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Copyright © 1994-2007
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We will not be responsible for damage to equipment, your ego, blown parts,
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of this material.
(From: Kenny Greenberg (kenny@neonshop.com).)
While it is true that warm air pushes the arc up the ladder, there is also
the typical 'high leakage' or reactance curve of the transformer contributing
to the effect. The transformer will happily arc across the bottom as long as
Paschen's Law will allow. Once this arc is struck the current in the arc will
actually increase to the transformer's preset limit. The heat is also
creating higher resistance.
Normally the transformer would try choke the voltage down as current
increased. But just above the arc exists a path that the transformer
can easily maintain and which in fact will lower its current. Voila.
At the top of course we are not only at the upper limit of the transformer
but it is also where the current is very low and so all the fun breaks apart
only to reignite down below.
A very interesting variant existed in the 1930's which used separate
horizontal electrodes at various points along one side instead of a
continuous vertical line. Each electrode is attached to a separate
neon unit. They are tied together and return to the HV transformer.
The convection current was optimized by placing this in a housing with
vents at the bottom and top. The result is an animator with no moving
parts.
The downside is the nitric acid that gets produced so keep it away from
things that may be corrode.
Make sure that no one can come in contact with this - particularly curious
onlookers. Separating the potential victims from any possible contact
with the high voltage is really the only foolproof way of protecting
against fools or the unaware - and you from a lawsuit. People not
familiar with high voltage phenomena (or aware only through grade-C
sci-fi movies) can be incredibly naive.
A GFCI (Ground Fault Circuit Interrupter) is of no use in protecting against
HV contact since the secondary of a neon sign transformer is isolated from the
line but its centertap is usually connected to the case - which should be
grounded. However, a GFCI would be a good idea in any case when you are
working with line connected equipment.
12,000 volts will jump approximately anywhere from 3/8 to 3/4 inch in dry air,
with sharp points and edges generally but maybe not quite always favoring
longer distances. This distance occaisionally varies unpredictably with
humidity. Don't forget that 12,000 VAC is approximately 17,000 V peak. Neon
sign transformers have current limited outputs - 30 mA is typical - but that
is still highly dangerous - lethal under the wrong conditions.
You can build a small Jacob's Ladder using a high voltage transformer
of lower capacity or a DC-AC inverter using a TV flyback transformer.
While these would be less dangerous, there is little room for carelessness
when working with any type of high voltage device. Even if there is no
resistive path, the stray capacitance can permit enough AC current to flow
to give you a painful experience!
Electrical discharges in air are also a producer of ozone which may be a
health hazard. See the section: About Ozone Production.
They also can produce significant Radio Frequency Interference (RFI) so the
FCC may come calling if you run the thing for an extended period of time.
You will need 12 to 15 kVAC at 20 to 30 mA. However, the exact values are
not at all critical. A neon sign (luminous tube) transformer is the usual
source for this power though an oil burner ignition transformer will work in
a pinch (some say better and cheaper) or you could build an inverter type
power supply.
Typical ratings: 12,000 to 15,000 VRMS current limited to 20 to 30 mA.
Larger ones are available - up to 60 mA or even 120 mA - but this is not
required. More information on neon sign transformers can be found in
the chapter: The
Home-Built Laser Assembly and Power Supply of
Sam's Laser FAQ.
Typical ratings: 8,000 to 10,000 VRMS current limited to 10 to 25 mA.
Inverter voltage and current ratings will be similar to their transformer
counterparts, above. However, since the frequency of operation is in the
10s of kHz range instead of 50/60 Hz, behavior for the Jacob's Ladder will
differ somewhat. For example, if they run on filtered DC (internally),
there will be none of that 50/60 Hz buzz associated with those classic
sci-fi movies! Perhaps, more of a starship engine sound. :)
Having said that, see the section: Notes on Really BIG
Jacob's Ladders for some more information.
Connect the high tension output of the
transformer to the two rods using high voltage insulated wire unless
the routing is such that there is no chance of arcing where you don't want it.
DO NOT use automotive ignition cable for this unless it is the non-resistive
type. Some adjustment of the spacing at the bottom (to get the arc started)
and at the top (to determine when the arc is extinguished and how fast it
rises) may be required (but do so only with the power off!). If starting
is problematic, a pair of sharp electrodes made from thin wire may be added
at the bottom with a spacing just slightly smaller than the main gap. The high
electric field at their points will help in initial ionization. But careful
shaping of the main electrodes usually makes this unnecessary.
Depending on the voltage and power rating of your high voltage source, these
dimensions may vary considerably. Spirals and other more creative
configurations are also possible.
IMPORTANT SAFETY NOTE: Essential line fuse, power switch, and power indicator
lamp not shown. Centertap (case) MUST be connected to Safety (earth) Ground!!
A Jacob's Ladder works on the principle that the ionized air in the arc
is a lower resistance than the air around it and heated air rises. The
arc strikes at the point of lowest breakdown voltage - the small gap
at the bottom. The heated plasma rises and even when it is an inch or
more in width is an easier path for the current to follow. Eventually,
the gap becomes too wide, the arc extinguishes and is reestablished
at the bottom. For best results, shield the whole thing from drafts but
don't use anything that can catch fire!
You know how critical the gap between the electrodes at the bottom of
a Jacob's ladder can be. Too wide and the arc won't strike, and too
narrow and it won't make it all the way to the top.
Because I live in the UK I'm more or less saddled with a 10 kV limit on
the maximum neon transformer available. Since the lower voltage makes
the gap even more critical, I designed a slight enhancement that works
really well.
It's simply a third electrode placed between the strike gap at the
bottom of the vee. It is connected to either one of the main electrodes
via two 1M ohm high voltage resistors.
When an arc should occur, the following happens...
I've never seen anyone use this on a Jacob's ladder before so what do I call
it? How about the Gabriel electrode (in keeping with the biblical theme).
Similar comments apply to the use of utility pole or substation transformers.
A word to the wise: at some point, bigger is just stupid. Sorry.
Microwave oven transformers make excellent Jacob's Ladders and Tesla Coil
drivers with a properly chosen series cap in the primary for power factor
correction. One of the major problems with the hollow "E" core current
limited neon sign and furnace ignition transformers is that they do not source
enough current to maintain a plasma on larger Jacob's Ladders, and the current
limiting makes them lousy as pulsed laser drivers as well.
However, the high current from a microwave oven transformer will source a 10"
inch arc in nitrogen/oxygen mixtures once it has been started at about 1/2".
There are several European web sites that show back to back microwave oven
transformer powered ladders. A key problem is that you have to mount the
transformers on Lucite or Lexan because the center tap is grounded on the
secondary side. For a advanced safety conscious experimenter it is not a
problem to use these, provided you use either 1" diameter rods of copper or
carbon rods for the ladder rails. While I would not approve of just anybody
playing with these either, it can be done and produces spectacular results, if
you don't mind the electric meter spinning at Mach 1!
They also make nice stable cores for high current low voltage transformers
when rewound, I use them to power 25 amp 3.2 volt laser tube filaments all the
time with out blowing breakers.
Snock's
High Voltage Page has a number of articles and links relating to large
Jacob's Ladders and other high voltage projects.
One of those old(bad) movies had a huge Jacob's Ladder inside a large
transparent (glass, I assume - Plexiglass wasn't around then) hollow column
resonant at 60 Hz. Unbelievable sound! Always wanted to build one like this
but never had the time, the big resonant column or the really huge neon sign
transformer that would have been needed.
(Assuming 1,100 feet per second for the speed of sound in air, a column closed
at one end would be a 1/4 wavelength resonator resulting in an actual height
of about 4.6 feet for a 60 Hz fundamental. --- sam)
(From: Pamela Hughes (phughes@omnilinx.net).)
The arc is a plasma of hot ionized gas. Molecules like O2 are broken down to
the atomic level and ionized. when these ions collide with the surrounding
air, they cause chemical reactions... the O can combine with nitrogen and form
small amounts of nitrogen oxides, and with O2 to form ozone (O3). However,
the high temps in an arc also tend to destroy these molecules too so you'd
probably only produce trace amounts if it weren't for the UV given off by the
arc. Ultraviolet seems to be the main mechanism for producing O3 as it can
ionize in the air far enough from the arc that it will be cool enough for
ozone to exist (a spark gives off UV and ionizes the air around it) A glow
discharge is better at generating ozone than an arc though, since it maximizes
the UV and the pressures and temps are much lower (i.e., put a conductive
coating on the outside of a glass tube and a wire down the center of it, then
apply enough voltage to produce a glow discharge inside the tube as you pump
oxygen at low pressure through the tube. Shortwave UV lamps will produce it
too (they use these as sterilizers in dairy barns).
-- end V1.36 --
All Rights Reserved
DISCLAIMER
Jacob's Ladders - especially large ones using line powered transformers - can
be both deadly, destructive, or both.
Introduction
What is a Jacob's Ladder?
A Jacob's Ladder is the type of high voltage "climbing arc" display seen in
many old (and usually bad) Sci-Fi movies. Jacob's Ladder come in all shapes,
styles, and sizes. Here is info on a common type that is easy to construct
with readily available parts. However, read the section:
Jacob's Ladder SAFETY before attempting to power any
high voltage project of this type.
How Does a Jacob's Ladder Work?
The simple explanation is that an arc starts at the bottom and due to the
fact that hot air rises, tends to move up the diverging rods until they are
too far apart for the voltage provided by the power source. A more complete
explanation is given below:
Jacob's Ladder SAFETY
WARNING: See Safety
Guidelines for High Voltage and/or Line Powered Equipment before firing
up any Jacob's Ladder or other high voltage or line powered project!
Building a Jacob's Ladder
Basic Components
There are only two major parts to a basic Jacob's Ladder: a high voltage
power source and a pair of rods arranged in a narrow V configuration on an
insulated and fireproof support.
Construction
Take a pair of thin metal rods - the steel wire from old metal coat hangers
works quite well. Straighten them out and mount them on an insulated
non-flammable support with a gap of about 1/4 inch at the bottom and 1 to 3
inches at the top forming a narrow tall 'V'. Mounting locations should not
be in the path of the rising arc. Since the electrical forces on the rods
will tend to pull them together, adequate support especially at
the top is critical unless a moving sculpture is the desired result. :)
A free-standing metal coat hanger wire will not be rigid enough. Again,
the support must be non-flammable and not in the path of the arc.
___ 1-3 inch gap or more at top.
^ \ /
2-3 feet | \ /
or more | \ / 1/4 inch gap at bottom.
_v_ / \
+----X-' '-X----+ X is mount attachment.
| Insulated, |
HV | non-flammable |
+----+ mounting base |
||( |
Hot o---+ ||( |
)||( Luminous tube | HV insulated wire
)||( CT transformer |
AC Line )|| +---+ (12 kV, 30 mA, |
)||( | typical) |
)||( | |
Neutral o---+ ||( | |
||( HV | |
Case | +----------------------+
| |
Safety Ground o---+------+
Clive's Gabriel Electrode to Help the Arc to
Strike
(From: Clive Mitchell (clive@emanator.demon.co.uk).)
It's simple, but works perfectly.
Why NOT to use Microwave Oven Transformers
Using multiple high voltage transformers from microwave ovens to construct a
Jacob's Ladder is a very bad idea for several reasons:
I bet you are also going to try to run them on 220 VAC to double the output
voltage as well, huh??
Additional Information
Notes on Really BIG Jacob's Ladders
(From: Steve Roberts (osteven@en.com).)
And Now for the Audio Feed
(From: Norman E. Litsche (nlitsche@worldnet.att.net).)
About Ozone Production
If the Jacob's Ladder is large, significant ozone is an inherent byproduct
unless you run it in an inert gas which might be an interesting experiment
though I don't know how performance will be affected.
Jacob's Ladders and the FCC
Operating Jacob's Ladders (as well as Tesla coils) are broad-band RF sources
and can interfere with radio, TV, maybe cell phone, cordless phone, and
other communications equipment. There have been reports of the FCC tracking
down and fining people for this interference - I don't know if these stories
are true but it is definitely something to think about before running your
creation for an extended period of time, at least. The FCC would probably
confiscate your setup as well - which would likely be more traumatic than the
fine (as hefty as it might be!) Perhaps, consider a Faraday cage in addition
to the Plexiglass ozone shield :-).