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Tesla Coil
Invented by Nikola Tesla in the 19th century, and much beloved of science fiction film makers of the 30's and 40's, as well as hv experimenters of today, the Tesla coil is an inexpensive way to generate very high voltages (megavolts) at radio frequencies. A tesla coil consists of an air core inductor that acts as a quarter wave transmission line shorted at one end and open at the other. The shorted (low impedance) end is driven by link coupling from a high power RF source at the resonant frequency of the transmission line. The impedance transformation of the quarter wave line turns the high current, low voltage at the shorted end to very high voltage, low current at the open end. The most popular RF source is a resonant LC circuit excited by an impulses created by a spark gap.
Corona
Corona is caused by the electric field next to an object exceeding the breakdown value for air (or whatever it is immersed in). Since the magnitude of the field is inversely proportional to the radius of curvature, sharper edges break down sooner. The corona starting voltage is typically 30 kV/cm radius. Dust or water particles on the surface of the object reduce the corona starting voltage, probably by providing local areas of tighter curvature, and hence higher field stress.
The easiest case to analyze is that of a sphere. The magnitude of the electric field at the surface of a sphere in free space is simply the voltage/radius. Note that if the sphere is near another conductor, the field is no longer uniform, as the charge will redistribute itself towards an adjacent conductor, increasing the field.
Since corona is fundamentally a breakdown phenomenon, it follows Paschen's law: the voltage is a function of pd. Double all the dimensions and halve the gas pressure, and the corona voltage will be pretty much the same.
Jacobs Ladder
The basic construction and principle of a Jacobs ladder is that a metal vee is formed from two bits of wire and a high voltage is applied across them. The electricity arcs across at the bottom of the vee where the electrodes are closest together. The air that the spark is passing through is ionised making it a preffered route for the arc, so when it heats up and begins to rise it drags the arc up the metal vee pulling it wider as it goes. In a correctly set up ladder the arc will travel all the way to the top where the wires have a sharp outward bend in them to pull the arc apart to the point that it extinguishes and the whole process starts again at the bottom of the vee.
As with tesla coils, it's the wattage that gives the long arc. A 2000V
transformer might only be able to START a spark over a tiny fraction of an
inch gap, but once the spark has "lit", the bent wire electrodes can draw
it out into a long flame. A 9,000v transformer might work better than a
30,000V transformer, if the 9,000V unit has a much higher wattage
capability. Also, wattage is usually proportional to transformer size, so
the more iron in the transformer, the longer an arc you'll be able to
make. Power-pole transformers (if wired backwards to 120V and limited
with a large AC heater in series!) can generate discharges several feet
long.
Marx Generator
The Marx generator is a voltage multiplier working on a completely different principle than all other multipliers presented in here. Roughly, the principle is: charge a set of capacitors in parallel, discharge in series. One way to achieve this would be straight-forward via lots of switches. The Marx generator, however, is a clever way of doing this switching automatically, via spark gaps.
Marx generators are mainly used to simulate the effect of lightning on technical high voltage components. Voltages above 1MV are easily reached. An advantage over cascades is that there is only one cap per stage (and no diodes at all, making it a rather robust device). However, there is one drawback: the maximum spark length is reduced with respect to DC, because a spark needs time to develop.
Capacitors for a Marx generator must ne suitable for pulsed applications. Many cheap high voltage caps (MKT-types) are not suitable. Caps with PP or PE dielectric are good. The resistors can be a lot smaller than in my example, actually they might be substituted by chokes. A large charging resistor is recommended, however, as it protects the DC source and makes the generator controllable by slowing down the repetition rate.
Fly Back Transformers
In contrast to Oil Burner Ignition Transformers (OBITs) and Microwave Oven Transformers (MOTs), flybacks cannot be connected to the mains directly. They work at a frequency of about 20kHz, whereas the mains has only 50Hz. The higher frequency has many advantages, such as smaller, lighter cores, smaller caps for rectifiers etc., but it makes an electronic control circuit necessary.
Flybacks can be found in all types of monitors and screens that use a cathode ray tube (CRT), e.g. TV sets, computer monitors etc. A flyback serves several purposes in a TV set, mainly the generation of the acceleration voltage for the CRT (typically 20-30kV), and of several auxiliary voltages. Very roughly, three types can be distinguished:
Flybacks for use with a cascade (voltage multiplier). This type is common in (old) large color TV sets, as these need a higher acceleration voltage. The flyback output is usually around 8-10kV (peak to peak), which is often tripled by the cascade to 24-30kV DC.
Flybacks which are connected to the CRT via only a single rectifier. This type is common in (old) black and white TV sets. It outputs around 20kV (peak to peak).
So-called diode split transformers, which are usually found in more modern devices. They have a secondary which is split into several parts of lower voltage by embedded diodes, which at the same time rectify the output voltage. It therefore gives directly DC (positive polarity) without need for external rectifiers, but also without possibility to multiply the output further by a cascade.
One major difficulty is to find the useful ones out of the vast number of connector pins a flyback usually has. The high voltage output is in most cases obvious (the single lead coming from the secondary). With normal flybacks, i.e. not internally rectified, the other end of the high voltage winding can be found using an Ohmmeter or circuit tester. The rest of the pins can be grouped as belonging to several different windings by the same method. Which winding and of this, which taps are best suited for input must be tried.
The original primary winding of a flyback is often designed for about 150V, i.e. the circuit which produces the necessary 20kHz signal works at 150V DC. Voltage pulses produced thereby across the primary measure easily 700V and more. The simplest (though not best, for safety reasons) way of producing such high working voltages is directly from the mains. If you don't want to take the risks of working with mains voltage, or if the device must be independent of the mains, you will have to add a new primary winding. One winds about 10 turns (try out optimum number) 1mm diameter enamelled copper wire around the core, preferably directly underneath the secondary winding. If there is no room, it often helps to rip off some of the unused original windings. However, such a new primary winding with a low number of turns will be less efficient than the original winding.
Oil Burning Ignition Transformers (OBITs)
Oil burner ignition transformers (OBITs) can be found in oil and gas heatings, where they are used to produce an arc which ignites the flame. they are directly connected to the mains (230V primary winding in Europe), while the secondary winding gives 10kV(eff) between the two high voltage leads, and 5kV(eff) between each one and the iron core. This is called a center-tapped secondary (mid-point grounded). The iron core is connected to the green/yellow earth lead and must be connected to the PE (protection earth) contact of the socket.
Obits are current limited to about 25mA between the HV leads, i.e. they may be short-circuited on the secondary side. However, the short-circuit time must be kept below 1 minute, and the obit must cool down for 2 minute before the next short-circuit period.
It is possible to parallel several identical units (on the primary as well as on the secondary side), to obtain a higher current output. You have to pay attention to correct phase when connecting the HV leads. This is checked by bringing the leads to be connected near each other. If no arc can be jumps between them, they may safely be connected. Otherwise, either the primary or secondary leads must be swapped. The photo shows a wooden case containing 9 obits in parallel, giving an output current of more than 200mA.
It is also possible to cascade obits for higher voltage, but it is not possible to use the straight forward method paralleling the primaries while connecting the secondaries in series. This shorts out the secondaries, as they are all "midpoint grounded". The trick to be used instead is illustrated in the circuit diagram below. Note that only one transformer is actually connected to the mains. All others are fed by one half of their secondaries, while the other half feeds the next in turn. This way, every obit adds only 5kV to the output voltage.
Sparks
Sparks, in contrast to arcs, are defined as single, very short electric discharges across an air gap. They are accompanied by a more or less impressive bang and a flash of light. After the spark, the voltage source must first be recharged before the next event can take place.
Physically, the bang and flash of light are produced when a tube of very hot, ionized air (a so-called plasma) forms and expands rapidly. The ionization is first caused by the acceleration of single random charged particles in a strong electric field, resulting in new ionization when the particles hit air molecules. Once a conductive channel has formed, it is strongly heated by the resulting short-circuit current from the voltage source, which can be very high if a capacitor is involved.
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Plasma Sphere/Globe
The globe typically contains an inert gas or an inert gas mixture at a reduced pressure. Pressures somewhat below atmospheric pressure favor longer sparks, or a less severe high voltage requirement, with little loss of spark brightness or appearance. If the pressure is too low, the sparks will be dim and fuzzy. I would guess that a typical gas pressure may be around 1/10 or 1/20 of an atmosphere.
The central electrode is sometimes surrounded by glass. In this case, the globe usually consists of two concentric bulbs that join at the base. The inner bulb is filled or largely filled with metal, and the space between the bulbs contains the inert gases.
High voltage is applied to an electrode in the center of the globe. This high voltage must be high frequency AC or high frequency pulsating DC in order for any current to get through the glass of the globe and surrounding air by capacitive coupling. Typical voltages are around a few thousand volts for most commercial plasma globes, sometimes around 10,000 volts for some homebrew ones. Typical frequencies are from a few to a few tens of kilohertz.
A continuous oscillation of near or over 100 kilohertz is not recommended unless the current is limited to around a milliamp. Otherwise, if you touch the globe, excessive current may flow and overheat your finger or that spot of the globe.
Most plasma spheres seem to contain xenon, krypton, or a mixture of at least one of these with neon. Xenon and krypton favor more lightning-like sparks rather than fuzzy streamers. Xenon is especially good for this. Xenon and krypton (especially xenon) conduct heat the least and confine heat toward the sparks, which favor any continuously maintained sparks rising upward like the arc in a Jacobs ladder.
However, xenon is particularly expensive. Plasma spheres containing xenon probably have the lowest pressure that is favorable to lightning-like sparks.
Van de Graaff
The Van de Graaff generator is constructed of two Textolite columns, six feet in diameter, each with a 15 foot hollow aluminum sphere at the top. In its original state, rubber conveyer belts ran through each column. Metal comb-like brushes sprayed electrical charge onto the belts which carried the charge from the bottom of the machine to the top, where another set of brushes distributed it on each of the spheres. A tube with a target in it ran between the spheres. One sphere was charged positively, the other negatively, until a discharge between the two occurred, hitting the target in the process. Laboratory equipment in each of the spheres was used to examine and study what occurred as the particles were smashed. Each sphere could be charged to approximately 2.5 million volts, resulting in a 5 million volt discharge.
Lightning
During thunderstorm conditions the turbulence in the cloud causes the charges to separate in such a way that the negative charges concentrate in the base of the cloud. Since like charges repel, some of the negative charges on the ground are pushed down away from the surface, leaving a net positive charge on the surface.
Opposite charges attract, so the positive and negative charges are pulled toward each other. since the negative charges (electrons) are many thousands of times smaller than the positive charges (ions--charged atoms) they move much more easily and cover most of the distance. This first, invisible stroke is called a stepped leader.
As soon as the negative and positive parts of the stepped leader connect there is a conductive path from the cloud to the ground and the negative charges rush down it causing the visible stroke. The channel created by the stepped leader is full of relatively static charge, like a line of cars at a red light. When the two parts join, it is like that light turning green, and just as the cars near the light start moving first, so do the charges near the join. Since it is the fast-moving charges that create the light, the visible stroke actually travels upwards, even though the charges are moving downward!
Automotive Ignition Coils
Ignition coils are used in cars to produce a spark which initiates the combustion of the fuel-air mixture in the cylinders. They can be had cheap at the scrap yard, or expensive from car parts dealers. Depending on manufacturer and type, their properties differ somewhat. In particular, so-called high-power types have a reduced internal resistance, allowing higher primary currents.
An ignition coil, like a transformer, consists of an iron core with a primary and a secondary winding. The turns ratio between secondary and primary is in the order of 100:1. Both windings are connected at one end, so that the secondary is automatically grounded through the primary circuit. Arbitrarily many coils may be paralleled for higher output current, but cascading secondaries is not possible due to the internal connection to the primaries. However, just like with Microwave Oven Transformers (MOTs), two coils may be anti-paralleled on the primary side. Through the internal connection, this automatically puts the secondaries in series, i.e. they produce different polarity output. The maximum voltage difference thus possible is around 60kV, which is already enough to jump about 10cm air gap.
Ignition coils are usually operated on a DC supply, and just like flyback transformers they need a driver circuit. The simplest circuit is shown above. With the switch closed, an increasing DC current flows through the primary, producing a magnetic field inside the iron core, in which energy is stored. The final current is limited by the internal resistance of the coil, usually a few Ohms. When the switch opens, the current is interrupted and the magnetic field collapses, releasing the stored energy in the form of a large voltage pulse (a few hundred volts across the primary winding). This voltage pulse is multiplied the turns ratio, resulting in a peak voltage of around 30kV. The cap across the switch limits the ultimate peak voltage (see also the chapter on flyback transformers) by slowing down the collapse of the magnetic field, turning the singular transient into a damped high frequency oscillation. Without this measure, an arc would form in the switch after opening, possibly damaging the switch and slowing down the collapse even more, resulting in a much reduces output voltage. The additional resistor prevents welding of the switch contacts when it is closed again and the cap discharges through it.
Microwave Oven Transformers (MOTs)
Microwave ovens contain a very powerful high voltage transformer (MOT = microwave oven transformer), see photo. A typical output voltage is 2kVeff, at around 1000W power. This is equivalent to about 0.5Aeff output current @ 2kVeff output voltage. The short-circuit current is even higher.
However, Microwave Oven Transformers (MOTs) are not internally current limited (like Oil Burning Ignition Transformers (OBITs) are). And as an arc is pretty much a short-circuit for the secondary winding, the output current should be externally limited for the purpose of drawing arcs. This can be done by inserting a resistive or inductive load into the primary or secondary circuit, see figure. When using a transformer (e.g. another MOT) as an inductive load, the secondary winding of this transformer may be short-circuited to reduce it's inductive impedance. Without current limiting, chances are good that the mains fuse will blow when drawing an arc. With or without current limiting, the secondary will probably overheat when arcing takes place over long time.
In spite of not being noticeably current limited, most MOTs could draw an arc without the fuse blowing, but the winding heats up within seconds!
When connecting a MOT to the mains (i.e. switching it on), there may be a very high current for some very short time. This may be enough to blow the fuse. This problem can be avoided by using a so-called switch-on current limiting circuit, see figure. Such a circuit is used in all microwave ovens and should be saved as well when cannibalizing such a device. After switch-on, the resistor limits the current to a sensible value for the short time the relay needs to switch.
Correct grounding is important with MOTs. The inner end of the secondary, which is near to the core, should be connected to the iron core. In many MOTs, this is already the case. The reason is that the insulation between core and winding is usually insufficient to withstand the full output voltage. Therefore, like OBITs, MOTs cannot be connected in series to increase output voltage. Only two MOTs can be used for double output voltage, when the cores are connected and the primaries anti-parallel, see figure below. However, in principle arbitrarily many MOTs may be paralleled for higher output current (although usually not more than two can be run on the same mains outlet).
Arcs
Arcs, in contrast to sparks, are continuous discharges. They are initiated as a spark, but then keep burning. The voltage across a burning arc is much lower than the voltage needed to ignite it, typically only a few hundred or thousend volts. Whether an arc forms or not depends on whether the voltage source can deliver enough current after the initial spark. If some minimal current can not be maintained, the arc is quenched (stops burning).
Due to their high power and current capability, Microwave Oven Transformers (MOTs) are well suited for arc experiments. However, run time should be limited to a few seconds to prevent overheating of the transformers. To ignite this kind of arc, it is necessary to bring the electrodes so close together that the relatively low voltage (2kV to 4kV) jumps over the gap, and pull them apart afterwards. This can be done by moving the electrodes via a sufficiently long insulated handle. Alternatively, a piece of wire, mounted on an insulated handle, can be used to bridge the gap and ignite the arc.
Flyback transformers are also well suited to make arcs, though by far not as spectacular as MOTs. On the other hand, they are also less dangerous. It is possible to melt and even burn iron and copper wire by using it as one electrode of the arc. The wire first melts into a brightly glowing droplet at the tip, which, when further heated, starts to oxidize (burn), producing an effect similar to a sparkler.
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