Spark Gaps|Tesla-Coil.com

Spark Gaps are normally two connection devices which arc over at some preset voltage. The voltage is set by choosing the gap width and the gas pressure inside the device. These are set at the time of manufacture and are not field changeable. EG&G has a website describing their range of gaps although some of them are not allowed to be shipped out of the U.S.A. without an export license.

By adding a third electrode to a spark gap one can make triggered spark gap, turning the over voltage spark gap into a triggered switch that behaves much like a thyratron or ignitron. The triggering voltages tend to be quite high (perhaps 25% of the SV Static Breakdown Voltage) but nevertheless they make effective switches and do some jobs better than the other devices.

The best material I've found on these industrial gaps is on the EG&G site. Take a look at Miniature Triggered Gaps, Triggered Gaps, Triggered Vacuum Gaps, Standard Trigger Transformers and Series Trigger Transformers, Spark gaps have one major disadvantage over thyratrons and ignitrons in that each time they fire, some material is used up. Like in an automotive spark plug, the firing action erodes a small amount of material each time. Calculation is needed to determine the energy of each pulse for the estimated life.

The Over Voltage Spark Gap

The Over voltage spark gap is essentially just two electrodes with a gap between. When the voltage between the two electrodes exceeds the breakdown voltage of the gas, the device arcs over and a current is very rapidly established. The voltage at which arcing occurs in these devices is given by the Dynamic Breakdown Voltage, which is the voltage at which the device will breakdown for a fast rising impulse voltage. Note that this voltage may be as much as 1.5 times greater than the static breakdown voltage (breakdown voltage for a slowly rising voltage.) how much greater than the static breakdown voltage the actual breakdown voltage is will be depends almost entirely on how rapidly the voltage rises, a shorter rise time means a higher breakdown voltage. Commutation times for these devices are exceptionally low (sometimes less than 1 nanosecond).

Overvoltage gaps are primarily used for protection. But in combination with the other devices mentioned here they are commonly used to sharpen the output pulses (decrease the rise times) of very high current pulses form triggered switching devices, e.g., Thyratrons.

The size of these devices is almost entirely dependent upon how much current/voltage they are intended to switch, There is really no limit as to the size of these devices they can be as small as krytrons, however they can also be very big, and devices intended to switch MA will be just that.

Triggered spark gaps

The triggered spark gap is a simple device, a high voltage trigger pulse applied to a trigger electrode initiates an arc between anode and cathode. This trigger pulse may be utilized within the device in a variety of ways to initiate the main discharge. Different spark gaps are so designed to employ one particular method to create the main anode to cathode discharge. The different methods areas follows-

Triggered spark gap electrode configurations:

i) Field distortion: three electrodes; employs the point discharge (actually sharp edge) effect in the creation a conducting path

ii) Irradiated: three electrodes; spark source creates an illuminating plasma that excites electrons between the anode and cathode.

iii) Swinging cascade: three electrodes; trigger electrode nearer to one of the main electrodes than the other.

iv) Mid plane: three electrodes; basic triggered spark gap with trigger electrode centrally positioned.

v) Trigatron: trigger to one electrode current forms plasma that spreads to encompass a path between anode and cathode.

The triggered Spark gap may be filled with a wide variety of materials, the most common are, Air, SF6, Argon and Oxygen.

Often a mixture of the above materials is employed. However a few spark gaps actually employ liquid or even solid media fillings. Solid filled devices are often designed for single shot use (they are only used once- then they are destroyed) Some solid filled devices are designed to switch powers of 10TW (10,000,000,000,000 Watts) such as are encountered in extremely powerful capacitor bank discharges. Except (obviously) in the case of solid filled devices, the media is usually pumped through the spark gap. Some smaller gaps do not use this system though.

Usually Gas filled spark gasp operate in the 20-100kV / 20 to 100kA range though much higher power devices are available. I have one spec for a Maxwell gas filled device that can handle 3 MA - that's 3 Million Amperes! But then it is the size of a small car!! More commonly gas filled devices have dimensions of a few inches. Packages are often shaped like large ice pucks though biconical, tubular and box like structures are also seen.

Spark gaps are often designed for use in a certain external environment (e.g., they might be immersed in oil). A system for transmitting the media to the appropriate part of the device may sometimes be included. Common environments used are: Air, SF6 or Oil.

Spark gaps are damaged by repeated heavy discharge. This is an inevitable consequence of such high discharge currents. Electrode pitting being the most common form of damage. Between 1 and 10 thousand shots per device is usually about what is permissible before damage begins to severely degrade performance.

EG&G makes miniature triggered spark gaps specially designed for defense applications. these devices are physically much smaller than normal spark gaps (few cm typical dimensions) and designed for use with exploding foil slapper type detonators.

Laser switching of spark gaps. The fastest way to switch a triggered spark gap is with an intense pulse of Laser light which creates a plasma between the electrodes with extreme rapidity. There have been quite a few designs employing this method, chiefly in the plasma research area.

Triggered spark gaps tend to have longer delay times than Thyratrons (their chief competitor, at least at lower energies) However, once conduction has started, it reaches a peak value exceptionally rapidly (a couple of nanoseconds commutation).