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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 4th Dec 2019, 6:51 pm   #1
merlinmaxwell
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Default Help with inductance value on ferrite core

Dear all,
I have some binocular cores made of 4B1 material thus...
https://uk.farnell.com/ferroxcube/mh...ical/dp/242494

Material spec here...
https://elnamagnetics.com/wp-content...cification.pdf

What is the Al value?? i.e. inductance per turn at say 1MHz, I want to make a transformer for RF and knowing this means I can calculate the minimum number of turns. I know that L is proportional to n squared and a turn on a binocular core is once through both holes.

Thanks
MM
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Old 4th Dec 2019, 7:19 pm   #2
kalee20
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Default Re: Help with inductace value on ferrite core

You're right, however on checking the data sheet, this is a core made for RFI suppression. So the ferrite grade used will be deliberately lossy (to convert RF energy to heat, and to damp any spurious resonances so there is no artificial peak enhancement).

That's why no Al value is specified. People just won't want to use them for transformers!
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Old 4th Dec 2019, 7:43 pm   #3
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Default Re: Help with inductace value on ferrite core

The material data sheet does specify balun and wideband transformers, I only need a 1:1 with a minimum frequency of 200kHz (very approx), any ideas of number of (bifilar?) turns? Edit, at 100 Ohms, so an impedance of 200 Ohms at 200kHz will be fine.
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Old 4th Dec 2019, 10:29 pm   #4
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Default Re: Help with inductace value on ferrite core

Al = u0.ur.A.l

u0 = permeability of free space
ur =relative permeability of that material
A = mean cross sectional area
l = mean path length.

The path is along the centre limb, then splits into the two side limbs, then recombines into the centre limb. So the side limbs are effectively in parallel.

What an unhelpful datasheet, giving you everything except for mean lendth, mean area and Al.

Have a look and see if they do other cores of the same shape/size with mean dimensions or Al quoted,

David
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Old 4th Dec 2019, 10:34 pm   #5
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Default Re: Help with inductace value on ferrite core

Actually, some of what are normally seen as RFI ferrites like Fair-Rite number 43 make rather good HF transformers.

For any ferrite, there is a low band where it makes insufficient inductance to bother with, then there is a band where it makes useful inductors of low loss/high Q. Then there is a band where the losses have grown, but it is still useful as a reasonably efficient transformer, and in the highest band losses have grown to make a good RFI absorber.

Different materials just put the boundaries in different places.

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Old 5th Dec 2019, 12:09 am   #6
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Default Re: Help with inductace value on ferrite core

It is really all about the minimum number of primary turns you can use and the source impedance of the circuit/signal you are using to feed the transformer and how much power, which relates to the peak currents in the windings you want to transfer.

It is fairly easy to find out experimentally. Initially just with some turns, say start with 10, to make a choke and connect it like an unloaded transformer, across your 200kHz signal source and look at the level with the scope. Hopefully it won't be too loaded down, remove turns until the load of the "choke" causes about a 5 or 10% drop in the the level and no more. Then that would be about the minimum inductance you could "get away with" as a "primary" for that core and that frequency and that applied voltage level, if it were a transformer.

If it requires more turns than about 10 turns likely the core is not big enough and or the ferrite material not suited to the frequency range and the applied signal level. I use a lot of type 77 small binocular ferrites that I get from Communications Concepts in in the USA. They are superior to the type 43 I find for operation at 1500kHz, where in a push pull output stage for a pantry Tx I can get away with a 1 or two turn primary, but that would not be the case at 200kHz.

One of my favorites is the RF-400:

http://www.communication-concepts.co...0-transformer/
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Old 5th Dec 2019, 9:49 am   #7
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Default Re: Help with inductace value on ferrite core

For SW frequencies the best cores seem to be dust-iron, not ferrite.

What I do with unknown cores is wind a turn or two on it and use an aerial analyser to measure the impedance. Ferrite cores usually look purely resistive and dust-iron looks inductive.
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Old 5th Dec 2019, 2:01 pm   #8
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Default Re: Help with inductace value on ferrite core

I did a test with 6 turns bifilar wire wrap wire, fed and terminated in 50 ohms, half voltage at about 100kHz, that will do.
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