My Little Marx Generator

SoCal Teslathon July, 2000

NorCal Teslathon July, 2001
(click any picture to enlarge)

The little Marx generator I built produces around 600KV maximum, and something like 18" to 20" sparks or so (so far), very bright white, once every second or two. I'm probably only running at 400kv at the moment until I solve some of the discharge "problems" I mention below.

The design is a wonderful combination of power conversion and inversions technologies, starting with 6ea "C" batteries (chemical power) to 6KV AC at 40KHz, to 60KVDC, to the final voltage of 600KVDC via the Marx. The total average power is about 10 watts at the moment. The C batteries are good for 5 or 6 hours continuous duty.

The physical design is entirely dictated by the components. The capacitors for the Marx are 40kv, 2000pf each. I needed two in series to get to 1000pf, 80kv per bank, and the "hockypuck" style of the caps clearly wanted to be put into this form factor.

The static gaps are all spherical chrome drawer pulls from Home Depot ... the cheapest available at $1.50 each (but very nice). The clear uprights can either be polycarbonate or acrylic if cost is a problem. I'm trying out acrylic at the moment and haven't seen any problems so far (I would probably use polycarbonate for "museum grade" or commercial installations though).

When one gap fires (the closest one) it creates a UV pulse that triggers all the other gaps. This is the reason I stacked all the gaps with clear view to all the others. The parallel charging resistors are all 300Kohm, 10watt which dictate the maximum power of 10 watts. Other values will work, but I wouldn't go much lower than 300K since the max output voltage and pulse width gets limited by the resistors.

Capacitors = 2* 2000pf at 40kv
= 1000pf at 80kv max
= 1000pf/10 & 80kv * 10
= 100pf at 800kvdc max (.0001uFd)

Resistors = 300K ohm 10watt = 300K * 10 = 3.0Megohms in series
Note that 3.0Mohms at 600kv ... about 200ma of fixed load!

Larger caps will take longer to charge, more power (faster rep rate) will require larger resistors. Watch out for resistors that can't take the 60kv (metal clad, exposed metal, etc).

I've been using 300Kohm, 10watt ceramic resistors for my Marx. Anything up to 1Meg should be usable for a first pass. The 10 watt limit is a pain, since resistors grow a lot in size as power increases. Inductors are probably the part of choice, but they would need to withstand the 40kv/80kv and perhaps more voltage. The resistors are hardly 60kv rated on my system so I have hope that they only need a "BIL" sort of rating, not an operating rating (and they float so they have no ground reference).

As far as the rest of the design, the biggest problem you will face in putting a system together is making it robust enough to survive the kickback from the discharge. The pulse is fast enough that it overpowers even a really good local ground (exactly like lightning).

What this means is that every time you fire this puppy off, every component in the "low voltage" side of the supply, (even your batteries) will suddenly be raised to about 80kv or so due to the ringdown.

All that energy has to go somewhere, so the ground used to receive the "zap" must be an isolated earth ground, and the low voltage portion of the supply must have a different isolated earth ground.

I take some nice hits from the 60kv generator box, even 6 inches away. This is an area I'm still working on, so I don't have any perfected suggestions or solutions yet. In addition, the voltage is high enough that objects even several feet away will pick up a noticeable charge, especially since this is DC.

Remember that at these voltages everything looks like ground. I've actually zapped a lady taking pictures with a video camera on a tripod who was about five feet away.

Until the ringback problem is solved, you will probably be unable to use "real" power supplies to replace the batteries for the 60KV inverter. I damaged two different supplies (ouch) in the process of testing the unit out.

A Yahoo search for Marx Information: +"Marx Generator" +Schematic
Yield: 42 hits (more than double from a year ago!)

There are dozens if not hundreds of web pages about Marx generators, all very good, often with schematics and great pics, so don't be afraid to look around the web.