Triggering a Marx generator.

[Desmond]

Hi, I'm new here and was hoping that someone could help with a really special project I have in mind
I have built a simple Marx generator and have a more powerful version on its way which will hopefully give me 20cm sparks.


At the moment it charges and discharges about twice a second but I want to be able to trigger it when I take a photo.
I do a bit of high speed photography and want to add something to my photos and get pictures at around 1/1000th sec with a giant spark in them.
Last night I tried wiring up an ignition coil to an old flash which gives me about 20 000 volts as I take a picture. I tried opening up the first spark gap to prevent the automatic discharge and adding the 20 000 volt spark when I take a picture to actually trigger the spark when I want it.
But it isn't happening. Can anyone suggest a way that I can trigger this Marx generator with my camera flash circuit please?

 

[Desmond]

Here's an example of a spark at 1/4000th sec through sugar.



This was the flash circuit I used to trigger that spark.


And this is an SCR dumping 300 volts from a 3300uf capacitor through a piece of copper wire at 1/1000th sec.

 

[hevans1944]

The DC resistance of the ignition coil secondary winding appears to be shorting out the first stage of the Marx generator. You need a high impedance there, i.e., a spark gap, to allow the downstream stages to charge up. Then, when you create a low-impedance spark, it causes a domino effect, triggering all the spark gaps downstream as the capacitors upstream become connected in series and voltage across each successive spark gap increases. For this to work, you need a low impedance in all the spark gaps, created by the ionized air when the gap fires. The secondary resistance of the ignition coil doesn't satisfy the requirement for low impedance. The way this is usually done is with a pulse transformer whose secondary has low resistance and is in series with the first spark gap. After the Marx generator charges, a pulse applied to the primary of the pulse transformer triggers the first gap and away you go.

I don't know anything about pulse transformer design, but I would experiment with winding a few hundred turns of copper wire on a ferrite core to form a primary and an identical secondary. Discharge the photoflash capacitor into the primary and see if enough voltage is developed in the secondary to trigger the first spark gap. Pay attention to polarity to ensure the pulse adds to the voltage already across the spark gap.

How is it your 15 kV capacitors aren't blowing up if you charge them from a 30 kV power supply?

Nice image of incandescent copper droplets flying off the copper wire.

 

[Desmond]

Thanks very much for the input When it is "freewheeling" it pulses quite nicely. Regarding pulsing the spark gap I see what you mean about the possibility of it backfiring through the generator, however I did actually have the spark jumping the gap several times without the system discharging.
Today my ZVS flyback driver and flashover transformer arrived which will give me a more consistent power supply compared to the small generator that overheats in a minute. The little generator was advertised as being "400kv" though with about an inch gap it's probably 1/10th of that I'm not sure why my caps aren't popping but my other order of 10 X 30kv caps arrived today and I will soon put them to work on a better model.
Of course I got all excited about it last night and ordered a full size 10 stage 20cm spark Marx generator which may be here in a month.
My next experiment will be with a camera flash tube - I may try making my first spark gap a flash tube though there is the issue of the 240v cap connected to it conflicting with the 15 kv cap. It seems like I have a lot of work to get my plan in action.

Here is a video of the generator in action, "freewheeling".

 

[davenn]

@Desmond

I have deleted that last post with the microwave video

you are really treading on seriously dangerous ground and I cannot permit stuff like that to be posted
where some one else mite try it and kill themselves or others

The other problem is, I also have no idea what your electronic experience is with high voltage ?
for all I know, this may be your first experiments with such

I recommend lots of care ... don't want to read about you in the deceased column of the newspaper

Dave

 

[Desmond]

Ok, I have wired up an 800 volt 11000 amp SCR to dump a 500 volt 3300uf capacitor through a piece of wire. I'm an automotive electrician and regularly work with over 50000 volts. I have a lot of experience with high voltages.

 

[Desmond]

I just got home from work for a more comprehensive reply.
I have a blog on my photography . I've published an ebook on photographing explosions, I have a blog article from my book on wiring up an SCR to the camera flash circuit, and I've made a Trigatron and wired it up to my camera flash circuit.
Just to confirm that I do have experience with high voltages and dangerous projects.
This was my last photo shoot with 25000 volts from an HID headlight kit.



My current endeavours are in line with my new invention called "superspeed photography" and you will see
one of my images in National Geographic.


I do know how to work with high voltages.