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Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
ferrite is quite conductive at these high voltage, so no. always use aircore for marx. wirewound is also no go, failure mode under high voltage is usually short and burn.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Electroholic wrote ... wirewound is also no go, failure mode under high voltage is usually short and burn.
Wirewound resistors fabricated "by hand-winding 22-gauge Nichrome wire on a one-mm thick fibre reinforced plastic (FRP) sheet" and given "Enough insulation and mechanical rigidity...introduced by epoxying these resistors." is described in
Development of a 300-kV Marx generator and its application to drive a relativistic electron beam Y CHOYAL, LALIT GUPTA, PREETI VYAS, PRASAD DESHPANDE, ANAMIKA CHATURVEDI, K C MITTAL+ and K P MAHESHWARI in Sadhana Vol. 30, Part 6, December 2005, pp. 757–764
The tail resistors in this case are 50 ohms for a 150 - 200 nS Marx output pulse.
The paper also describes the construction of "a shielded copper sulphate (CuSO4) resistive voltage divider having divider ratio 1000:1."
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
I had trouble with resistors when designing mine. I now use 4x ordinary 220k 1/4W metal film glazed resistors in series (for each drawn resistor) and for a good measure they are placed inside a length of heatshrink tubing, to prevent corona, or flashover. I have had no failures yet.
It is possible to use inductors, but realy thats the preserve of medium-large marx gererators. I once tried it as an alternative to resistors, using 30 swg wire wound on 1inch x 2 inch forms, but even then the inductance was still a little too low for good performance.
If physical size isnt important, then using inductors is probably the way to go, but figure winding each one on at least 2inx2in forms with 30 swg ecw.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
They may do, but there are no guarantees either way. It is pretty much a trial and error thing. Carbon comps have a tendency to go high or low, in high voltage service. Metal-film tend to flash over or go open circuit alltogether.
Which ever you choose, I do recomend using 4 or 5 lower values in series rather than having a single resistor handle all the stress, and heatshrink is definately a good idea.
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Marx Generator
