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Re: Bypass Caps



> From: Scott Myers <scotty-at-wesnet-dot-com>
> To: tesla-at-grendel.objinc-dot-com
> Subject: Re: Bypass Caps

(much deleted)

> A few last points on the chokes.  Constructing them should be done on a fairly large
> powdered iron core.  It is important that core saturation by magnetic flux be avoided.
> The cores available from Hosfelt electronics for $1.49 each, part number T-250-B, are a
> good choice.  You will be in no danger of saturating those units, if a hundred turns or
> so of wire are used.  Care must be taken when you construct these chokes that you don't
> burn them out between layers or turns.  This is a matter of experimentation.  I recently
> posted information on how I wound my chokes with these cores.  Air core chokes are
> worthless in these circuits due to core saturation and provide no protection to speak of.
> 
> OK.  I'm done.  I hope this answered your question about bypass capacitor value selection. 
> 
> Scott Myers
> 

Well said.  Here's the equations I was referring to the other night when we talked:


L section of filter:

              L/2 
0-----------0000000-------------0
                        l		                   
                        l
                     -------   C/2
	             -------
                        l
                        l
0-------------------------------0


Low-Pass Constant K

Z=line impedance
f=cutoff frequency (-3dB)


L = Z/(pi x f)
C = 1/(pi x f x Z)
Z = sq rt (L/C)
f = 1/(pi x sq rt (L/C))

Don't forget to refer to the L/2 and C/2 in the diagram.

Not to be nit-picky, but you stated that air core inductors are
worthless because of core saturation.  Air core inductors don't
saturate.  When a core material saturates it loses its permeability.
The permeability of a core material is in comparison to an air
core with a permeabilty of one.  You can't drive the permeabilty
of an air core any lower by increasing the flux density.  The only
way to lower it would be to insert a material having a permeability 
of less than one, like brass.

I don't really see a problem with using an air core inductor if 
somebody wants to wrap enough wire to come up with an effective value
of inductance.

Getting back to flux densitys and saturation, the more I think about
it the more I'm convinced that the voltage component of the flux density
equation is the voltage DROP across the inductor. 

-Mike McCarty