K1TTT Technical Reference

Subject:        Re: Toroid permeability ???
From:   cwhiffen@atl.mindspring.com (Clay Whiffen)
Date:   Sun, 21 Jan 1996 19:07:39 GMT

hduff@humnet.humberc.on.ca (Hugh Duff) wrote:

>Does anyone know how to measure the permeability/loss of a toroid core
>(without too much difficulty !) ?
>They are difficult to get locally ( I usually have to order them from
>Amidon in Cal.) but some surplus stores in Toronto have a wide
>assortment of bins of unmarked toroids. It would be a nice
>cost-effective altenative to having to order new ones if we knew what we
>had sitting at our doorstep.

>A friend on a local BBS originally posed this question to me...
>I couldn't really answer him ...I am also curious !
>I thought that winding some wire onto them and taking impedance
>measurements at various frequencies may work but how does one accomplish
>this ?


>Hugh Duff  VA3TO  Toronto

>  NFX v1.3 [000]
OK, I'll give it a try.  Measuring Permeability, Pb, isn't too
difficult but it requires measuring inductance.  I use a little unit
from Jan '81 QST that, in turn, needs a frequency counter.  So if you
have these then the easy way is probably to  measure the dimensions of
the core and find it in the Amidon catalog.  Then wind enough turns
to cover about 3/4 of the core; with ferrites probably about 10 turns
but more may be needed to get a good inductance measurement with
powered iron cores.  Measure inductance L in microhenries.  Then
calculate the ALratio by:  Alratio=L(uH)x10000/N  where N is the
number of turns.  (If you measure inductance in millihenries use:
ALratio=L(mH)x1000000/N) You can then go back to the Amidon catalog
and find the type of core closest to your unknown. .  

I f  in Amidon you can't find a core of the size you are testing you
can calculate  Pb by:  Pb=L(uH)/(.0319xNxfactor) where: 
factor=Hx(OD-ID)/(x(OD+ID)).  H is core height, OD is outside
diameter and ID is inside diameter.

The numbers you will get will not be exactly  the same as the Pb in
the catalog but the material types are sufficiently different that you
can make a pretty good guess.  (at least for the ferrites.)  To
identify powered cores exactly you need the loss data.

Loss data is more difficult to get. You need to be able to measure the
Q of the test windings.  I won't attempt to get into that here.  

Good Luck OM, 73  Clay, KF4IX