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Spark gaps -- making and triggering

I

Ignoramus29428

Jan 1, 1970
0
I finally picked up the caps from Fermilab. Each of them weighs about
40 lbs and they look nice, very cute color also.

So... I would like to make some sort of a pulse discharge thingy from
readily available materials. Can crushing would be my first choice.

My plan is as follows, to use copper pipe as conductor. I bought some
HV diode stack (4 7.5 kV diodes) and a HV probe.

The first thing to make would be a reliable system for discharging the
caps safely, and test it at low voltages. I would probably want to use
two 12 MEG Victoreen resistors in series a permanently attached
bleeder.

With four 1 uF caps, RC would be 2*12,000,000 * 4/1000000 = 96, which
is hopefully acceptable, and power dissipation in the resistors will
be about 3W per resistor.

My main questions concern the spark gap. How to practically make a
decent spark gap, and also how to safely trigger it.

For instance, would 3/4" brass balls such as McMaster item 9617K47, be
adequate? If not, I have 3/16" tungsten electrodes. Would they make an
adequate spark gap?

If not, can I use sections of, say, 1" copper pipe, placed at some
distance with their axes perpendicular?

Also, what is the best way ot safely triggering a spark gap. My
understanding is that there is a rule of one inch per 15 kV. Would
making the gap 1.5" and injecting some argon into it be a safe
and effective triggering method?

I am going to do a lot more reading and I will post my "design" here
and get it to pass some consensus before implementing it.

i
 
L

Lloyd E. Sponenburgh

Jan 1, 1970
0
Ignoramus29428 said:
Also, what is the best way ot safely triggering a spark gap. My
understanding is that there is a rule of one inch per 15 kV. Would
making the gap 1.5" and injecting some argon into it be a safe
and effective triggering method?

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
 
D

Dave Hinz

Jan 1, 1970
0
My main questions concern the spark gap. How to practically make a
decent spark gap, and also how to safely trigger it.

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.
For instance, would 3/4" brass balls such as McMaster item 9617K47, be
adequate? If not, I have 3/16" tungsten electrodes. Would they make an
adequate spark gap?

Probably bigger.
If not, can I use sections of, say, 1" copper pipe, placed at some
distance with their axes perpendicular?

I think with HV you want to stick with a sphere.
Also, what is the best way ot safely triggering a spark gap. My
understanding is that there is a rule of one inch per 15 kV. Would
making the gap 1.5" and injecting some argon into it be a safe
and effective triggering method?

Slide 'em towards each other until they touch. Do this not with your
body parts.
I am going to do a lot more reading and I will post my "design" here
and get it to pass some consensus before implementing it.

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

steamer

Jan 1, 1970
0
--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.
 
I

Ignoramus29428

Jan 1, 1970
0
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
 
I

Ignoramus29428

Jan 1, 1970
0
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
 
F

Fred Bartoli

Jan 1, 1970
0
Ignoramus29428 said:
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

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

Ignoramus29428

Jan 1, 1970
0
Maybe have 3 spheres. 2 to make your spark gap, the 3rd slowly rolling on
nylon rails right between the two others...

I like this idea. How would one go about calculating the sphere size
given the energy stored and perhaps voltage?

i
 
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
 
B

Boris Mohar

Jan 1, 1970
0
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) http://www.viatrack.ca

void _-void-_ in the obvious place
 
R

Rene Tschaggelar

Jan 1, 1970
0
Ignoramus29428 wrote:

Also, what is the best way ot safely triggering a spark gap. My
understanding is that there is a rule of one inch per 15 kV. Would
making the gap 1.5" and injecting some argon into it be a safe
and effective triggering method?

I once saw some guys using car spark plug for
triggering the arc between bigger copper spheres.
This means at least one of the spheres was at
GND potential. The spark plug was mounted on the
inside of one sphere and made almost even, located
close to where both spheres are closest.

Rene
 
K

kell

Jan 1, 1970
0
Ignoramus29428 said:
I finally picked up the caps from Fermilab. Each of them weighs about
40 lbs and they look nice, very cute color also.

So... I would like to make some sort of a pulse discharge thingy from
readily available materials. Can crushing would be my first choice.

My plan is as follows, to use copper pipe as conductor. I bought some
HV diode stack (4 7.5 kV diodes) and a HV probe.

The first thing to make would be a reliable system for discharging the
caps safely, and test it at low voltages. I would probably want to use
two 12 MEG Victoreen resistors in series a permanently attached
bleeder.

With four 1 uF caps, RC would be 2*12,000,000 * 4/1000000 = 96, which
is hopefully acceptable, and power dissipation in the resistors will
be about 3W per resistor.

My main questions concern the spark gap. How to practically make a
decent spark gap, and also how to safely trigger it.

Check out www.4hv.org
 
I

Ignoramus29428

Jan 1, 1970
0
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.

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

i
 
D

Doug White

Jan 1, 1970
0
Keywords:

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
 
I

Ignoramus29428

Jan 1, 1970
0
OK, what about using "hitch balls" from walmart for spark gaps? They
are cheap and I could buy them locally.

i
 
B

Bert Hickman

Jan 1, 1970
0
Ignoramus29428 said:
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.jpg
http://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
--
-------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, our "Captured Lightning" Lichtenberg
Figure sculptures, and Out-of-Print technical Books.
Visit Stoneridge Engineering: http://www.teslamania.com
-------------------------------------------------------
 
I

Ignoramus29428

Jan 1, 1970
0
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.

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.
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)

What size balls? They have 3/4" diameter balls pretty cheaply.
and copper end caps on copper pipe should work as
long as you're not going to do thousands of shots.

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?
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.

All I have is 3/16" electrodes.
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 like the idea of copper pipe with end caps.
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.

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.
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,

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.
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.

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

Will do! Thanks a lot!

i
 
I

Ignoramus29428

Jan 1, 1970
0
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
 
R

RoyJ

Jan 1, 1970
0
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" copper pipe with smooth end caps would be fine, they have flat heads
and rounded corners.
 
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