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Old 22nd August 2004, 07:53 PM   #11
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Default bifilar primaries

"That's most surprising. I'd expect safety problems "

Yes, I agree. I was surprised to find it was wound that way too. Probably why it was being sold surplus. It was coated in epoxy so was not obvious until I tore it down. I mentioned it to someone in the power supply industry and they thought it was a poor idea too. Since then, however, I have been even more shocked to find that the low cost E-I dual bobbin transformers sold by the likes of Signal, Stancor, Magnetek ... etc all have their dual 120V primaries wound bifilar on one of the bobbins. These are most likely all made in China. I also had a Jameco low cost E-I isolation xfmr which buzzed and had high common mode capacitance, so I tore it down to see how it was made, and it had restamped laminations full of holes and cutouts with no insulation between primary and secondary. Probably from India. Apparently old scrap laminations are shipped over there and they punch out smaller lams from the pieces. Transformer quality is getting scary.

For that reason I have been accumulating some large toroid cores from the local scrap yard with the idea of re-winding them myself for audio outputs (bigger core, fewer turns). The secret of getting large bandwidth from toroids is actually well known and published, and far simpler than the interleaving complexities of E-I xfmrs. Each winding must be wound using progressive or chevron winding technique. This is done by dithering the core back and forth slightly as the winding is done in a single overall pass around the core (on automated equipment). The winding piles up in short chevron shaped layers with the result that all turns are always near turns with little voltage difference. This minimizes distributed capacitance. For DIY, can just use a lot of multi-layered 1/2 inch wide mini-sections with an insulating sheet between adjacent section ends. Good insulation between the overall windings (layered) (and core too) is needed to minimize common mode capacitance. By making each chevroned or mini-sectioned winding cover 95+% of the core, leakage inductance is minimized. The toroidal core with no lamination gaps allows one to use higher maximum flux density as well (10,000 G versus 7500 G ,M6, for E-I) for higher power rating (with same linearity) and has higher effective permiability for higher primary shunt inductance. You can expect to get near 1/2 MHz bandwidth this way.

Don
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Old 22nd August 2004, 10:21 PM   #12
EC8010 is offline EC8010  United Kingdom
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A very interesting post - although hugely alarming. I may have to investigate some of my transformers...
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Old 23rd August 2004, 11:40 AM   #13
hacknet is offline hacknet  Singapore
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Default Re: Re: Re: Re: commercial power toroids

Quote:
Originally posted by EC8010


Who knows? A toroidal main transformer will certainly be cheaper than an output transformer designed for the job. I suspect in the end that it will be down to whether you can keep those anode currents in balance. If you can, then the HF performance will probably be better.

well, toroidals arn`t particularly cheap.

are they implimentable in SE?
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Old 23rd August 2004, 08:27 PM   #14
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Hi,

Quote:
are they implimentable in SE?
They could be provided you make sure there isn't any DC flowing through them....as in a parafeed design for instance.

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Old 24th August 2004, 04:11 AM   #15
jlsem is offline jlsem  United States
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Quote:
They could be provided you make sure there isn't any DC flowing through them....as in a parafeed design for instance.
Or just use a gapped toroid. They are available, believe it or not. I'm guessing Plitron uses them in their single-ended line.

John
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Old 24th August 2004, 11:34 AM   #16
hacknet is offline hacknet  Singapore
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gapped toroid? wheres the gap?
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Old 24th August 2004, 11:46 AM   #17
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Steve Bench has used a small powersupply Toroid as an output
transformes in one of his many unusual amps. Sorry I didn`t book
mark that page.

Woody
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Old 24th August 2004, 12:50 PM   #18
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Quote:
Originally posted by jlsem


Or just use a gapped toroid. They are available, believe it or not. I'm guessing Plitron uses them in their single-ended line.

John

Quote:
Originally posted by hacknet
gapped toroid? wheres the gap?

If you read the links to Sanjay Maniktala's web presentation which I mention above (the notes are in a PDF file) you will find the gap equations. Do you expect your mother to do your homework?
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Old 24th August 2004, 12:55 PM   #19
garbage is offline garbage  Singapore
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Quote:
Originally posted by woody
Steve Bench has used a small powersupply Toroid as an output
transformes in one of his many unusual amps. Sorry I didn`t book
mark that page.

Woody

Steve Bench's page
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Old 24th August 2004, 12:59 PM   #20
hacknet is offline hacknet  Singapore
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Quote:
Originally posted by jackinnj






If you read the links to Sanjay Maniktala's web presentation which I mention above (the notes are in a PDF file) you will find the gap equations. Do you expect your mother to do your homework?

the notes din`t really make sense since i have no formal ee education
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