, by putting the fuse into the 
> appropriate one you set the mains voltage 230 or 115 volts.  However I 
> can't figure this out, by transformer have two windings on the primary, 
> with 240V you have to connect them in series and for 120 in parallel, 
> how can this configuration be made using two fuse holders, one empty the 
> other setting the primary configuration!?
> 
> Its been bugging me, but I'd like to do it that way if I could figure it 
> out!
> 
> Cheers
> 
> Ed
> 
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