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@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.)
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