How to wire a 'Buck' transformer

[johnm]

Wasn't quite sure were to post this so apologies first if this is in the wrong section.

OK I have 248-9V mains, and I want to get that down to around 235V. I have a 50VA 240V - 2 x 12V transformer lying around idle.

I assume I can just use one of these 0-12V sections and create a 'buck' transformer to shave some volts of my high supply here?

My question is how to I wire this up please? I'd like to put it in a box with IEC connnectors so I can easily just plug my CD player (or whatever) into this bucking trannie, and then that into the mains.

Any diagrams for Mr mentally challenged here would be awsome

Cheers,

- John

[zigzagflux]

See attached.

The output you will get is closer to 236, but you get the idea.

If you use both 12V windings in parallel, you will end up with a VA capability of about 1000. That's the beauty of an autotransformer at the expense of lost isolation.

If there is any question about polarity of the windings, energize just the 240V winding with your 248V line voltage. Measure from the common point of the winding (bottom node) to the very top (dot end of the 12V winding). If you have greater than 250V, it is wired correctly.

edit: recommend a 4 or 5 amp fuse in both primary input legs

[johnm]

Thanks for that that's great

Just to make sure I am doing this safely, the cd player consumes 60W, and at 240V I guess that's only 0.25amps? So this 50VA buck trannie is well up to the task.

Is it safe to only use one pair of the secondaries, and just leave the other two unsoldered? (they're terminals not wired).

[johnm]

I'm still unsure how to wire this thing up properly/safely. Have attached a photo of the transformer in question.

A picture of how to wire this up would be grand - many thanks

P.S. please ignore existing wires - this is from a previous project.

[zigzagflux]

Transformer is actually 100VA. If you only want to use one secondary, you will end up with a 50VA size. Connected as an auto, the size increases to 1000VA.

The black areas are to be soldered at the transformer lugs. Green wires are 248V input. Red wires are 236V output. You will have no issues running a CDP off this; the capability is 4 amps at 240V.

Leave the other 12V winding unconnected.

[johnm]

Is this correct please? Thanks

[johnm]

Posting at same time haha! I'll check ur reply now - thank you

[johnm]

Looks like I got it correct then - whew!

I take it I could safely run not only my CD player, but also amplifier (Stereo 70) and pre-amp (KingRex Pre) off the same transformer without having to use the second secondary? The Dynaco St70 consumes 190 watts which is around 0.8amps. Not sure of the current draw of the tiny KingRex pre but I think it's negligable...

Thanks for posting the pictures - very helpful to this layman!

[johnm]

Just taken off the lid on the CD player (A shanling CD-T80, valve output stage job) and the mains transformer has written on it 75VA...

The buck transformer is only rated 50VA, so I assume I can't use it now?

Thanks,

- John

[zigzagflux]

Originally posted by zigzagflux
Connected as an auto, the size increases to 1000VA.

You can run plenty of devices off a small transformer when it is connected as an auto. Your CDP has consumed 75VA of your total capability of 1000. Get it?