Marx Generator

'Simulating lightning...'

A Marx Generator is commonly utilized in museums to simulate and demonstrate lightning effects on various things. Basically it uses a very clever way of charging capacitors and discharging them to produce a massive gain in voltage.

Schematic courtesy of Mike's Electric Stuff

The schematic is shown to the left. It shouldn't be too hard to see that the input supply (into left-most terminals) charges up the capacitors in parallel. The terminals in between each 'stage' is a spark gap.

This means that all the capacitors are now charged up to the supply voltage. However, the spark gaps are set to a distance so that they will fire at this instant. When the spark gaps fire, they basically assume no resistance, and thus because of the way they are arranged, the capacitors are essentially discharged in series. The voltages on each capacitor adds to the output voltage, which appears across the discharge terminals. Yep, very simple.

Anyway, on to the Penguin's Lab Marx Generator...

I had ordered 20 pieces of 1nf, 20kV rated capacitors from somewhere, and they arrived in perfect condition. Excellent. Knowing that they were made in China, and they probably wouldn't stand 20,000V before arcing over, I knew there was preparation work to do.

Grabbing some oxygen free copper cable, I stripped bits of it off for the insulation, and threaded the capacitor leads through [right]. A perfect fit, and would definitely insure me against spark overs.

I went for a simple approach. All parts crudely soldered onto each other on top of a fibre ceramic board (good insulator). Each 1M resistors was made out of two 470K, 1W resistors in series. Close enough to 1M, who cares.

Spark gaps are cleverly constructed using the long leads of the resistors carefully bent into U shapes to minimize coronal loss.

And here is a test run with 6 stages and very low spark gap. voltage settings. So far, so good.

Oh, here is the supply I was/am using. Same driver as for the ignition coil driver, except frequency has been changed to suit the flyback transformer (actually this one came from an old broken monitor we found at the uni).

Output is approximately 10kV. I shall be ramping this voltage up later.

After a while, I ran out of resistors. Which is sad, because I only got up to 10 stages. I'll go buy some more later...

Anyway 10 stages should give me an output voltage of approximately 100kV (input voltage x number of stages). Not too bad. Lets put that to the test...

Ohhhh yeah! Ruler indicates ~9cm spark length. Which means approximately 99kV. Pretty close.

The Marx fires about twice a second. Oh, here's a short video to give you an idea of what happens...

Download the Marx Spark video HERE (160 Kb MPEG)

And in the air... This was a 2.5 second exposure and there are 5 full length sparks visible, indicating that the firing rate is indeed approximately 2 times per second.

And I just could not resist. Mr. Penguin is fortunately made out of plastic, although you can see that one of the sparks has leaked a bit of corona just under his beak... The sparks tend to snake around the penguin...surface tracking.

Dull sparks, but I really like this one. A perfect demonstration that plastic/acrylic sheeting or other common household insulators are completely useless at such high voltages. The spark snakes around a bit and goes straight through the plastic film as it if wasn't there.

Thicker piece of perspex used to demonstrate the same effects. This time it just surface tracks along the plastic.

The extended Marx:

Got the new resistors, and added ten extra stages. Marx Generator now houses 20 stages. There is significant loss evident, probably from corona. The hissing noise of the corona is very audible, and the spark length less than the theoretical.

The extra stages add on an extra 4cm to the previous spark length. A maximum of 14cm spark length now indicates an approximate output voltage of 150kV.

Polystyrene balls are great indicators of static electricity, and are commonly seen used in 'rub-the-plastic-ruler-with-wool-and-pick-up-stuff' experiments. Download the video to see what happens when the marx generator charges up a whole container of these!

Download the polystyrene ball video HERE (1.8MB WMV)

~Lightning Simulation~

Some examples of how marx generators can simulate lightning strikes... ground wire is connected to the body of the object (object must be conductive).
	One of my old toy cars being zapped the hell out of. Oh by the way, I've always wondered why on earth people think that the rubber tires of a car is what saves them from being stricken by lightning. Because they're wrong! Come on... the lightning bolt has travelled more than a kilometer through the sky, is one inch of rubber going to do much? Lightning cannot strike you in your car because the metal chassis of the car acts as a faraday cage- the charge flows on the outside of the metal rather than the inside, which is where you are!
The Eiffel tower having a bad day. I now realise I shouldn't have changed to a black background, it would have looked much better with the white background like with the car.
	To everyone who hates ipods! This one was kindly donated to me by Charles. It was already broken through, had a cracked screen, and I had already salvaged the hard drive off it earlier, this is a 'mock' photo with the turny thingy and screen put back in place. The fact that it was already broken may be good or bad news depending on whether you like or dislike ipods... I'll keep my opinions out of this one...
The setup showing how wires are connected during the lightning simulations. The yellow wire is at high voltage (~150kV), whilst the white one just visible down the bottom is the ground wire, which must touch the metal casing of whatever is being zapped.

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