"Phil Allison" wrote in message
...

Gareth Magennis wrote:


... and check that the creepage distances, clearances, insulation etc. of
the 120V PSU's isolation barrier are adequate for 240V operation.


The 120v PSU is receiving 120v from the autotransformer.


** That claim is only generally true for isolation step-down transformers.

With an auto-transformer, it DEPENDS on getting everything right all the
time.

The supply neutral must remain connected to the neutral side of the auto
tranny AND the neutral side of the appliance's AC connection.

A simple loose connection on the neutral pin of the plug means that 240V AC
appears on each AC conductor in the appliance. Sure, the appliance does not
work but that is no barrier to insulation or AC side capacitor failures.

I would never suggest using an auto tranny step-down with any appliance that
did NOT have a 3 wire lead with safety ground connection.



.... Phil



Thanks to all.

My use for this method would be convert low power audio equipment, mostly
designed for both 110v and 240v use, and has a grounded IEC inlet. e.g.
keyboards, rack mount equipment etc.
Sometimes the 110v versions that find their way to the UK are not
convertible without changing the mains transformer or entire SMPS.

Having to cart an autotransformer around to gigs is inconvenient, and unless
you change the IEC socket to something else you risk connecting the 110v
unit to 240v and blowing it up.
So it makes more sense to me to incorporate an autotransformer inside the
unit itself, if there is room to safely do so.



Cheers,


Gareth.