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Marx Gen proposal

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Steve Conner

Fri Feb 10 2006, 10:06AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Ohh! That is a pretty smart idea 8) So you're making one long water resistor with taps for each stage? It neatly solves the problem of bubbles accumulating at the ends, by only having one end.

My only worry would be that with the U-tube arrangement, the water in the bottom of the U is essentially shunted across the charging supply?

About the more stages vs. fewer controversy: I remember Dan recommending a maximum of five stages too. But it was in the context of a Marx generator designed to have a fast rise time for impulse testing. If you don't care about pulse shape, and just want to see some big sparks, then a large number of stages with lower voltages seems to work fine. I saw Mike Harrison's setup at Cambridge, with 40 stages each charging to 25kV, and it was firing off some very impressive discharges over 3ft long.

HV Enthusiast

Fri Feb 10 2006, 12:49PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

About the more stages vs. fewer controversy: I remember Dan recommending a maximum of five stages too. But it was in the context of a Marx generator designed to have a fast rise time for impulse testing. If you don't care about pulse shape, and just want to see some big sparks, then a large number of stages with lower voltages seems to work fine. I saw Mike Harrison's setup at Cambridge, with 40 stages each charging to 25kV, and it was firing off some very impressive discharges over 3ft long.

Yes, that is correct. And i have seen quite a few amateur marx generators perform quite well with 10-15 (and sometimes more) stages at around 15kv-30kv per stage.

Dan

Hazmatt_(The Underdog)

Sun Feb 12 2006, 03:39AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

When my TC is done and I've finished the specs. I am going to look into this.

I was proposing that I make some resistors and even sample some out to you guys on the other forum. I am still willing to do this, but the problem is that:

1. I need some time to perfect my glassblowing technique
2. I don't know if I can make a satisfactory copper/glass seal but I really want to try to make some damm nice resistors
3. I can work with you on your specifications
4. I only have 7 feet of compatible glass to play with, which isn't nearly enough!
I may end up breaking 3' in the process of learning the seal and annealing. Im trying to work out the annealing process, but I don't yet have a kiln for that, so I have to set the toaster oven to meltdown.

I have vacuum pumps, can perform minor sealing operations, now I have Uranium glass for Tungsten seals in Borosilicate, and once I can do things reliably with a good technique, I would really like to help some of you guys solve these problems. I like the challenge!

Matt

HV Enthusiast

Sun Feb 12 2006, 03:43AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

If you are planning on building a marx with up to 40kv per stage, than you can use simple 1Meg, 3 watt carbon resistors. They don't make them anymore, but you can usually pick them up surplus or on ebay. I've used these types in a number of marx generators and usually only blow one here and there. Sure, you have to replace a few here and there, but its much easier and cost effective than custom building complex high voltage resistors which really aren't necessary.

Hazmatt_(The Underdog)

Sun Feb 12 2006, 07:41AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

well...yea.....but....where's the fun in that?

Marko

Sun Feb 12 2006, 01:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Few posts before I asked about vulcanized (black, non-insulating) rubber track resistor, seems simple and nice for marx (ofcourse not cool as water) has anybody tried that??
Sorry for double-posting (sort off)..

Dr. Shark

Wed Feb 15 2006, 08:30AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Here is a picture of the current progress of the marx, so you get an idea what my drinking straw resistors look like. Be sure to notice all the lovely brass spheres.
The heatsink next to the marx contains the IGBT-bridge that I want to use for powering my to-be-constructed HV transformer for powering the marx. The ruler is in cm.

The main advantage I see in using these water resistors is that I can get them to stand as much voltage as I want, just by making them longer. I'll see if I can go for a 100kV stage voltage once I get my high voltage diodes to build the CW. If not, I'll change to ordinary wire-wound 5W resistors, I think they handle a few kV quite well.

HV Enthusiast

Wed Feb 15 2006, 03:53PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

I'll see if I can go for a 100kV stage voltage once I get my high voltage diodes to build the CW.

Even providing you get 100kV per stage, your present design imaged there will:

a) not be able to house 100kV capacitors per stage (too small)
b) not be able to withstand 100kV per stage

Your marx generator would have to be significantly larger.

Dr. Shark

Fri Feb 17 2006, 03:17PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

OK, I think I will forget about this 100kV per stage dream and cut down to 60kV /stage. Now I am planning to get some 1" acrylic because I think it looks really ugly with the cheap wood. Since I dont want to mess that up, I should figure out the distance between stages required to avoid flashover. I think the 1kv/mm figure for air will not help me here, since I will have surface tracking discharges. Would something like 5mm/kv be sensible? This would equate to 30cm (12") per stage, so the whole thing would end up being huge! Could I get away with something like 4" stage separation, or is that asking for trouble with flashover?

Marko

Fri Feb 17 2006, 07:49PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If you are trying to force capacitors to their exact rated voltage then its bad practice, use 5kV or less for two capacitors in series, because at 60kv you have very large possibility of dielectric breakout, you dont want these knobs dead

You muts take care of your resistors not to flash-over, capacitors can be mounted close to them as if happens breakout will be over resistor surface..
Use about 5 cm between each, aprox. length of your resistors, maybe a cm more for safety.

Capacitors are usually made to withstand their rated voltage without surface breakout..

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