From: "james moritz" <j.r.moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 23 Dec 2004 11:31:09 -0000
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In-Reply-To: <HKEKINMDBIDLPDGEIOBMIEKLCDAA.g4wgt@tiscali.co.uk>
Subject: LF: RE: M0BMU mods for G0MRF 300 W Tx
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Dear Gary, LF Group,

You need one without an air gap - the air gap will drastically reduce the
winding inductance, unfortunately. There is nothing magical about the LOPT
cores - I just used them because they were a suitable component available
for free! The original specified cores should also work fine with the same
number of turns. Try to keep the windings as compact as possible to minimise
the leakage inductance. 

In theory, you will get less loss if the strands of enamelled wire are
twisted together in small groups which are then twisted into bigger groups -
the idea being that each strand is on the outside of the bundle for an equal
fraction of the overall length, giving each strand an equal impedance and
making the current distribution more even. But in this case, the main reason
for doing it was because it is easier to wind a stranded wire than very
thick solid wire - the "litz" construction can't hurt though.

In my example of the G0MRF circuit, doing these mods led to increased power
output (about 400W @ 37V if I recall), while at the same time the improved
efficiency reduced the heating of the output transistors and output stage
components in general. If you have an oscilloscope, you should see a cleaner
waveform at the drains of the MOSFETs, although there will still be
considerable switching spikes present.

Good Luck with the mods,

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Gary - G4WGT
Sent: 22 December 2004 20:44
To: LF (Rsgb LF Group)
Cc: Jim Moritz (M0BMU)
Subject: LF: M0BMU mods for G0MRF 300 W Tx


Hi Jim & LF,

I am about to carry out the M0BMU current-fed mods for the G0MRF 300 watt
class D Tx.

I have salvaged several cores from monitor flyback/line O/P transformers &
they all have an air/di-electric gap on the round leg of the core. Is this
OK or should I search for one with no gap?