>>or more important question is there an easy way to get over the
>>limitation? so I can take advantage of the 110Vac selection in
>>240Vac line to get a few more AC output.
> 
> 
> short answer you cannot.
> 
> The reason is the primary of the 110V winding does not have enough
> inductance to limit the current to a safe value, and so the transformer gets
> hot and burns out - as you have discovered.
> 

> However, as you apparently have an endless supply of these wall warts, you
> could use two, with the secondaries in series

These wall warts are getting rare :-).  No, not because I am burning out 
them mindlessly but because of the switcher type wall warts.

I remembered vaguely I had success before with an old wall warts.

Guess I have to go with capacitive voltage doubler at the output, or 
dual transformer, or a heavy variac.  Shall have to skip Russel magical 
L n C suggestion as I don't intend to buy up the remainding transformer 
wall warts left in the town.

Thanks for all the replies.

Ling SM


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