Maker Pro
Maker Pro

Spark gaps -- making and triggering

I

Ignoramus29428

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.

Sounds like a time for a trip to Ace Hardware!

i
 
W

Winfield Hill

Jan 1, 1970
0
Ignoramus29428 wrote...
True, but at a thousand volts, they would hold about 1/2 joule of
energy, which could be discharged with just a crowbar.

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!
 
I

Ignoramus29428

Jan 1, 1970
0
Ignoramus29428 wrote...

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!

Win, thanks, I did not realize that there were HV relays and did not
think aboout high power HV resistors. I will check it out... Thanks a
lot...

i
 
D

DoN. Nichols

Jan 1, 1970
0
According to 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.

Hmm ... I remember a high voltage test cage which I used to have
to calibrate at a place which made high-voltage rectifiers (among other
semiconductors) was set up in a metal electronics rack with a door of
1/2" Plexiglas. The door was held closed by a long screw cut with a
buttress thread (lots of slope on the side towards the free end, and
almost vertical on the side towards the knob which turned it. The
female thread was a hole in two plates with a spring wire between them
pinching on either side of the screw.

To close it -- all you had to do was push.

To *open* it, you had to spend a couple of minutes turning the
knob to unscrew the screw -- and about halfway during the time a cam
would swing a safety grounding bar into contact with the high voltage
terminals, to make *sure* that there was no high voltage there when the
operator reached in to change the device under test.
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?

The larger the radius, the shorter the gap will need to be,
because charge concentrates on small radius points. I've got no idea
whether the TIG electrodes could handle that much power (but I'm not sure
how many caps you intend to hook in parallel for this.)
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?

Does argon have a lower dielectric constant or breakdown voltage
than air? I would expect it to be somewhat more insulation than air.

I think that you'll have to inject a very high voltage pulse
between the two with the electrode closer to the ground side, but
capable of ionizing the air to make the gap enough shorter to dump
everything from the caps -- with something like a flash lamp trigger
transformer. (Hmm -- perhaps a spark coil from a car?) Part of the
problem is keeping the trigger transformer from being a consumable. :)

Perhaps set the gap small enough so at near your full voltage it
will break over on its own, and simply set it up and then start
charging, letting it trigger itself -- though you won't know exactly
when.

Perhaps UV illumination will ionize the air enough to trigger
the gap.
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.

A good idea -- this strikes me as a very dangerous experiment.
(But possibly fun. :)

There is no way I would generate that much of an EMP near all of
my computers. :)

Enjoy,
DoN.
 
J

James Waldby

Jan 1, 1970
0
Ignoramus29428 wrote:
....
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?

Thoriated tungsten is used in some spark gaps - the slight amount
of radiation making the trigger level more consistent. (Beyond that
interesting item, I know nothing else about tungsten in spark gaps)
Also, what is the best way [of] 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?

Probably safe but not effective. However, here's an argon-based
method I thought of that might work, with some work: Put round metal
contact plates at opposite ends of a sealed cylinder (say 1" across
and 1" apart). Through one valve, fill with high-pressure argon before
charging caps (or at least before arming the discharge circuit). When
ready to fire, exhaust the argon via second valve into a several-
gallons evacuated tank to rapidly drop the pressure. See Paschen
curves (discharge V vs. P*Distance or vs. P) for argon and for air
at http://www.seas.ucla.edu/Chang/papers/Chang-Hicks-RFPlasmas.pdf
and http://www.duniway.com/images/pdf/pg/Paschen-Curve.pdf eg.

Also see http://members.tm.net/lapointe/Pseudospark_Switches.html
re a low-pressure spark-gap switch triggered by light or by an
electrical trigger. It says, "The neat part is that a spark does
not form. Instead, a diffuse discharge occurs" and "A simple switch
based on [figure] can operate at potentials of 35 KV and handle
currents of 10 KA with rates of current rise of 4 X 10^11 amps/second.",
then refers to a Maxwell Labs patent for 1 MA "Coaxial Pseudospark
Discharge Switch" and Tetra Corp patent for 50 KA "Linear Aperture
Pseudospark Switch".

-jiw
 
R

Robert Latest

Jan 1, 1970
0
["Followup-To:" header set to sci.electronics.design.]
On Wed, 05 Apr 2006 01:16:10 GMT,
in Msg. said:
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

If I wanted to build this I'd look into using a small pneumatic cylinder
to bring the electrodes together.

BTW, a setup like this is really more like a relay, not a spark gap.
It's a relay that happens to spark a lot ;-)

robert
 
R

Robert Latest

Jan 1, 1970
0
["Followup-To:" header set to sci.electronics.design.]
On Wed, 05 Apr 2006 01:55:09 GMT,
RoyJ said:
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.

Why not bring them together all the way? To avoid them welding together?

robert
 
C

Cydrome Leader

Jan 1, 1970
0
In rec.crafts.metalworking Ignoramus29428 said:
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?

can crusing coils will try to flatten themselves. They don't explode.

Trying to contain giant bangs in plastic boxes is called making a bomb.
Although a bit different, it's the same reason gunpowder is shipped
in really flimbsy containers, not tough containers.
 
J

John Fields

Jan 1, 1970
0
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!
 
I

Ignoramus26024

Jan 1, 1970
0
["Followup-To:" header set to sci.electronics.design.]
On Wed, 05 Apr 2006 01:16:10 GMT,
in Msg. said:
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

If I wanted to build this I'd look into using a small pneumatic cylinder
to bring the electrodes together.

BTW, a setup like this is really more like a relay, not a spark gap.
It's a relay that happens to spark a lot ;-)

I actually do have an air cylinder,so I could try that.

i
 
B

Bert Hickman

Jan 1, 1970
0
Ignoramus29428 wrote:
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.

That should work fine. Or you could use copper or aluminum flashing cut
into wide strips, or extruded aluminum. I wouldn't recommend using steel
in the high current path because of high skin effect losses.
What size balls? They have 3/4" diameter balls pretty cheaply.

3/4" is pretty small - something like 1" - 1.5" might work better for a
variety of reasons.
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.

Using 1" pipe end caps should work fine.
Just what forces act upon the coil? Do I need to wrap it in fiberglass
cloth with epoxy to make it safe from exploding?

By using thick wire or tubing, a large diameter coil, and applying
limited energy, there's no risk of the work coil exploding. There are
radial "magnetic pressure" and Lenz's Law forces that try to expand the
coil's diameter, and there are axial forces that squeeze the turns of
the coil together. All of the forces act in a direction that tries to
increase the inductance of the coil (i.e., increase diameter and
decrease axial length).

If you use heavy gauge copper wire (#6 - #8 AWG) or 1/4" copper tubing
and 3 - 5 turns, the coil will have sufficient mechanical strength. If
you use copper tubing or bare copper ground wire, you'll need to slip
some heat shrink or vinyl tubing over it before winding. Using a space
wound coil using bare wire will likely fail since axial forces may close
up the inter-turn gaps, shorting out the coil.
Yes, I saw your solenoid triggered gap, and wondered to myself,

1) does high intensity discharge impact the solenoid

It doesn't seem to have any effect, the gap simply fires as the
electrodes are being brought together. I do use a tightly twisted pair
of wires going to the solenoid - this helps to prevent any induced EMF's
from the pulsed magnetic fields in the area from creating any problems.
2) how do you switch the solenoid without exposing yourself to
possible shock

The solenoid is electrically isolated by distance and by using good
insulating materials. You could use an air cylinder to actuate it instead.
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.
Sorry - the correct URL is:
http://205.243.100.155/frames/Newgap2a.jpg
If you want a free hostname, like coincrushing.algebra.com, let me
know.
Thanks!



Will do! Thanks a lot!

i

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
-------------------------------------------------------
 
B

Bert Hickman

Jan 1, 1970
0
Robert said:
["Followup-To:" header set to sci.electronics.design.]
On Wed, 05 Apr 2006 01:16:10 GMT,
in Msg. said:
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


If I wanted to build this I'd look into using a small pneumatic cylinder
to bring the electrodes together.

BTW, a setup like this is really more like a relay, not a spark gap.
It's a relay that happens to spark a lot ;-)

robert

Nope - it's still a true spark gap switch. I neglected to mention in my
earlier post that the electrodes approach, but do not touch, each other.
There's a residual air gap of about 0.050" when fully energized.
Although the contacts wouldn't weld under a high power shot, they could
under low power - leaving a small air gap prevents this. I chose an
electrical solenoid, but a small air cylinder would also work.

BTW, there's an intentional "hairpin loop" in the electrical path. This
causes the high current arc to be magnetically blown upwards, rapidly
moving the incandescent cathode spot(s), thereby helping to reduce
electrode evaporation and wear. A similar technique is used is some high
current utility circuit breakers. The added gap inductance doesn't cause
any problems for my applications.

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
-------------------------------------------------------
 
B

Bert Hickman

Jan 1, 1970
0
Ignoramus29428 said:
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

These should work fine - relatively inexpensive and easily replaceable.
I use silicon bronze bolts for bolting the work coil to the bus bars,
and the threads have lasted for thousands of shots. Just make sure that
the threaded section is locked down to prevent arcing.

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
-------------------------------------------------------
 
R

RoyJ

Jan 1, 1970
0
Yep. If they don't touch you get arc erosion. If they do touch you will
get metal transfer at the least, fully welded at the worst.

Robert said:
["Followup-To:" header set to sci.electronics.design.]
On Wed, 05 Apr 2006 01:55:09 GMT,
in Msg. said:
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.


Why not bring them together all the way? To avoid them welding together?

robert
 
B

Bert Hickman

Jan 1, 1970
0
Ignoramus29428 said:
Win, thanks, I did not realize that there were HV relays and did not
think aboout high power HV resistors. I will check it out... Thanks a
lot...

i

Win,

Are you using high energy bulk resistors (such as Kanthal, or HVR
Advanced Power) to rapidly absorb the energy? I've seen high power
wirewounds fail spectacularly under similar conditions.

BTW, HVR has a nice thermal modeling tool for designing rapid energy
dump solutions:
http://www.hvrapc.com/thermalapplicationpage.asp

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
-------------------------------------------------------
 
B

Bert Hickman

Jan 1, 1970
0
Cydrome said:
Trying to contain giant bangs in plastic boxes is called making a bomb.
Although a bit different, it's the same reason gunpowder is shipped
in really flimbsy containers, not tough containers.

Although the BANG from a high energy air spark gap is extremely loud, it
doesn't generate volumes of high pressure gas like chemical explosives.
The container must be capable of handling the shockwave and acoustic
pressure. A number of manufacturers (General Atomics, R. E. Beverly)
currently use Lexan polycarbonate housings for their high energy spark
gaps.

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

Ignoramus26024

Jan 1, 1970
0
Ignoramus29428 wrote:


That should work fine. Or you could use copper or aluminum flashing cut
into wide strips, or extruded aluminum. I wouldn't recommend using steel
in the high current path because of high skin effect losses.
Great!


3/4" is pretty small - something like 1" - 1.5" might work better for a
variety of reasons.


Using 1" pipe end caps should work fine.

Good, then, that's what I will do.
By using thick wire or tubing, a large diameter coil, and applying
limited energy, there's no risk of the work coil exploding. There are
radial "magnetic pressure" and Lenz's Law forces that try to expand the
coil's diameter, and there are axial forces that squeeze the turns of
the coil together. All of the forces act in a direction that tries to
increase the inductance of the coil (i.e., increase diameter and
decrease axial length).

If you use heavy gauge copper wire (#6 - #8 AWG) or 1/4" copper tubing
and 3 - 5 turns, the coil will have sufficient mechanical strength. If
you use copper tubing or bare copper ground wire, you'll need to slip
some heat shrink or vinyl tubing over it before winding. Using a space
wound coil using bare wire will likely fail since axial forces may close
up the inter-turn gaps, shorting out the coil.

That sounds simple enough. I will indeed use copper tubing, I think.

I appreciate your advice.

As for triggering the gap, at first I could try simply setting a good
distance. That would not get me all the way up to 13 kV that my power
supply could make, but maybe I can get close.

i
 
B

Bert Hickman

Jan 1, 1970
0
Ignoramus26024 wrote:

That sounds simple enough. I will indeed use copper tubing, I think.

I appreciate your advice.

As for triggering the gap, at first I could try simply setting a good
distance. That would not get me all the way up to 13 kV that my power
supply could make, but maybe I can get close.

i

A properly adjusted fixed gap will work. However, you may see
significant shot-to-shot gap breakdown voltage variation. Personally, I
prefer to be in control as far as WHEN the spark gap will fire. I really
don't like sudden (loud!) surprises in the high voltage lab... :^)
http://205.243.100.155/frames/kaboom.jpg

More importantly, you should plan to disconnect your charging power
supply before firing the device. If the gap happens to fire while
charging you run the risk of blowing up the HV diodes in the charging
supply. Also, consider connecting 10-20k current limiting resistor(s)
between the charging supply and the capacitor bank. These should be
100-225W wirewounds for sufficient worst case standoff voltage. This
will limit forward diode current during ringing, and should prevent
backfeeding high current back into your charging supply if you happen to
encounter any HV diode failure(s).

If you ground the charging input after firing, these resistors can serve
as bleeder resistors to dissipate any residual bank energy, or for an
emergency shutdown. Similar to Win, I leave a set of bleeder resistors
across the bank at all times (other than charging) to prevent any
surprises.

Did I mention, I really don't like surprises in the HV lab? :^)

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
-------------------------------------------------------
 
M

Martin H. Eastburn

Jan 1, 1970
0
Some years ago - just after horse and buggy :) - we used A hollow sphere that
was fixed and another on a rack and pinion. The unit was a voltage regulator
for a large X-ray machine. It was Nasty to say the least. Adjust that xxxx
thing from no arc to an arc - air blast.

The method I worked out - professor wasn't thinking straight I think -
Set rack to number x - and move out and away - turn on and measure voltage.
Calibrate X to value - after a table was generated then we could set the voltage
we wanted to set.

The point is a low voltage arc point. It easily gives off electrons and breaks down.
The larger a sphere the higher charge density and potential difference possible.

So I'd try for basket ball or so size.

Martin

Martin Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
NRA LOH & Endowment Member
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member
 
Top