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LAN_403

Thu May 31 2007, 08:37AM

Registered Member #809 Joined: Wed May 30 2007, 08:59AM
Location: Melbourne, victoria
Posts: 114

hi, i am going to build a marx generator using 10 kv 100pf ceramic caps from rs components and was wondering if it was possible to put resistors across the caps to discharge the voltage when it was switched off or would this affect operation. also, would around about 12 of these wired up in a marx generator configuration with an input from a car ignition coil create a lethal charge? thanks for all the help

Electroholic

Thu May 31 2007, 08:57AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

short answer is no, no and no.
1 don't bother trying to put bleeders
2 iggy won't work
3 it will hurt quite a bit, but not even close to lethal, not to normal ppl anyways.

do ur homework before asking questions.

Sulaiman

Thu May 31 2007, 09:26AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I see no reason why an ignition coil wouldn't work ... provided you use a hv diode on the output

sparky

Thu May 31 2007, 02:25PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

Ignition coils - or any high voltage transformer will work PROVIDED that they are able to have rectified output to DC

Dave Marshall

Thu May 31 2007, 08:21PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Actually, I have to disagree with Electroholic on all counts.

Bleeders will not only work, I highly recommend them. I've been hit hard by a largish marx generator that had a broken connection on its bleeder (use more than one!). Judging by how far from the terminal my finger was when it hit me, I'd estimate it in the 30-50KV range. I took it from hand to hand, was knocked to the ground, and blacked out for a few seconds. I don't know that I lost conciousness, but I can't recall anything after feeling a slight tingle. Within a few seconds I regained my senses to my friend standing above me saying "Dude! You got lit up like a Christmas tree!"
For my carelessness I had a minor burn, and a wicked head and neck ache for several hours.

The only caveats are
1) the bleeders should be very high resistance (probably something like the sum of the charging resistors in series)
2) you don't need one on every stage. One every few stages will allow the caps to discharge through their charging resistors and the bleeder in series and ensure that even if one or two fail open or become disconnected, eventually the thing will still discharge completely.

Using one on every stage, or too low of a resistance will slow charging pretty dramatically, so keep that in mind.

Alternatively, you could just use a chicken stick to short each cap. Just be careful you short EVERY stage obviously.

Saying that a car ignition coil is not capable of being lethal is just absurd. I have posted video in the past of a pair of old oil filled ignition coils in anti-parallel producing several hundred watts at 10s of kilovolts for short durations. Plenty of power to really ruin your day, or earn you a pine box and a granite slab.

In their typical duty, Iggies are enough to make you drop the wire very quickly, but they are certainly capable of a lethal charge under the right (or wrong) circumstances. I'd consider the coil itself a bigger threat than the capacitors in this case, but I'm a firm believer in the 'Treat it like its always loaded' theory. If you treat all high voltage as if it might be lethal, you won't get surprised. With some medical conditions, even a relatively minor shock can be life threatening.

Anyway, an ignition coil will work just fine. 10KV is well within the ability of a reasonably designed driver circuit, and the relatively low current demand of the marx means that even a smallish coil would work fine. You might try and drive it at the highest frequency possible, or use a couple coils as a sort of center tapped transformer to make a smoother output. It should speed up charging quite a bit. Your only challenge will be finding some decent HV diodes for the output of the coil(s).

Dave

Sulaiman

Thu May 31 2007, 08:55PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

In normal (flyback) operation an ignition coil is non-lethal (less than 100mJ), it will make you jump though.

Once you add a diode and capacitor(s) you MAY have a lethal combination.

100pF charged to 10kV is 5mJ, so you'd need about 1000 in parallel for a lethal jolt.

Fri Jun 01 2007, 03:38AM

Registered Member #809 Joined: Wed May 30 2007, 08:59AM
Location: Melbourne, victoria
Posts: 114

well i don't really think i'll have 1000 caps, so that makes me feel better. i'm intending to run it off one ignition coil and a driver circuit. i sort of estimated the output of the ignition coil because it makes a spark around 1 cm long and around 10 kv for every centimeter (roughly). and so do i just put a high votage diode on the output of the igntiton coil?
thanks

Mod Merged Double Post - read the rules

here is the circiut i made up. when i hook the ignition coil up how do i stop it from continuosly sparking across the spark gaps?. does this circuit look right? and i don't really know how many caps i will be using yet but just made the circuit anyway.

teravolt

Fri Jun 01 2007, 05:59AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

safty should be your first consideration with energy storage. those little caps may not kill but will give you a nasty shock. I use a peace of wire that will reach across each cap and connect that to a ground. make shure that your shorting bar is on a plastic or wood dowel. my marx puts out 2.5 ft spark so I use a long stick. Marxes are fun but should be respected.
If you use a 2n3055 flyback circuit and about 15 1n4007 it will charge prety fast. If you use an ignition circuit use a circuit that has a 2n3055 oscilator. your circuit looks good just use 1 hv diode or a batch of 1n4007's just remember your diodes should have about 1.5 times the rated voltage than you expect to see. use your circuit there but don't bother using bleeders they will just suck charge energy. if you use high voltage you will get bit so do your best to minimise this good luck and show us the results

Fri Jun 01 2007, 06:48AM

Registered Member #809 Joined: Wed May 30 2007, 08:59AM
Location: Melbourne, victoria
Posts: 114

for the ignition coil setup i am using this kit i got from jaycar. the way i guessed 10 kv output from the coil is 10 kv = 1cm spark length (roughly). Will this circuit work?

ragnar

Fri Jun 01 2007, 08:04AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Yep, that Jaycar ignition coil driver kit will do the trick. You can only run it from ~15V without cooking the 555, though.

A long string of 1N4007s on the output of the ignition coil will allow you to rectify it.

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