[RFI] Re: Ferrite Beads

[Tim Groat]

Unless the ferrite has very high mu (in the thousands), the gap effect 
isn't large. A quick estimate: assume a core having .50" ID, 1.00" OD, 
2.36" mean magnetic path length (Le), and mu=100. Reluctance is 
proportional to Le/mu, (.0236). If you have a total air gap of .002" (.001" 
per gap), the gap reluctance is proportional to .002 (mu = 1 for the gap), 
less than 10% change. That's probably less than the temperature effects for 
an under-hood application.

Now do the same calculations for a core with mu=2000. For the core alone, 
Le/mu = .00118. A .002" gap makes the total of gap and core Le/mu = .00318, 
or 270% of the core alone (37% of the original inductance). For this 
high-mu core, a small gap is significant. That's why high-mu E and U cores 
often have mirror-smooth polished mating surfaces: it allows the gap to 
made very small, preserving the high permeability of the material.

In short, don't let reasonably tight gaps worry you for #33, #43, #61 etc. 
in RFI suppressors. It's important only for high-mu materials such as #73, 
#77 and Magnetics Inc. type "W".

72,
--Tim (KR0U)

>Jim Smith <jimsmith at shaw.ca>:
>
>I think that if you end up with any air gap at all between the toroid
>faces that the inductance will drop dramatically due to the large
>reluctance of the air gap in series with the flux in the toroid.
>(Reluctance is to magnetic flux as resistance is to electric current.
>Kirchoff's Laws can be applied to magnetic circuits as well as electric
>circuits.)