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Re: Ferrite rings

Hi all

oops! it seems i goofed on my last post on ferrite rings. I still couldn't
understand how some people had managed to get some fairly high inductances
with a few turns untill i discovered the page i was looking at for the 
info was, in fact, for Iron Dust Cores.

These are used mainly for R.F Baluns and transformers and as my background
is radio, every core i had in my junk box was Iron powder.

To find out more i raided the scrap bin at work and came out with a well
blown switch mode P.S.U. and pulled it apart. On board were multivaryous 
ferrite ring cores, some filtering others pulse transformers ect. I just
spent the evening pulling off the old windings and putting on a rough twenty
turns
of pvc coated wire and came up with two of the cores giving around 3mH
for twenty turns. These cores are only 1.2 inch diameter so i put two 
together and wound fifteen turns and got 4.5mH with a single layer of wire
with no overlaps. 

For the final finished choke i'll use two cores together with a couple of 
layers of pvc tape then fifteen turns of slightly thinner wire, wound with an
even gap between each turn and maybe a couple of coats of polyurethane 
varnish to hold it all together. I was thinking of coating it in epoxy
but this would make any re-winding impossible if it flashed over.

Further investigation into ferrite materials and mixes revealed that
peremebilitys range from around 40 (better than the best iron powder
that i could find listed) right up to a material called " 'j'/75 " 
with a value of 5000.

Several of the rings in the psu gave very low readings so i assumed they were
iron powder too. The rings that gave me the best reading were wound as a 
common mode choke i.e with two equal numbered turns windings on either side of
the core. 
 
*******************************************
Ian Hopley ---->  i_hopley-at-wintermute.co.uk
Aberdeen
Scotland
*******************************************