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Re: LF: Coil Losses

To: [email protected]
Subject: Re: LF: Coil Losses
From: "Derek Atter" <[email protected]>
Date: Tue, 10 Apr 2001 19:25:31 +0100
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
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" <[email protected]>
To: <[email protected]>
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





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