Toroidal transformer primary/secondary turns combinations?

[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?

 

[srizbf]

Re: Toroidal transformer primary/secondary turns combination

...the primary coil must be half the number of turns as the secondary right.

it cannot be.

...both primaries in parallel, but instead of feeding 115V in to it I feed 230V..

it will burn the transformer.

srizbf
1stjune2010

 

[betwixt]

Re: Toroidal transformer primary/secondary turns combination

The nearest you will get is to use it as an auto-transformer. Connect the primaries in series and connect 230V across the ends. Take the 115V from one end and the center. Leave the secondary windings disconnected completely.

Please be sure to understand that an auto-transformer does not isolate the inputs and outputs, there is a direct connection between them.

Brian.

 

[uengin]

Re: Toroidal transformer primary/secondary turns combination

Thanks a lot Brian. That was very helpful indeed.

Just another question. I will be using this transformer for a 31W device. So, does this mean that the transformer I imploy for this application must be 31VA or greater?

Thanks in advance.

 

[IanP]

Re: Toroidal transformer primary/secondary turns combination

Because the primary winding is connected in series and the load is connected to one of it’s halves, they (the halves) have to be rated for the same power, that is 31VA+31VA, so total >62VA ..

IanP

 

[uengin]

Re: Toroidal transformer primary/secondary turns combination

Thanks for that Ian. Ok, so, when reading the specs in a datasheet of a toroidal transformer, and it gives you the rating that a particular transformer is rated at 50VA, it is safe to assume that each winding is rated at 50VA?

 

[IanP]

Re: Toroidal transformer primary/secondary turns combination

Thanks for that Ian. Ok, so, when reading the specs in a datasheet of a toroidal transformer, and it gives you the rating that a particular transformer is rated at 50VA, it is safe to assume that each winding is rated at 50VA?

50VA is the combined power of all primary windings or all secondary windings (in ideal transformer)) ..
If you have 2 primary windings - 115V each - then each is rated for 25VA ..
Together, 25VA + 25VA = 50VA ..

IanP