I found the picture below on the Internet about a double primary winding transformer. I understand how a simple primary double secondary winding transformer works, but I don't see the point in making a transformer like the one found below (output is at the bottom of the picture, input at the top of it).
Is it the same thing when you put two voltage sources in parallel so you can get twice the current at the output (so you get twice the power)?
Some countries use 110VAC mains, others 220VAC. So, transformers would need to be constructed differently depending on the country from where it is used.
To avoid that, some manufacturers are selling transformers that have two primary windings. In 220V coutries, you wire the windings in series (as if the number of turns would double, thus doubling the voltage ratio), and in 110V countries, you wire them in parallel (as if the number of turns was the same, but with double wire section, thus double the current carrying capability).
With a single transformer design, they can sell everywhere.
Profit!
Moreover, with some switch arrangement, you can make the transformer primaries configuration selectable, so the same end product can be sold everywhere in the world (provided that the switch is configured correctly depending on where the product is distributed).
Profit again!
This is so that in 110 V land, you connect the primaries in parallel, and in 230 V land you connect them in series. In each case, each primary winding takes about the same current, so the same heating and same transformer action.
One problem with this setup is that if you connect the 110 V configuration to a 230 V supply, you get smoke.
Modern laptop power supplies with a 'universal' voltage input from 90 V to 260 V work in a completely different way, without using a mains input transformer like this.
The transformer you've illustrated would be easy to re-wire for 220V use.
At 110V, wiring the primaries in parallel reduces the current through each winding.
At 220V, wiring the two secondaries in series doubles the primary voltage rating. The current should be roughly halved when operating on 220V, so no the primaries will not be overloaded.
