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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