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Old 9th November 2017, 09:38 AM   #21
mchambin is offline mchambin  France
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Air core transformers, or inductors are usual at RF frequencies.
I have not seen it at lower frequencies.
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Old 9th November 2017, 10:32 AM   #22
DF96 is offline DF96  England
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Surely an air-core transformer would be better at passing on HF noise - so worse isolation?
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Old 9th November 2017, 12:35 PM   #23
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by analog_sa View Post
How would the absence of a core help?
The usual kind of mains transformer has a laminated steel core. Steel is a conductor, hence there's stray capacitance between primary and secondary via the core. With an air cored transformer, this path is absent so there is only the direct capacitance between the windings. When I made some rudimentary measurements on a split bobbin transformer, the removal of the core reduced the interwinding capacitance by around two thirds (from memory).
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Old 9th November 2017, 12:48 PM   #24
mchambin is offline mchambin  France
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Indeed.
However, there is no way to make a mains 50 or 60Hz transformer without an iron core.
Not that is impossible...., The size of an air core mains tranformer would be humongous. Anyone to calculate such a transformer ?
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Old 9th November 2017, 12:54 PM   #25
abraxalito is offline abraxalito  United Kingdom
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There is - use ferrite. But that's jolly inefficient because your maximum flux is extremely limited. So its only good if your power requirements are really modest. Power amps are out. The number of turns on the primary is scary large, not one I'd want to wind myself.
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Old 9th November 2017, 01:03 PM   #26
mchambin is offline mchambin  France
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Ferrite was used in radio receivers at 445 KHz for the intermediate frequency amplification.
Typically ( 120db gain ) two tuned circuit stages implemented with ferrite core transformers, the cores where used for fine tuning.
I think, this became obsolete with PLL ( Phase Lock Loop ) techniques.
AM intermediat frequency is 445 KHz.
FM intermediat frequency is 10.7 MHz.

I have seen ferrite used at lower frequencies to implement high Q filters in the audio band with ferrite pots.
I have seen ferrite pots for DC to DC converters. That is close to power supply. But frequencies far higher than 50 Hz.
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Old 9th November 2017, 01:18 PM   #27
abraxalito is offline abraxalito  United Kingdom
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Sure, no-one wants to use ferrites at 50 or 60Hz unless there are compelling reasons. And in general there aren't, for power applications. Signal transformers though are another matter - I've been winding ferrite cored signal transformers for a number of years. They're easy to diy and much cheaper than nickel/steel cored ones. Of course the flux limitation is still the major issue but with signal trafos we care much less about copper losses so using more turns of thinner wire isn't normally a big problem.

NiZn ferrites might be an alternative to air-cored transformers as NiZn material has high resistivity. I've not played with any cores made from this though so I'm just speculating. Of course to transfer any power it would need to operate well above mains frequency.
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Old 9th November 2017, 01:35 PM   #28
DF96 is offline DF96  England
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Interwinding capacitance can be reduced by using an interwinding screen. The core can be grounded. More to the point, a typical power transformer iron core is poor at magnetic coupling at high frequencies due to eddy current losses. Air core (or ferrite) would not suffer so more interference could get in, not less. People already complain that toroidal transformers let though more rubbish than old-fashioned EI etc.
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Old 9th November 2017, 01:37 PM   #29
mchambin is offline mchambin  France
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I did not master ferrite technology. There are many materials to choose from for different applications, frequencies.
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Old 9th November 2017, 01:38 PM   #30
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Poor magnetic coupling doesn't affect the rejection of common-mode noise, only normal mode.
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