Spark gaps -- making and triggering

Dry argon is not conductive at STP. Consider making a "trap" above the spark gap that drops a short section of, say, 36awg wire across the gap to initiate the arc.

LLoyd

I saw a coin-crusher guy's website a while ago that had a sliding arrangement that put two large brass balls, or maybe a ball and a disk, into contact with each other.

Probably bigger.

I think with HV you want to stick with a sphere.

Slide 'em towards each other until they touch. Do this not with your body parts.

If you can't find that coin shrinking website let me know and I'll dig around until I recognize it.

Don't have any real info for you, but you may want to contact the guy who runs this site:

seems to have some really fun HV stuff

Tom

Ignoramus29428 wrote:

--Time to cruise over to sci.physics.electromag which is where the folks you need to gab with hang out. Good idea using pipe for conductors; when I was messing with mass drivers I discovered that floppy, unsecured wires will thrash about when pushed to their limits; it can be a little disconcerting, heh.

Thanks, Dave, Surftom, steamer, and Lloyd. I am going to subscribe to that newsgroup now. I have Bert's website in my bookmarks now.

http://205.243.100.155/frames/shrinker.html i

OK... I realized that I have two steel bearing balls stolen two decades ago from a Soviet factory. About 2" in diameter. Their downside is that they are very heavy and thus would require some support, with the attendant issues pertaining to HV. McMaster has some balls, but relatively expensive.

i

"Ignoramus29428" a écrit dans le message de news:snzYf.3$ snipped-for-privacy@fe49.usenetserver.com...

Maybe have 3 spheres. 2 to make your spark gap, the 3rd slowly rolling on nylon rails right between the two others...

96 seconds is a pretty long time for the RC discharge time but much better than nothing. Remember that 3 rc times will be about 4.5 minutes and the caps will still have about a thousand volts on them. Be careful out there.

Dan

Ignoramus29428 wrote:

You can experiment triggering a gap by firing a photo flash at it.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

void _-void-_ in the obvious place

Check out

True, but at a thousand volts, they would hold about 1/2 joule of energy, which could be discharged with just a crowbar.

i

The proper gadget for triggering a discharge is a special vacuum tube called an "ignitron". The ones I used to play with were about the size of a beer can. I have no idea where you could get one surplus. They basically have a pool of mercury in them, with a small electrode just over the pool, with one terminal connected to the pool, and one terminal on top. You discharge a relatively small HV capacitor between the electrode and the pool, which creates mercury vapor. The main high voltage then discharges through the vapor. It produces a very low loss HV switch. They used to show up occasionally at the surpls yard in Los Alamos, where I grew up, along with all the other HV toys you could imagine for 35 cents a pound. Those were the days...

Doug White

OK, what about using "hitch balls" from walmart for spark gaps? They are cheap and I could buy them locally.

i

For your application, keep it simple. You can use heavy flexible cabling (such as welding cable) to make the interconnections. A little bit of cable jumping from magnetic forces won't cause you any problems.

For the spark gap, there's no need for pressurized gases, inert gases, etc. - air at STP works just fine. Copper or brass balls will work great (from McMaster-Carr), and copper end caps on copper pipe should work as long as you're not going to do thousands of shots. Tungsten or copper-tungsten would also work, but because of the high peak currents, I'd recommend using electrodes that were at least 1/2" in diameter. I'd avoid using steel electrodes since steel tends to generate a messy spray of pyrotechnic sparks. For similar reasons, avoid aluminum electrodes. Aluminum would erode rapidly, and the spray of sparks and flash from oxidizing aluminum vapor might make quite an impression. Do think about enclosing the spark gap to help deaden the sharp "BANG!" when it fires.

I previously used a triggered gap (a home-built Trigatron) that used a modified spark plug and 40+kv pulse to fire the gap. However, I've recently converted to a simple solenoid triggered gap with brass electrodes, since it provides much wider operating voltage range and I don't require precise switch timing. Solid brass electrodes show relatively little wear even after thousands of 50-100kA shots. The only downside is a bit of zinc oxide film on nearby objects, apparently from some zinc leaching out and oxidizing during the discharge. After thousands of shots, the surface of the brass looks similar to a dry river bed, with little islands of copper surrounded by small fissures.

A picture of my solenoid gap can be seen here. The Lexan case helps dampen the noise somewhat. It's STILL loud, but not as sharp. You could rig something similar using a (long!) string to pull the electrodes together, or you could swing a third electrode between the main pair to fire it. http://205.243.100.155/frames/gallery/newgap5a.jpghttp://205.243.100.155/frames/gallery/newgap2a.jpg As someone suggested, you could also use a pulse-rated ignitron. These use a molybdenum anode (instead of a graphite one) to handle current reversals and 100 kA peaks on an infrequent (1/minute) basis. However, although an ignitron is quite a bit quieter when firing than a spark gap, there's no real performance advantage. In fact, an ignitron may actually be significantly lossier than a well designed spark gap for your application. Unfortunately, pulse-rated ignitrons tend to be scarce and costly even on the surplus market... and you never know how much life remains on a used pull. However, if you do want to pursue this route, some pulse-rated 20 kV ignitrons include the 7703 series, NL1037H, and NL1039.

BTW, if you are near Fermilab, you are also near me. Contact me off Usenet if you'd like to see the coin shrinker in action or discuss some of this in greater detail.

Good luck, and play safely!

Bert

Bert, thanks. I am going to make a big Gnumeric worksheet with all values, etc, to calculate stuff. Prior to that though, I would like to get a "feel" of things. When you talk about heavy cables, what size cables would apply to 4 uF charged at 11-13 kV, discharging through a spark?

I think that for paralleling the caps, I will simply use aluminum or steel angle, which I already have.

For connecting paralleled caps to the actual can crushing setup, I would indeed use some cabling. I have some six gauge wire, for instance.

What size balls? They have 3/4" diameter balls pretty cheaply.

Definitely not thousands, so, it would be great to not need anything.

If I can buy copper tubing (what diameter?) and some end caps, and be in business, that would be awesome.

Just what forces act upon the coil? Do I need to wrap it in fiberglass cloth with epoxy to make it safe from exploding?

All I have is 3/16" electrodes.

I like the idea of copper pipe with end caps.

Yes, I saw your solenoid triggered gap, and wondered to myself,

1) does high intensity discharge impact the colenoid 2) how do you switch the solenoid without exposing yourself to possible shock

Otherwise I like the solenoid idea a lot, a push solenoid would not be that expensive. I will start with something simplier though.

That's what I will do, indeed!

the second ipcture does not work.

If you want a free hostname, like coincrushing.algebra.com, let me know.

That's a little above my head at the moment.

Will do! Thanks a lot!

i

How about McMaster item 94050A475 (1/2" silicon bronze carriage bolt) for spark gaps? The nice thing about them is that they can be adjusted, unless they would weld themselves to threads after a few discharges.

i

I haven't played with these but I think you can just stop the contacts a few thousanths short of contact. At that voltage, it just doesn't make any difference. Anything between .010" and .060" should work. using some

1" c> >

Sounds like a time for a trip to Ace Hardware!

i

Crowbar, smobar. When I interlock my high-voltage systems I use a HV relay with a low-value HV resistor to discharge the caps by five time constants in under a few second. The relay is normally closed and is opened only by the charging circuit. This means the HV caps won't have voltage unless the system is in its active state, ready to pour energy into the system. And it means that whenever I think it's off the capacitors are already discharged.

BTW, there's no reason to mamby-pamby around when discharging HV capacitors. In one of my instruments the caps are discharged in 50us to quickly remove any voltage from some electrodes for the experiment. No big deal, it could have been microseconds. Let's get those big caps discharged before they can hurt someone!