180kV Marx Generator
Marx generators are quick, simple, and easy to construct voltage
multipliers consisting of only passive components; capacitors,
resistors and spark-gaps. Because of their design they are
suitable to provide high current, high voltage pulses which last only fractions of a second.
A Marx generator works by
capacitors in parallel via resistors, and discharging them in
series. Once the capacitors have charged enough the first gap
breakdown, and the voltage across the two capacitors will have doubled.
Since the resistance of the ionized air in the spark is much
lower than the resistance of the resistors the capacitors are
effectively in series. The next gap will breakdown immediately due to
the increased voltage across it's gap and this chain reaction continues
up the ladder. The voltage across the chain of capacitors will be
the sum of
their voltages, and enough to spark to either ground or another Marx
generator if in a bipolar setup.
Cheap high voltage capacitors can be found easily on ebay, for my
generator I purchased 20 ceramic caps from China. Proper HV resistors
are much harder to
find, especially in large quantities. Fortunately they can be
constructed by putting several regular 1W resistors in series.
used 3x 330k 1W carbon film resistors which were soldered together and
then sealed with heat-shrink tubing as shown in the photo. When
constructing the actual circuit the components should be soldered
together free-form to aid voltage stand-off. The resistors were
soldered together in long
chains, with capacitors joining each chain and simultaneously forming
gap. Once completed I found it necessary to encase the capacitor's
legs in additional epoxy to prevent arc-overs.
For the high voltage source I used a flyback transformer which
was run from the
"555 driver basic", which could provide about 20kV. The voltage
provided by the source isn't too important as it is limited by the
first spark gap. The whole setup was
then glued onto an ikea cutting board for easy use. Although a
theoretical maximum of 300kV is possible (20 stages * 15kV across each
capacitor) I'm not
too keen on pushing the capacitors right to the limit, especially since
arc-overs started when the spark gap was still set much below 10kV.
There's no room for larger sparks
anyway. :-P The spark in the photo is 18cm long, which is the maximum
achievable with the spark gaps set to where they are. I would assume
the spark's voltage to be slightly under 180kV. Photos taken later with
my new digital camera have revealed that the repetition rate is
actually close to 14Hz (upto 7 sparks in a 0.5 second exposure), not too shabby for
something using the cheapest parts available!