[RFI] Fair Rite #31 Mix Material

[Larry Benko]

Paul,

A good first order approximation for all cores is:

Effectiveness = (n^2)*u*length*log(OD/ID)
where n^2 = #turns squared , u = permeability (43 matl.=850, 31 matl.= 
1500), length = dimension that is perpendicular to the hole, OD and ID 
are the outside diameter and the hole diameter (all in same units).

I agree that for your application #31 matl. is probably best but if you 
have a bunch of #43 matl. cores and there is enough room for an extra 
turn or 2 the difference may be pretty small.  I have a hundred or so 
#31 type cores and maybe a thousand #43 cores.  Over the years many 
measurements I have made confirms the above formula pretty well.  Bottom 
line is try for at least 1000 ohms of total impedance for any frequency 
you are interested in.

I am probably preaching to the choir but the impedance of the circuit is 
just as important as the impedance of the choke when it comes to 
reducing common mode currents.  A choke with 20 ohms of impedance will 
halve the common mode current in a 20 ohm circuit as will a  200 ohm 
choke in a 200 ohm circuit.  The circuit impedance is not easily 
measured but a study done by a company in Britain about 10 years ago 
said that 95% of the circuits had impedances ranging from 10 ohms to 300 
ohms.  Also the circuit impedance varies with frequency which should be 
obvious since external wiring is involved.

Another thing that can be done if you have a cooperative environment to 
do the tests is to pick a frequency and gradually ramp up the power 
until you see (hear) the problem.  For example if you have a problem at 
the 200W level, but you want to run 1500W, the current induced in the 
wires is 2.74 times more at 1500W than at 200W.  Therefore the common 
mode choke needs to have a high enough impedance compared to the circuit 
impedance that the common mode current is reduced by more than 2.74 
times.  BTW 1000 ohms of choking impedance in a 300 ohm circuit will 
reduce the current by slightly more than  4 times.  If by some rare case 
you find than you need to reduce the current by more than 10 times 
(20dB) you may not be successful with common mode chokes alone and may 
need some shunt capacitors from my experience.

Hope you are successful.

73, Larry, W0QE


Paul Christensen wrote:
> I am looking for small toroid cores in a #31 mix to construct my own 
> bifilar-wound CM chokes for audio RFI abatement applications.  From what I 
> glean on the Fair-Rite website, #31 material is only available in a snap-on 
> core type, elongated cable cores, and large donut cores.
>
> Anyone know a source of small toroid donut cores in #31 material?  Do they 
> even exist?  Is #31 material exclusive to Fair-Rite?  In addition to 
> Fair-Rite, I tried Amidon and CWS ByteMark - and no luck there either.
>
> Paul, W9AC 
>
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