In a recent transaction, I got an american version of the well known Yaesu NC-29 quick charger. Logically it was constructed to be operated at 125 volts AC (to be more precise, at 117 V AC). Here in Europe we use 220 volts, so the charger stayed some days in the junk box. But the charger would be very useful to me, because I had some Yaesu batteries laying around, so I decide to change the charger's internal transformer to a 220V version.
Once I removed the six screws at the bottom of the charger, I
realized the original transformer have two primaries windings rated at
117 volts, connected in parallel. This is nice, because two windings in
parallel driven with 117 volts, produces the same magnetic flux as two
117 volts windings connected in serial driven with 234 volts (2x117).
Same magnetic flux will result in same output at the secondary winding,
so the charger could be used with 220 volts mains after change a pair of
This is the original connection of the transformer in the original
american version. We can see both windings are connected in parallel and
This is the modified wiring to permit 220 volts operation. As you can
see, now both windings are connected in serial and again, in phase:
Remember to connect both windings in phase, otherwise you will have
problems. Once you preform this little modification, the charger works
perfectly on 220V mains.