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IGNITRON

Ignitron

An ignitron is a type of gas-filled tube used as a controlled rectifier and dating from the 1930s. Invented by Joseph Slepian while employed by Westinghouse, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron". Ignitrons are closely related to mercury-arc valves but differ in the way the arc is ignited.

ignitron, electron tube functioning as a rectifier to convert alternating current (AC) to direct current (DC). Each conduction cycle is started by an external voltage applied to the igniter, a small electrode touching the tube’s cathode, which is a pool of mercury. Electrons released by the igniter from the surface of the mercury initiate a conducting arc through the tube. The arc lasts until voltage on the ignitron’s plate has been reduced to the point that the arc can no longer be sustained.

Large ignitron devices may be built inside vacuum tanks instead of tube envelopes. Ignitrons are very limited with respect to their physical orientation. Because of the pool of mercury, the device cannot lean more than two or three degrees from the vertical.

Construction and operation

An ignitron is usually a large steel container with a pool of mercury in the bottom that acts as a cathode during operation. A large graphiteor refractory metal cylinder, held above the pool by an insulated electrical connection, serves as the anode. An igniting electrode (called the ignitor), made of a refractory semiconductor material such as silicon carbide, is briefly pulsed with a high current to create a puff of electrically conductive mercury plasma. The plasma rapidly bridges the space between the mercury pool and the anode, permitting heavy conduction between the main electrodes. At the surface of the mercury, heating by the resulting arc liberates large numbers of electrons which help to maintain the mercury arc. The mercury surface thus serves as the cathode, and current is normally only in one direction. Once ignited, an ignitron will continue to pass current until either the current is externally interrupted or the voltage applied between cathode and anode is reversed.

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Fig.1: Ignitron (1) Anode, (2) Cathode, (3) Ignitor, (4) Mercury, (5) Ceramic insulators, (6) Cooling fluid

Applications

Ignitrons are used where power control of high voltages or currents is required. Electrical welding equipment incorporating an ignitron as a heavy-duty relay is probably the most common application. Ignitrons were long used as high-current rectifiers in major industrial and utility installations where thousands of amperes of AC current must be converted to DC, such as aluminum smelters. Ignitrons were used to control the current in electric welding machines. Large electric motors were also controlled by ignitrons used in gated fashion, in a manner similar to modern semiconductor devices such as silicon controlled rectifiers and triacs. Many electric locomotives used them in conjunction with transformers to convert high voltage AC from the overhead lines to relatively low voltage DC for the traction motors. The Pennsylvania Railroad's famous GG1 ( or for example Russian ВЛ-60 freight locomotive ) passenger locomotive plus its freight locomotives carried on-board ignitrons. For many modern applications, ignitrons have been replaced by solid state alternatives.

Because they are far more resistant to damage due to over current or back-voltage, ignitrons are still manufactured and used in preference to semiconductors in some installations. For example, specially constructed "pulse rated" ignitrons are still used in certain pulsed power applications. These devices can switch hundreds of kilo-amperes and hold off as much as 50kV. The anodes in these devices are often fabricated from a refractory metal, usually molybdenum, to handle reverse current during ringing (or oscillatory) discharges without damage. Pulse rated ignitrons usually operate at very low duty cycles.

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They are often used to switch high energy capacitor banks during electromagnetic forming, electrohydraulic forming, or for emergency short-circuiting of high voltage power sources ("crowbar" switching).

Comparison with mercury-arc valve

Although the basic principles of how the arc is formed, along with many aspects of construction, are very similar to other types of mercury-arc valves, ignitrons differ from other mercury-arc valves in that the arc is ignited each time a conduction cycle is started, and then extinguished when the current falls below a critical threshold. In other types of mercury-arc valve, the arc is ignited just once when the valve is first energised, and thereafter remains permanently established, alternating between the main anode(s) and a low-power auxiliary anode or keep-alive circuit. Moreover, control grids are required in order to adjust the timing of the start of conduction. The action of igniting the arc at a controlled time, each cycle, allows the ignitron to dispense with the auxiliary anode and control grids required by other mercury-arc valves. However, a disadvantage is that the ignition electrode must be positioned very accurately, just barely touching the surface of the mercury pool, which means that ignitrons must be installed very accurately within a few degrees of an upright position.

Welding Ignitron:

The ignitron used as a fast-acting switch for the transmission of one or more current pulses of defined magnitude in type of welding known as resistance welding is the simplest mercury tube. Apart from the mercury cathode and a graphite anode it contains a third electrode, the semiconducting ignition rod. The cathode spot is formed by an ignition pulse at the start of each period, and moves over the surface of the mercury starting from the line of contact between the mercury surface and the ignition rod. The spot moves 1-2cm in 1/100 second (half a period). This means that spot never gets the chance to reach the wall of the tube, since it only moves for half a period at most and ignition rod is situated at least e.g. 3 cm from the wall. It is clear that it is undesirable for the discharge to reach the junction between the mercury pool and the wall of the tube. Ignitron tubes operate on the principle of ionization of mercury vapour.

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Fig.2: Construction of a ignitron for welding purposes

Operation of Welding Ignitron:

The length of time the welding current flows through the two pieces of metal to be welded is so important. Therefore, the device used to turn the current on and off is the critical part of the system. A relay or hand operated switch might be considered as a switching device, but either would be unsuitable because of the relatively slow speed of operation.Therefore, some electronic device with no moving parts should be used. Two such devices are available. The ignitron tube, which has been used for many years, is one, and the SCR, a more recent development is another. Both operate by virtue of fact that a small electrical signal applied to the device allows it to turn on in a small fraction of a second and conduct a large amount of current. Removing the electrical signal allows the device to turn off again. Fast turn-on and turn-off are possible because there are no mechanical moving parts. Large ignitron devices may be built inside vacuum tanks instead of tube envelopes. Ignitrons are very limited with respect to their physical orientation. Because of the pool of mercury, the device cannot lean more than two or three degrees from the vertical. Ignitrons are used where power control of high voltages or currents is required.

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Importance of Ignitron:

 Ignitrons are ignitor-fired mercury pool rectifiers with very high peak and average current handling capability. Low frequency ignitrons were the backbone of the welder control and electroplating industries for years. These low-cost devices are still in use today in large motor speed controls and many welder panels around the world. Richardson Electronics offers the following Ignitrons and Accessories: Pulse Power Ignitrons National Electronics' high voltage switching ignitrons are used in a wide variety of pulse power applications, such as capacitor discharge, laser switching, magneforming, magnetizing and crowbar circuits. They are used in switching service for currents ranging to 700,000 Amps with voltages ranging to 50 kV. Solid State Replacements Several "Welder" type ignitrons can be replaced with SSRIs (Solid State Replacement Ignitrons) designed to exact specifications and replace a pair of low frequency resistance welder-style ignitrons. Richardson Electronics may be able to offer an SSRI for your application. Ignitron Connectors & Accessories Ignitrons are unique devices and sometimes require ancillary parts for connecting or mounting.

Ignitron Symbol:

References:

http://www.britannica.com/EBchecked/topic/282374/ignitron

http://en.wikipedia.org/wiki/Ignitron

http://ignitron.askdefine.com/

http://www.rell.com/products/Ignitron/Ignitron.html

http://www.electronic-symbols.com/index.htm?url=/pictures_electronic_components/vacuum_tubes_img2.htm

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