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Re: Ferrite types and choke formulae (?)
From: Edward V. Phillips[SMTP:ed-at-alumni.caltech.edu]
Sent: Monday, November 17, 1997 1:33 PM
To: tesla-at-pupman-dot-com
Subject: Re: Ferrite types and choke formulae (?)
"Does the Wheeler formula work only for air core solenoids, or can I use
this also for coils wound on a ferrite rod, just by multiplying by the
appropriate permeability?
"
Your method will work if you know the real permeability.
"I am looking for a practical formula for choke inductances. I have an EE
degree, and I have plenty of electromag texts, but I am finding that the
purely mathematical formulas for inductance of a fixed length solenoid
(via. Biot-Savart law) gives results that are so far from the actual
measured values of the inductors produced, that the calculation is
essentially useless. "
Something wrong there! Wheeler's formula is usually good
to within 2% for any SINGLE LAYER solenoid you are likely to build.
(Function of length/diameter). Your trouble is somewhere else...
"Also, does anyone know the difference between the different types/grades
of ferrites? I am most interest in the frequency ranges and
*permeability* values. Does the tesla list have a nice table for this,
as it does for dielectric constants? My surplus catalogs have ferrites
listed as materials HF-70, 43, 61, 70, 73, 77, white mix, and orange mix.
They all seem to indicate that 43 and 43 Mixes are the most widely
applicable, good up to 200MHz. What type is found in TV Flyback cores
(that's what I am using now for my RFCs)."
Try a web search for Amidon. Think it is as simple as
www.amidom-dot-com (have the right address at work but not here).
They have an on-line listing for the properties of many ferrites
and powdered iron cores. Give it a try.
Ed