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Re: Arnold Toroidal Ferrite?? Cores

Hi Scott,

	Re:Combination of 60Hz and 100kHz signals
in Choke Design.


The formula you have for Flux Desnisty computation
if just a modified form of Faraday's Law of 
Induction: V=NA dB/dt where we assume a sinusoidal
form for both V and B, ie B=B0sin(2pi ft) and V0 is
90degrees out fo phase from B so that 
V=V0cos(2pi ft). This gives the formula:

V0=NA x B0 x 2pi x f

Or its rearranged form:

	V0
B0= ---------------------
	2pi x f x N X A

Where 	V0 is the peak voltage 
	B0 is the peak flux density
	f is the excitation frequency
	N number of turns of wire
	A core area

The factor 4.44 which appears in the popular
formula comes from multiplying 2pi by 0.707 so 
that V0 can be expressed in RMS voltage.

Key point:
So, it is perfectly legitimate to "add" the voltages
of various frequency components to obtain a total
Bmax=B0. [If you want, you can go back to the 
deriviative form to prove it to yourself.]

Ah, but one hitch:
How do you know what are reasonable voltages for
each frequany component? [60Hz and 100kHz]

I don't think your guess of 5000 Vrms at 60 Hz is
reasonable. If your chokes have 2mH of inductance=
.75ohm impedance at 60 Hz. If you had 5000 Vrms
across this choke at 60Hz, it would be pulling
6600 amps! Most likely, you'd have at most a couple
volts across the choke at 60Hz. So I don't think
the 60hz contribution is important in the flux
density calculation.

I think the real question is:
How much of the oscillation voltage at 100kHz (or
whatever) appears across the chokes and how much
appears across the the supply transformer.

If this were at DC, it would be a simple matter of
an inductive voltage divider. The neon/pig has much
more inductance than the 2 or 10mH of the choke, so 
most of the voltage should appear across it - not
the choke.
However, I don't think we have a good feel for what
the actual high frequency impedance of a neon/pig is.
There is so much interturn capacitance in a big
high voltage transformer that I would expect its
high frequency impedance to be near that of the 
chokes people often use (to apparently good effect).

Malcolm ? Anyone?


-Ed Harris