Gas Discharge Tube Overview
Today’s microprocessor based electronic equipment are increasingly more vulnerable to lightning induced voltage surges and electrical switching transients because they have become more sensitive, and complex to protect due to their high chip density, binary logic functions and connection across different networks. These devices are critical to a company’s communications and information processing and typically can have an impact on the bottom line; as such it is prudent to insure them against these potentially costly and disruptive events. A Gas Discharge Tube or GDT can be used as a standalone component or combined with other components to make a multistage protection circuit – the gas tube acts as the high energy handling component.
Protection for PC Board Level Components
GDT’s are typically deployed in the protection of communication and data line DC voltage applications because of its very low capacitance. However, they provide very attractive benefits on the AC power line including no leakage current, high energy handling and better end of life characteristics.
GAS DISCHARGE TUBE TECHNOLOGY
The gas discharge tube may be regarded as a sort of very fast switch
having conductance properties that change very rapidly, when breakdown
occurs, from open-circuit to quasi-short circuit (arc voltage about 20V).
There are accordingly four operating domains in the behavior of a gas
The GDT may be regarded as a very fast acting switch having conducting properties that change very rapidly when breakdown occurs and transforms from an open-circuit to a quasi-short circuit. The result is an arc voltage of about 20V DC. There are four stages of operation before the tube fully switches.
- Non-operating domain: Characterized by practically infinite insulation resistance.
- Glow domain: At the breakdown, the conductance increases suddenly. If the current is drained off by the gas discharge tube is less than about 0.5A (rough value that differs from component to component) , the glow voltage across the terminals will be in the 80-100V range.
- Arc regime: As the current increases, the gas discharge tube shifts from glow voltage to the arc voltage (20V). It is this domain that the gas discharge tube is most effective because the current discharge can reach several thousand amperes without the arc voltage across the terminals increasing.
- Extinction: At a bias voltage roughly equal to the glow voltage, the gas discharge tube covers to its initial insulating properties.
Protecting a two-wire line (for example a telephone pair) with two 2-electrode gas discharge tubes may cause the following problem:
If protected line is subjected to an overvoltage in the common mode, the dispersion of the sparkover voltages (+/- 20%), one of the gas discharge tubes sparks over a very short time before the other (typically a few microseconds), the wire that has the sparkover is therefore grounded (neglecting the arc voltages), turning the common-mode overvoltage into a differential mode overvoltage. This is very dangerous for the protected equipment. The risk disappears when the second gas discharge tube arcs over (a few microseconds later).
The 3-electrode geometry eliminates this drawback. The sparkover of one pole causes a general breakdown of the device almost immediately (a few nanoseconds) because there is only one gas filled enclosure housing all the effected electrodes.