Message-ID: <002301c0c1f1$4cbce1e0$016501d5@j1r9b7>
From: "Derek Atter" <Datter@btinternet.com>
To: rsgb_lf_group@blacksheep.org
References: <3AD2D0CE.23045.117384@localhost>
Subject: Re: LF: Coil Losses
Date: Tue, 10 Apr 2001 19:25:31 +0100
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Mike,

Re your plan to try a loading coil with thinner Litz wire - CARC still have
plenty of  Litz in various thinner gauges if you need any.

                           Regards   -   Derek Atter G3GRO

----- Original Message -----
From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, April 10, 2001 9:22 AM
Subject: Re: LF: Coil Losses


> Yet again, many thanks to Jim for helping to clarify a technical point.
>
> > One problem with ferrite cores is saturation - if the magnetic flux in
> > the core exceeds a certain point, the core effectively loses it's
> > magnetism over the peaks in the current waveform in the coil,
> > which can give rise to harmonic generation as well as excessive
> > heating of the ferrite. If you put a ferrite core into a coil carrying a
> > particular current, the  magnetic flux density there is in the core
> > depends on the shape of the coil and core - a long, thin rod along
> > the axis of the core will be subject to much greater flux density than
> > a short, fat core.
>
> Ah. That is probably why my lumps of old broken pot core work well in the
> 73kHz coil - I actually used one with the wire still in it and this seemed
to have
> no bad effect!
>
> > Another factor that is important when deciding the type of coil to
> > make is the voltage it has to withstand. A 7mH coil with, say, 3A @
> > 136kHz going through it has about 18kV across its terminals, So
> > insulating an inductor wound on a pot-core would be quite a
> > challenge at this level, especially for outdoors use.
>
> I certainly had not planned to use a pot core as a former, rather to tape
bits of
> ferrite - probably of the SMPSU grade - to a plastic water pipe and place
it
> inside the existing drinks bottle former. I expect at least 70 per cent
air
> spacing.
>
> I am away at Easter but will do some tests later this month on practical
cores,
> and will report back to the group.
>
> I do have an alternative strategy, and that is to get some thinner Litz
wire so
> the same physical size coil can give greater inductance, but then
resistance
> may be an issue again.
>
>
>
>
> Mike, G3XDV (IO91VT)
> http://www.lf.thersgb.net
>
>
>