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Re: Ferrite chokes & saturation
From: Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Sunday, November 23, 1997 2:51 PM
To: Tesla List
Subject: Re: Ferrite chokes & saturation
Hi Gary,
> From: Gary Lau 21-Nov-1997 1839[SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent: Friday, November 21, 1997 4:46 PM
> To: tesla-at-pupman-dot-com
> Cc: lau-at-hdecad.ENET.dec-dot-com
> Subject: Re: Ferrite chokes & saturation
<snip>
> Thank you Malcolm, you obviously know what your talking about, but I'm
> still having trouble. One of my problems is I bought my torroids at a
> swap meet, no specs, vendor name, part number, etc, so I can't tell what
> the initial permeability is, I just bought the largest cores I could
> find. When you mention "work out the gapping...", are you suggesting
> cutting a gap in the torroids?
No. I was talking about using U-core or C-core halves. You can find
the initial permeability by winding say, 20 turns so that the whole
core is covered by the windings then measuring inductance. A more
useful figure than ue can then be calculated (Al) by:
Al = N^2 x L where N is number of turns and L is inductance (you can
either measure it directly or resonate it with a known capacitor to
determine L. The units for Al are nH/turn^2. Ferrites can go from Als
of 1000 or so up to maybe 10,000 or so. Iron powder can go from a
few tens up to a couple of hundred. You can see the effect of the
distributed airgap.
> My ferrite torroid cores are 3.25" OD,
> 2.25"ID, 0.50"T, insulated with .04" LDPE & hot melt glue, wound with 51
> turns LDPE insulated 22AWG, measuring 14mH. Could one generalize about
> the likelyhood of saturating with 250mA peak 60Hz current (15k/60mA w/
> resonant charging est.)?
Not without knowing you core characteristics. Try the above and see
where your Al value ends up.
> >I am leaning towards the use of series resistors and bypass caps only
> >given the cost and effort of building chokes approaching a Henry or
> >so. Chokes with significant Q can generate enormous spikes across the
> >small stray capacitances present. I will be testing these ideas
> >shortly as I am in a situation where I *have* to use neon transformers
> >for a job and they *have* to last.
>
> >Malcolm
>
> Do you know if ferrite core chokes have the same high Q's as air core
> units?
Depends on the losses in the grade of ferrite you are using, wire
size and operating flux density. For example, a high permeability core
with a few turns might look great at a few mA on an inductance meter
but be a total loser at a more substantial power level. An iron
powder core of he same size might look pretty lousy with the amount
of wire needed to get the same L but would fare better with real
power applied.
Malcolm