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A spark gap consists of an arrangement of two conducting separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductors exceeds the breakdown voltage of the gas within the gap, a electric spark forms, Ionization the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current". This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the of ionized gas. Usually, the action of ionizing the gas is violent and disruptive, often leading to sound (ranging from a snap for a spark plug to thunder for a lightning discharge), light, and heat.
Spark gaps were used historically in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, and . Their most widespread use today is in to ignite the fuel in internal combustion engines, but they are also used in lightning arresters and other devices to protect electrical equipment from high-voltage transients.
The Space Shuttle Main Engine hydrogen oxygen propellant mixture was ignited with a spark igniter.
Smaller spark gaps are often used to protect sensitive electrical or electronic equipment from high-voltage overvoltage. In sophisticated versions of these devices (called gas tube arresters),[1] Transient Protection Products, Gas Tubes, Hybrid Protection Systems | rated up to 20,000 amps ... sealed from dust and moisture ... some have small amount of amounts of Tritium Backfill Gas (10 microcuries) | Reynolds Industries Incorporated a small spark gap breaks down during an abnormal voltage surge, safely shunting the surge to ground and thereby protecting the equipment. These devices are commonly used for telephone lines as they enter a building; the spark gaps help protect the building and internal telephone circuits from the effects of lightning strikes. Less sophisticated (and much less expensive) spark gaps are made using modified ceramic capacitors; in these devices, the spark gap is simply an air gap between the two lead wires that connect the capacitor to the circuit. A voltage surge causes a spark that jumps from lead wire to lead wire across the gap left by the sawing process. These low-cost devices are often used to prevent damaging arcs between the elements of the electron gun(s) within a cathode-ray tube (CRT).
Small spark gaps are very common in telephone switchboards, as the long phone cables are very susceptible to induced surges from lightning strikes. Larger spark gaps are used to protect power lines.
Spark gaps are sometimes implemented on Printed Circuit Boards in electronics products using two closely spaced exposed PCB traces. This is an effectively zero cost method of adding crude over-voltage protection to electronics products.
and are the solid-state alternatives to spark gaps for lower-power applications. Neon lamp are also used for this purpose.
When a spark gap consists of only two electrodes separated by gas, the transition between the non-conducting and conducting states is governed by Paschen's law. At typical pressure and electrode distance combinations, Paschen's law says that Townsend discharge will fill the gap between the electrodes with conductive plasma whenever the ratio of the electric field strength to the pressure exceeds a constant value determined by the composition of the gas. The speed with which pressure can be reduced is limited by choked flow, while increasing the electric field in a capacitor discharge circuit is limited by the capacitance in the circuit and the current available for charging the capacitance. These limitations on the speed with which discharge may be initiated mean that spark gaps with two electrodes typically have high jitter.
Triggered spark gaps are a class of devices with some additional means of triggering to achieve low jitter. Most commonly, this is a third electrode, as in a trigatron. The voltage of the trigger electrode can be changed quickly because the capacitance between it and the other electrodes is small. In a triggered spark gap, gas pressure is optimized to minimize jitter while also avoiding unintentional triggering. Triggered spark gaps are made in permanently sealed versions with limited voltage range and in user-pressurized versions with voltage range proportional to the available pressure range. Triggered spark gaps share many similarities with other such as , , , and .
Triggered vacuum gaps, or sprytrons, resemble triggered spark gaps both in appearance and construction but rely on a different operating principle. A triggered vacuum gap consists of three electrodes in an airtight glass or ceramic envelope that has been evacuated. This means that, unlike a triggered spark gap, a triggered vacuum gap operates in the parameter space to the left of the Paschen minimum where breakdown is promoted by increasing pressure. Current between the electrodes is limited to a small value by field emission in the non-conducting state. Breakdown is initiated by rapidly evaporating material from a trigger electrode or an adjacent resistive coating. Once the vacuum arc is initiated, a triggered vacuum gap is filled with conductive plasma as in any other spark gap. A triggered vacuum gap has a larger operating voltage range than a sealed triggered spark gap because Paschen curves are much steeper to the left of the Paschen minimum than at higher pressures. Triggered vacuum gaps are also rad hard because in the non-conducting state they do not contain any gas that could be ionized by radiation.
(1990). 9781489921321, Springer Science+Business Media, LLC. ISBN 9781489921321
In this use the spark gap mechanism is often used in conjunction with a bait, such as a light, to attract the insect into the spark gap.
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