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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