Re: High voltage standing waves with a magnetron?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: "Philip Chalk" <phil-at-apsecurity-dot-com.au>
 >
 >         Simple enough - just a series resistor!  Note that the magnetron
 > volt/ampere curve is not like a Zener diode (battery in series with a
 > low-resistance diode, but that it also has a series resistance. I
 > couldn't find an oven magnetron characteristic data sheet in a Google
 > search but came up with a lot of hits on non-oven uses, primarily ham
 > radio.
 >
 > Ed.
 >
 > Hi Ed, all,
 >
 > Granted, but it would have shocking regulation.  In order to limit the
 > current to the extent required, (using the normal MWO supply) most of
 > the voltage would appear across the resistor, & it would need to
 > dissipate several hundred watts.
 >
 > [Clang -another late-nighter]
 >
 > Sorry, that's wrong.  To limit 4kV DC to 1mA (into a short circuit)
 > would dictate 4M Ohm. It would dissipate 4W. Horrendously inefficient
 > anyway - not that it matters.
 > However, as soon as the tube started drawing current (I agree it would
 > drop out of oscillation & behave as a diode at some lower threshold
 > voltage.), the bulk of the voltage would appear across the resistor; its
 > apparent resistance would be way below 4M resistor. So it wouldn't make
 > a practical oscillator.
 >
 > I was thinking more along the lines of a voltage & current regulated
 > supply, or the normal MWO supply followed by a current regulator, or
 > constant-current source.  Your series resistor approximates a constant
 > current source, but with awful voltage regulation.
 >
 > Phil Chalk.

	I think a series resistor will work OK.  The magnetron doesn't act like
a negative resistance device, just like a very high voltage Zener diode
with series resistance.  If you feed it from a voltage higher than the
"cutoff voltage" current will flow.  Based on some work I did with
smaller tubes a number of years ago I'd suspect the thing would start
oscillating with a current of a very few ma.  Easy enough to check.
Crystal diode rectifier and voltmeter will detect the oscillations even
if quite low power, which you want for safety's sake.  (All of the
cautions posted here are worth reading again!)

	This would take about two minutes to explain if we were standing in
front of a black board or sitting at a table with pencil and paper!

Ed