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Step down (balanced?) isolation transformer build
Step down (balanced?) isolation transformer build
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Old 14th June 2018, 09:07 AM   #1
Rodeodave is offline Rodeodave  Austria
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Step down (balanced?) isolation transformer build
Default Step down (balanced?) isolation transformer build

Okay, so I bought a Stax amp from Japan, hoping it would be possible to rewire the transformer's primary from 100VAC to 230VAC. For some inexplicable reason, however, the transformer only has a single primary winding, so I'll have to run it off a step down transformer. No biggie. The 50Hz/60Hz discrepancy should not be an issue, because both frequency standards are being used within Japan.

Digging through various boxes, I came up with two 180VA EI-core transformers with 220V (they're NOS from the 80's) primaries and multiple secondaries/taps. It looks like the transformers have an electrostatic screen which is connected to the mounting brackets, so mounting the transformers to a grounded enclosure without isolation would ground both the screen and the core. I also found a fitting enclosure and various other parts. The Stax amp draws 55W, so the transformers should run well within specs.

The plan is to run the primaries in parallel (in phase), and to run the 2x24VAC secondaries in series, effectively creating a 96V secondary (probably 100VAC irl with 230V mains). See schematic.

My question now is the following: Is a balanced isolation configuration preferable? The only device running off the transformer(s) will be the Stax amp, which has its enclosure properly grounded (that is, through mains earth, no enclosure connection to neutral).
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Old 14th June 2018, 10:27 AM   #2
Pano is offline Pano  United States
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Step down (balanced?) isolation transformer build
Good question, wish I had a good answer.
I remember when the balanced power thing started to be popular in recording studios, they raved about how quiet it is. I've played with commercial units at shows, and didn't hear much, if any, difference. I also built a couple of 220V biphase to 115V biphase transformers for my home system. Can't swear I heard any benefit there, either, but it didn't hurt.

All the devices in my system were connected to safety earth, so no worries there.

Is your mains system a bonded neutral? (bonded to earth)
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Old 14th June 2018, 10:42 AM   #3
Rodeodave is offline Rodeodave  Austria
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Step down (balanced?) isolation transformer build
Studio audio applications are where I found most praise for the balanced isolation configuration, yup.

The only real drawback I could find so far is the 1/2 power derating of the secondary, since each half winding (as seen from the center tap) has to pass the full current. For the present application this should still be within specs though.
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Old 14th June 2018, 12:23 PM   #4
Pano is offline Pano  United States
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Step down (balanced?) isolation transformer build
So the question becomes whether or not to tie the center tap of the secondary to ground, right?
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Old 14th June 2018, 12:44 PM   #5
Rodeodave is offline Rodeodave  Austria
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Step down (balanced?) isolation transformer build
More or less, yes. In this particular application I don't need galvanic isolation between primary and secondary, I just want to go from 230VAC to 100VAC. So from a step-down point of view it doesn't matter if I leave the secondary floating, ground one end of the secondary or if I ground the center tap.

But from what I read, a balanced "isolation" transformer with a grounded center tap of the secondary should provide cleaner power.

Thinking about it, it really shouldn't be called isolation transformer in that instance (not without quotes at least), since the secondary has a ground reference.
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Old 14th June 2018, 07:31 PM   #6
Rodeodave is offline Rodeodave  Austria
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Step down (balanced?) isolation transformer build
Some progress:
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Old 14th June 2018, 08:43 PM   #7
Mark Whitney is offline Mark Whitney  Netherlands
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You might want to consider placing a fuse at each output.

Something like this would be good.
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Old 15th June 2018, 08:40 PM   #8
martin clark is offline martin clark  Europe
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It struck me there's another way to arrange this, given the pair of similar transformers:

trafo 1: 230v in to 24v out, both secondaries in parallel.

trafo 2 is fed from trafo 1
Trafo 2 configured 24VAC into 2 x 24VAC windings in series. (ie a 24vac winding efectively feeds a nominally '48vac' input)

Take AC output from '230' VAC winding of trafo 2: it will be 120VAc max - but see below.


This arangement allows you to ground mid-point of 24+24vac windings on trafo 2 (i.e. the loop in the middle). groudn also teh interwinding sheild on Trafo 1.

Now you have an ultra-isolation transformer, because the intermediate loop is at 'ground'/ mains 0V potential, and the coupling capacitance of the two transformers in series. You are free to add film caps across the inputs to trafo 2 to reduce AC noise too: and these do not need to be full line voltage, X2 rated, only say 100Vac: so easily found in fairly large values.

It also places the losses of the transformers in series, so the output side is at slightly higher impedance - but means under load, you should hit your 100-110VAC output voltage target. Throughput capacity remains at nominal 180VA, but call it 150 to account for losses.

It'd be easily implemented to test with 'choc-block' wire connectors anyway... HTH.

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Old 24th June 2018, 07:39 AM   #9
Rodeodave is offline Rodeodave  Austria
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Step down (balanced?) isolation transformer build
Yesterday I finally found the time and proper mindset for finishing the wiring of this step down transformer build. The wiring is just like in the schematic in post #1 with the secondary center tap tied to (earth) ground.

I fully agree that a resettable fuse would be a worthy addition to the build, but since this is more of a proof of principle build made from parts I had in stock, I didn't include one...

I also really like the idea of the ultra-isolation transformer, thank you for taking the time to put your thoughts in writing! I kind of regret not going for that scheme, but the wiring was already too far along to reverse, and I really wanted to finish the project. Maybe next time

The balanced output step down transformer measures and performs as expected. The voltage measurements were done unloaded, so I guess they'll drop closer to target with the Stax amp attached. I have yet to look at the waveforms of the transformer input/output with a scope, so I can't comment on objective performance. The Stax seems to be happy though.
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