It doesnt have to be complicated at all
you dont HAVE to have two windings in parallel as long as one of 
the windings can handle the additional current at 110 volt
Immagine the following

a winding for example 1 mm2 wire for 110 volt use + a winding 1/2 
mm2 in 110 volt used in series with the other one at 220 volt

fixed width font drawing:

/--w-thin----f-220-------------0
|                           |
|------------f-110----------/
|
\--W-thick---------------------0

f-220 = fuse position for 220 volt
f-110 = fuse position for 110 volt
w-thin = thin wire 110 volt winding
W-thick = thick wire 110 volt winding


----- Original Message ----- 
From: "Russell McMahon" <apptech@paradise.net.nz>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Tuesday, January 18, 2005 8:34 PM
Subject: Re: [EE] Mains voltage selection for transformer


>> 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!?
>
> I've never seen this done, but either of the following would 
> work.
> I'd guess they used the 2nd scheme and "wasted" a winding on 
> 110V.
>
> _______________________________
>
> You can do it with 2/1 IF you allow a fuse to bridge between 
> the ends of two
> other holders.
>
> View diagrams in fixed width font.
>
>          A
> |---WWW---o----fff---|
> |         .          |
> |         .          |
> |---fff---o ---WWW---|
>          B
>
> WWW = winding
> fff = fuse
>
> Insert both fuses and you have 110v operation
> INSTEAD insert a fuse from A to B and you have 230 V operation.
>
> IF the fuse holders are arranged so A is the end of one holder 
> and B is in
> line then a fuse can be inserted between the two holders A-B.
>
>
> I'd be unhappy with building anything like this !!!
> Too much chance of disaster.
>
> _______________________
>
> If you don't mind leaving one winding unused on 110V then it's 
> easy.
>
> |---WWW---o- fff--o-----| 110
> |         |             |
> |         o-WWW--0-fff-_| 230
>
>
> Insert top fuse for 110v
> Insert bottom fuse for 230V
>
> This is most likely what you saw.
>
>
>        RM
> -- 
> http://www.piclist.com PIC/SX FAQ & list archive
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