Re: Ferrite types and choke formulae (?)

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: Ferrite types and choke formulae (?)
From: Tesla List <tesla-at-stic-dot-net>
Date: Mon, 17 Nov 1997 01:17:36 -0600
Approved: tesla-at-stic-dot-net


From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Sunday, November 16, 1997 3:05 PM
To: 	Tesla List
Subject: 	Re: Ferrite types and choke formulae (?)

Hi Adam,

> From:   Adam[SMTP:absmith-at-tiac-dot-net]
> Sent:   Saturday, November 15, 1997 11:02 AM
> To:     tesla list
> Subject:    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?

Yes.
 
> INDUCTANCES:
> 
> 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.  (It's too ideal). On the other hand, the formula 
> for the ideal capactior (K*A/d) is very accurate; real world caps are 
> quite close to the ideal calculated value when pure and uniform materials 
> are used.
> 
> FERRITES:
> 
> 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).
> 
> -Adam

The TV flyback ferrites are Philips 3Cx (Siemens N27) types. Don't 
know the American manuf. equivalents sorry). They are high 
permeability and since they are used in flyback supplies, are always 
gapped with spacers. 

You are better off using iron powder if you contemplate using 
toroidal cores. Ferrites can easily saturate in a closed-loop core. 
They are fine if you have a large air gap (e.g. ferrite rods).
Iron powder cores have a distributed airgap built in and are designed 
for high levels of applied magnetizing force.

Malcolm