Common Types of Gas-Discharge Tubes

Lightning storms occur as a result of electrical hysteresis, resulting from the action of strong wind and rain that generates a significant amount of static electric charge between clouds and the earth below. These imbalances begin to manifest as high voltages, and once they can no longer be held by the electrical resistance of air, a major surge of current will occur between poles in the form of a lightning strike. What keeps lightning from occurring during normal weather conditions is the resistance of air, as in a gaseous state acts as an insulator. The overall action of lightning is highly comparable to the operations of a relaxation oscillator, that of which is an electronic circuit that works on the principle of a charging capacitor that is discharged each time the voltage reaches a critical threshold value.

In their simplest form, relaxation oscillators consist of three major components, those of which are a resistor, capacitor, and a gas-discharge tube in the form of a neon lamp. The neon lamp is just two metal electrodes that are placed within a sealed glass bulb and separated by neon gas, and they exhibit near infinite resistance when there is no induced voltage during room temperature conditions. However, upon a certain threshold voltage being exceeded, the neon gas will ionize, turning into a plasma with a significantly lower level of resistance. As such, the neon lamp acts like atmospheric air during a lightning storm, even emitting light when the discharge occurs.

Capacitors are designed to store electrical energy, and those within relaxation oscillators charge at an inverse exponential rate that can be determined based on the resistor’s size. As the threshold voltage is surpassed and the light is turned on, the capacitor will rapidly be discharged until a low voltage value is attained. At this point, the lamp will turn off, and the capacitor will begin building up a charge to repeat the cycle. The overall cycle of lamp flashing comes down to attributes such as the resistance of the resistor, battery voltage levels, the capacitance capability of the capacitor, and the threshold voltage of the lamp.

Gas-discharge tubes and lamps are quite popular when used as a source of illumination, though more sophisticated variants known as thyratron tubes are also available. Basically, thyratron tubes are a gas-filled triode tube where activation requires a small control voltage applied between a grid and cathode while deactivation requires reducing the plate-to-cathode voltage. These devices are quite similar to neon lamps, though they are more controlled versions that serve for switching a current to a load. With AC voltage provided as the power source for the load, current will periodically halt between half-cycles. This allows the gas within the tube to cool so that it can return to an “off” state. Then, conduction will occur once again when enough voltage is supplied by the AC power source. With their basic capabilities, such gas-discharge tubes have found use in some modern semiconductor components and demonstration circuits.

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