[Amps] 'red' cores versus 43 cores

[Jim Brown]

On Sun, 29 Mar 2009 17:43:55 +0200, Alex wrote:

>2) Cores for transformers are for the most part ferrite material and they
>have a much higher initial u factor. The high frequency cores rely more on
>transmission line characteristics, with the core being there only to
>increase the fringing inductance at the low frequency end of the range.

WRONG!  When a conductor is wound around (or passed through) a ferrite 
material, nearly all of the magnetic flux resulting from current in the 
conductor flows in the ferrite core. Any loss in the core will couple into 
the electrical circuit as resistance. Nearly all ferrite materials have low 
loss at lower frequencies and high loss at higher frequencies. That loss will 
also cause the ferrite material to get hot, and when it gets hot enough, it 
temporarily loses its magnetic properties! That heat can also melt insulation 
on the conductor. 

When a parallel wire "transmission line" is wound around a ferrite core in 
the form of a bifilar winding, a significant part of the magnetic flux leaks 
to the ferrite core (typically 30-40%). If the core is lossy at the frequency 
of that flux there will be heating and power loss. 

Most ferrite materials, including #43 and #61, will be efficient (that is, 
low loss, low heating) at lower frequencies, but will have high losses (more 
heating, more power loss) at higher frequencies. As Carl says, ANY discussion 
of ferrite materials MUST consider the operating frequency. 


>3) Usual u ranges are 120 for the HF range, about 15 to 25 for low VHF (6
>meters or so) and about 10 for UHF (100 to 500 MHz).
>What is more important is the core loss, lower in ferrites and the ability
>to withstand the volts/second stress at the frequencies of interest.
>73 and all the best:

WRONG!  Mu is NOT the most important property. LOSS is a VERY important 
property!  Also, ferrite materials are semiconductors -- that is, they have 
varying degrees of electrical conductivity depending on their chemical 
composition (the mix). Some are such good insulators (#44) that bare wires 
can be wound through them, while others are good enough conductors that bare 
wires would short out the inductance!  Ferrite materials also have 
permitivity -- that is, they act as a dielectric and add shunt capacitance to 
the inductor. 

The high equivalent circuit of a wire passing through a ferrite core, and of 
a coil wound around a ferrite core, is a parallel resonant circuit. At low 
frequencies, the equivalent circuit simplifies to shunt R and L. Typical Q 
values are less than 1. 

If a ferrite choke is handling power, voltage breakdown can also be a factor! 

Materials like Fair-Rite #31 and #43 are PRIMARILY designed for RFI 
suppression at VHF. Ferrites are good suppressors around their resonant 
frequency, where they are primarily resistive!  They CAN be used as 
transformers at lower frequencies where their losses are small enough that 
they don't overheat. How low is low? Study the Fair-Rite data. 

Materials like Fair-Rite #61 and #67 are PRIMARILY designed as HF 
transformers, but #61 is also very useful for suppression in the 200-1,000 
MHz range. In general, materials that have the least loss also have lower mu. 
#67 has less HF loss than #61, but also lower mu.  

In general, LOW power transformers can work fine on lossy materials (if you 
can tolerate the loss), but HIGH power transformers are likely to be 
problematic. If you're putting more than a few tens of watts into a core you 
need to consider loss. 

Repeating my earlier advice -- the Fair-Rite catalog is an EXCELLENT 
resource. You can learn a lot from studying it. It is online in pdf form. 
There is a materials section at the front that has graphs of the electrical 
properties of each of their materials. You will be better equipped to 
understand what you see in that catalog if you first study the section of my 
tutorial on ferrites.

http://audiosystemsgroup.com/RFI-Ham.pdf

73,

Jim Brown K9YC