Can I increase current by combining transformer secondaries like this?

[Haz]

Ok. Lets say I have a 18-0 18-0 225VA toroidal transformer with two 18v secondaries. So its 115.VA how do I connect them to get 225 VA on a single voltage rail +/-?

The included picture will give you an idea of what I'm trying to do. I originally purchased the transformer for use with positive and negative voltage supply for an active subwoofer LM3886 type project, But switch to a 300w class D amp instead.

The link to the transformer here:
Buy Toroidal Transformers Transformer Toroidal 225VA 2x18V RS 81559RS online from RS for next day delivery.

I always appreciate any help!!

 

[geraldfryjr]

The top picture is wrong as it puts them in series and shorted, it will burn up!

Series is good if you want to double the voltage,But this schematic has them shorted.

The bottom picture is the good and will work fine.

jer

 

[abraxalito]

If you download the datasheet on the RS page you linked, you'll see it says its suitable for parallel connection of the secondaries. So you have no need of two bridge rectifiers - just connect winding starts together, same with winding ends.

 

[Haz]

How can the top picture be wrong when the bottom picture is correct? all I've done is remove the two capacitors the connections to the rectifier and transformer are the same.

 

[abraxalito]

No, you also swapped polarities so you have + going to - and vice versa.

 

[geraldfryjr]

Because the negative side of the bottom rectifier is connected to the positive side of the top one and Vice Versa.

jer

 

[Haz]

Oh sorry I did not notice that!!

 

[Haz]

I did look at the data sheet I linked but was a little unclear about series and parallel connections. I know parallel connections would double the voltage which I don't want to do
I guess this must be a parallel connection then.

Is this what you mean?

 

[abraxalito]

No that just looks like yet another short

Winding start is indicated by a dot. Connect the two 'dotted' wires (black, orange) together. Connect the remaining two (yellow,red) together.

Parallel connections double the current - that's what you want isn't it?

 

[Haz]

Yes that makes sense to me!! just don't want any shorts it never hurts to ask.
Thanks for clearing that up.

 

[Haz]

No that just looks like yet another short

Winding start is indicated by a dot. Connect the two 'dotted' wires (black, orange) together. Connect the remaining two (yellow,red) together.

Parallel connections double the current - that's what you want isn't it?

Yes I want to double the current!! thanks

 

[AndrewT]

build up a bulb tester.
Use it every time you power up a new project, or power up a modified project.

It is really important for first time powering of mains transformers. Sometimes the wires colours are wrong, or just confusing.