Dual rails from 2 x single xformers ?

[DRONE7]

I have 2 identical transformers...240v ac input and 20v ac output.

I plan to wire the primaries in series so each transformer's output would drop to 10v ac.
Then wire the outputs in series to give a 10v-0v-10v ac dual rail output.

With the working voltage de-rated to half for each transformer I don't anticipate any load problems..

Use case is as dual rails for a DAC output stage that requires 12+ve-0-12-ve dc.

Would this work ? anything I need to consider ? do I need LDO regs or would a LM317/337 be ok dropping only 2 volts.

Thanks,
Bob.

 

[JonSnell Electronic]

That will be unstable and if, say, one rails draw is a higher current that the other, the voltage on the lower draw rail will rise.
Primaries in parallel with 2 x 20v AC out, will give 20 x 1.414v RMS DC x 2, that through a regulator on each side, will be fine as the regulators are OK with an input voltage of 35v.

 

[FauxFrench]

For the DAC, you may even manage with only one transformer and single rectification on the secondary side. Increase the capacitance of the decoupling capacitors and avoid having to find space for one more transformer.

 

[DF96]

Mains transformers with primaries in series only work if the load on each is the same throughout the AC cycle. This is difficult to guarantee even when the DC load is the same. Forget this method.

 

[AllenB]

You could start with 20+20 and bring them down as DC.

 

[DRONE7]

You could start with 20+20 and bring them down as DC.

Separate primaries each 240v but connect the secondaries in series for 20-0-20 then rectify and regulate... ?
or series after rectification for +28-0-28- and regulate ?

 

[AllenB]

Either way I think the point comes down to whether you can manage to drop what you don't need. It is an opportunity for good filtering.

 

[nigelwright7557]

I would stick them both on ebay and buy the correct transformer for the job.