[RFI] toroids

[Ian White, G3SEK]

Michael Tope wrote:
>Yes and no, Tom. You can wind a known number of turns on each
>toroid and then measure the inductance with an impedance analyzer
>like the MFJ-259B. From the physical dimensions of the toroid
>(cross section area, Ae and average circumference, Le) and the
>inductance measurement, you can calculate the initial permeability
>of the material. This will tell you which materials the sample
>is probably NOT. It may not, however, tell you exactly which material
>the sample IS, since there are a number of materials available with
>similar initial permeability. You also have to be careful about what
>frequency you use for the measurement as low frequency materials
>(like type 77) will start to look very resistive at higher frequencies,
>and higher frequency materials (like type 61) may not provide enough
>inductive reactance at low frequencies to generate an accurate
>reading on the impedance meter (in this case you add more turns
>to the test inductor or raise the test frequency).
>

Yes, all of the above... and then you need a selection of catalogs to 
try to match your measurements to the available data. And don't forget 
to match the obvious: the exact physical size and any color code.

DL5SWB has written an excellent toroid calculator program which contains 
built-in data from just about all known catalogs (including European 
types that you in the USA will probably never see).  This is the best 
toroid calculator of any I've seen:
http://www.dl5swb.de/html/mini_ring_core_calculator.htm

Also be aware that many toroids that you find in mass-produced 
equipment, for example computer power supplies and TV sets, may be 
'specials' that are not in *any* public catalog.

The DL5SWB program can also help you to back-track to the magnetic 
characteristics of the basic material. This is useful because the number 
of different magnetic materials is much smaller than the number of 
special shapes and sizes. But even then, there are variations between 
different manufacturers.

*Approximate* cross-references are given in:
http://www.catchnet.com.au/~rjandusimports/index.html  (links at 
right-hand side of page)... but remember that these are approximate, and 
someone's personal opinion. In fact there are *no* exact equivalents 
between different manufacturers - they don't even use the same 
materials.

The three basic families of materials can be distinguished like this. 
(these are GENERALIZATIONS - in particular, the ranges of permeability 
overlap considerably - so please don't give me hard time about the 
exceptions).

Iron dust: resistance too high to measure [*], resin-based material can 
be cut with a hacksaw, initial permeability usually low, usually found 
in HF inductors and tuned circuits.

Nickel-zinc ferrites: resistance too high to measure [*], very hard, 
initial permeability usually low to medium, usually found in HF 
inductor/transformer applications or as lossy VHF suppressors.

Manganese-zinc ferrites: resistance high, but measurable [*], very hard, 
initial permeability usually medium to high, usually found in LF 
transformer applications or as lossy HF/VHF suppressors.

[*] The resistance test is simply to place the prods of your DMM a few 
millimetres apart on the surface (scrape off any varnish, and keep your 
fingers clear). Obviously this is very rough-and-ready, but it will 
usually pick out a MnZn ferrite from iron-dust or NiZn.



-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek