Hi all, some more information on the ferrites and a question.
The Ginormous ferrite rod is quite spectacular. My solenoid, a 6in long coil
on 35mm waste pipe (abt 400t) measures 1.32mH, with a 10mm by 100m ferrite
rod inserted to flush with one end of the coil (so not in the middle for the
max) the inductance is raised to 3.25mH, with the big slotted ferrite the
fully inserted the inductance becomes 10.8mH. From memory with a bundle of
about 6 of the smaller rods I could only reach about 7mH.
The wrongly described choke mentioned in the earlier message is wound with
about 24 turns of a stranded, maybe litz, wire about 2mm o.d.. I decided as
the bobbin had a fair winding area that I would try winding it with a
smaller diameter wire, to increase to inductance. I then realised that the
"pedestal" that the coil was sitting on was not plastic but was in fact a
small permanent magnet !! Now the question....as I am not up in switched
mode PSU design....is it normal to magnetical "bias" ferrite inductors?? If
so what would be the reason?? is it to cancel the magnetic (saturating)
effect of the direct current the inductor may have to carry? If so would not
the winding have to be "polarised". These bobbins, with the permanent magnet
would sit nicely in the jaws of an electromagnet, making an electrically
tuned inductor perhaps.
P.S for the RX builders Order code 12-0274 is the GEC-Plessey SL6440 high
performance mixer, mis-described as a "complete radio IF chip". These are
very good performance double balanced mixers (Gilbert cell I think) as used
in the AOR 7030, and many high end receivers.( I think they mixed it up with
the SL644/1644 which is an FM IF chip.) These are very hard to get these
days and although a little old now, can only be outshone by the likes of
Colin Horrabin's H-mixer, I believe.
Cheers de Alan G3NYK
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