23 Jun
2004
23 Jun
'04
9:14 p.m.
> Then you'll know why the HV supply is almost
invariably voltage
> doubled AC and not rectified and smothed DC.
From: ard(a)p850ug1.demon.co.uk
I know it is (the magnetron itself is one of the
diodes in the doubler,
along with one capacitor and a semiconductor diode), but I don't actually
know _why_. Presumably it's to simplify the insulation of the transformer
or something.
Water molecules [1] have three resonant frequencies around 2.45GHz and
to couple the H field microwave energy into the water you need to be
operating at one of these frequencies. A magnetron when used as an
oscillator has a frequency that is voltage and output load dependant,
so having a varying supply will sweep the frequency across the whole
operating range. When you get near a resonant frequency the output load
changes and this bcomes the dominating effect until the voltage changes
enough to pull the frequency to the next resonant point. This way you
can be sure to couple most of the energy into the water.
From: Dwight K. Elvey
Also, once the magnetron gets to the threshold
voltage, it likes to
run at a constant volage. ... If you try to feed it with a fixed voltage,
it will destroy the tube.
Magnetrons, even those in microwave ovens, can be run from a constant
(fixed) voltage without any danger of destroying the tube. This works
whether using it as an oscillator or an amplifier.
Lee.
[1] other OH bonds will absorb energy in this band such as those in
carbohydrates, but not nearly as well.
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