Toroidal transformer Primary/Secondary turns combination?

[uengin]

Hi,
I would usually be able to work this out but due to some pre-occupations I have to depend on your help.

A toroidal transformer has:
- 2 primary coils rated at 115V each
- 2 secondary coils rated 25V each

What I am trying to do is find a toroid transformer with the right combination such that I can achieve 230V to 115V step down. Now for this to happen, very simply put, the primary coil must be half the number of turns as the secondary right. Meaning, if I connect the secondary coils in the above example in series and use only one primary, or both primaries in parallel, but instead of feeding 115V in to it I feed 230V, I should (given the numbers above) get 100V out right?

The ratio = 115/(25+25) = 2.3
Therefore Vout = 230/2.3 = 100V

Or will I burn out the primary coils because the current will be double what it was designed for?

 

[Resqueline]

You can't put 230V on a 115V winding. The current won't only be double, it'll be ten times what it can take and the transformer will burn.
Unless you need an isolated 115V output you only need to use the primaries in what is called an autotransformer fashion. Connect them in series as appropriate and pick out 115V from the center to either end. The available power will be approx. the double of the transformer VA rating.