Any reason why I cant..? Hypothetical transformer question

[gilmo789]

Mains voltage in the UK is 230V. Many of the old valve stuff here was sold as 110V with heavy step down transformers. like this filmosound projector
Say if I have 2 identical 110V power transformers, could I wire the primaries in series and the outputs in parallel to make a power supply that works for UK/EU mains power? I don't know what the measured wall voltage is in the US but seeing as my wall measures 234V I would imagine the US mains is a wee bit higher than the 110V nominal. I therefore don't see 2 x 110 as being very different from 230V in practical terms.

I'm thinking something like if you have 2 x 110V monoblocs and you wanted to use them as a 2 channel stereo amp, could you wire up the transformers as described to make a single power supply for the 2 of them or just use the 2mains transformers to make a very stout PT for a mono amp

thanks,

[J M Fahey]

Maybe, but both transformers must be identical twins.

No problem with the series connections but wiring secondaries in parallel can cause problems, unless they put out exact same voltage.

[Helmholtz]

It is essential that secondary voltages are equal and in-phase. Otherwise a short will result.
First test should be on a bulb limiter.

[gilmo789]

Can I ignore the secondaries of the second transformer and basically use its primaries to turn the first transformer into a step-down auto transformer? Would the point where they are joined function as center tap?

If i did connect the secondaries in parallel, how equal? +/- 0.5%, 1%, 5%?

thanks,

[loudthud]

Can I ignore the secondaries of the second transformer and basically use its primaries to turn the first transformer into a step-down auto transformer? Would the point where they are joined function as center tap?

Not just no, HELL NO !!!!!

Buy a 240 to 120 step down transformer or auto-transformer.

[gilmo789]

Buy a 240 to 120 step down transformer or auto-transformer.

I have one. This is just a question that occurred to me hence - Hypothetical.

[gui_tarzan]

Gilmo, one thing I learned a long time ago is the books usually don't tell you what you can't do, they tell you what you can do and what to do. They also don't always tell you WHY.

I often have questions about things too and was pretty much roasted (I can take it) for a question (which in hindsight was a dumb question - had I thought about it more before asking I wouldn't have asked) and learned that sometimes I should keep my mouth/fingers quiet until I really put some thought into it. Unfortunately I operate on the "stream of consciousness" type of thoughts where they just all dump out into a pile and I have to sort through them until they make sense. It allows for much creativity but sometimes confusion for everyone else.

I don't believe you will find an answer to your question in a text book since the engineers already knew not to do that so they wouldn't risk bringing it up. That's why these forums and these experienced, knowledgeable and occasionally funny guys here are vital, not only to help with the learning/helping process for others but also to answer questions like ours.

I mention textbooks often because that's where the theory and initial knowledge comes from. As much fun as it is to dive into anything without knowing the basics it ends up being frustrating and sometimes dangerous after a while. But hey, how many times did it take Edison to finally create the lightbulb?

[Helmholtz]

Can I ignore the secondaries of the second transformer and basically use its primaries to turn the first transformer into a step-down auto transformer? Would the point where they are joined function as center tap?

Certainly not.

As the 2 primaries are not on the same core, they are not coupled and there is no CT effect.

Rather the mains voltage will be shared between the 2 primaries according to their impedances.
To make sure each PT gets 50% of the voltage, primary impedances must be identical.
Now a transformer has no fixed impedance, rather it's primary impedance is essentially given by the reflected secondary load.
A transformer without a load has a very high primary impedance and would take most of the voltage.

So each PT must "see" the same load (don't forget the load varies with load current and the load currents of stereo channels are not identical).
Consequently secondaries must be paralleled.

When testing on the bulb limiter, you should verify that each primary gets the same 50% voltage.

[gilmo789]

Thanks for the replies. I don't think I'll be trying this anytime soon

[eschertron]

I gave you a like because I'm learning right along with you

[gilmo789]

much obliged

[35L6]

It's good to learn from your mistakes but it's less painful to learn from other peoples mistakes .

[Enzo]

AH yes, we make mistakes so you don't have to.

[35L6]

Thank you !