psu logo back to main page

 

Marx Generator 1

A small, basic Marx generator.

Derek has a photo of the Marx in action on his Gaussfest 2007 photo album page.

 

Specs

  • Based on Mike Harrison's "Quick & Dirty" Marx generator
  • Individual capacitors made from 2 x 3n9, 1600V, PE film in series to give 2n, 3200V caps. Eight of these composite capacitors form the Marx bank.
  • Charging resistors were 2 x 470k in series and these were of the 2W metal film type.
  • Ballast/charging resistor at the bottom of the chain was the same type of resistor.
  • High voltage charging supply generated by a flyback transformer circuit
  • This was powered by a 12V line from a AT type PC power supply with a 5 ohm load on the 5V line to bring the 12V up to the right value.

 

Performance

Initial performance was good, but getting the Marx to fire repeatably was a pain. Even when the spark gaps were adjusted it would take me to wave something near the bottom gap for it to trigger (a nail in the end of a hot melt glue stick in this case).

After a query on the 4hv.org High Voltage forum, I decided to redesign my sparkgaps and make the layout more compact. To change the layout, I shortened all the leads connecting the components and attached them using large blos of solder.

The spark gaps changed design from simple bent bits of wire to tiny balls of solder. To make the solder balls, I bent the very end of the wire through 90º and trimmed it so that less than 1mm of bent bit was remaining. This was enough to prevent the wire from falling through a bit of veroboard which was used to support the head of the wire.

Then I dropped a large blob of solder from my iron's tip onto the exposed bent head which formed a nice round ball due to the surface tension of the solder.

The performance is much improved, with more consistent triggering. Now the Marx fires about once a second and gives nice 2" sparks.

Occasionally the bottom gap forms an arc across it that can be blown out easily enough. This is probably due to the charging resistor at the bottom of the chain being low enough to allow the arc to form. Something to watch out for should I ever build a bigger Marx in the future.