Parallelling torodial transformer output for full output power

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The transformer you link to is not centre tapped secondary, it is 2x15V. In this case, you would put both secondary windings in parallel - black+orange, red+yellow.

If you have a transformer that is 15-0-15 (3 secondary wires) then you cannot parallel them - you would create a short (i learned this one the hard way as a kid!). Instead you use two diodes, one for each 15v tap, and use the 0v tap as your ground:

File:Fullwave.rectifier.en.svg - Wikipedia, the free encyclopedia
 
When parallelling two transformers for full power, I recommend You to make the parallell connection AFTER the rectifiers.
This means you put one rectifier in each transformer and paralell-connect the + rail and the - rail. 0V is good to connect them all together.

The reason is quite simple:
Two transformers may have a slightly difference in the secondary windings.
Not much, perkaps one round or so,.
When parallelling the secondary windings of two transformers in this way, there will run current between the two transformers, "stealing" power from You.
It is actually possible to look at it as if one winding is done and the short cirquited.

So I recommend You to use separate rectifiers and then paralell-connect the rails.
 
When parallelling two transformers for full power, I recommend You to make the parallell connection AFTER the rectifiers.
This means you put one rectifier in each transformer and paralell-connect the + rail and the - rail. 0V is good to connect them all together.

The reason is quite simple:
Two transformers may have a slightly difference in the secondary windings.
Not much, perkaps one round or so,.
When parallelling the secondary windings of two transformers in this way, there will run current between the two transformers, "stealing" power from You.
It is actually possible to look at it as if one winding is done and the short cirquited.

So I recommend You to use separate rectifiers and then paralell-connect the rails.

Agreed. The only time it's safe to parallel output windings is if they're bifilar wound.
 
Say I have a transformer such as this:

Avel Y236103 30VA 15V+15V Toroidal Transformer 122-605

I use the primary connections in parallel and input 115V, but at the output instead of 15/CT/15 I want only 15V at full power. Do I just parallel the two output windings?

And just to be clear, that means black/orange in parallel and red/yellow in parallel.

if you want just 15volts, series connect the primaries and parallel connect the secondaries...make sure you get your phasings correctly...
 
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