Gas Discharge Tubes (GDTs) (a.k.a. Gas Tube Arrestors (GTAs) or Surge Arrestors (SAs)) are transient voltage/current spike protection components used in electronic circuits.
2. Schematic Symbol and Designator
My preferred schematic symbol for 2 and 3 electrode GDTs are shown below:
|You may assume the dot in the schematic symbol indicates a polarity. But most gas discharge tubes are polarity insensitive, and the dot signifies that the device is gas filled (usually at some pressure lower than 1 atmosphere).|
I have noticed the following designators used for gas discharge tubes:
GDTx(Gas Discharge Tube)
3. How They Work
3.1. 2-Electrode Version
A GDT contains two conductive electrodes separated by a controlled gas (or gas mixture) and a specific pressure, usually less than atmospheric. GDTs are normally connected across voltage rails on the inputs to circuitry. Under normal operating conditions at "low" voltages, the gas acts as a almost perfect insulator and (almost) no current flows through the GDT.
When a voltage transient begins to appear, the voltage on the rail will start rising. At a certain voltage potential across the GDT, the electric field between the two electrodes becomes strong enough to ionize the gas. This makes the gas conductive, and a current begins to flow between the electrodes. This ionization causes an avalanche effect, once localized ionization occurs in the very close proximity to the electrode tips, much of the surrounding gas becomes ionized too[bib-bourns-gdt-white-paper]. At this point, the GDTs resistance rapidly falls, and hence so does the voltage, acting as a "crowbar" and protecting down-stream circuitry from excessive voltages.
The GDT remains in a low-resistance conductive state until the voltage drops below the extinction point, which is typically much lower than the sparkover voltage (you can think of this as a type of hysteresis).
3.2. 3-Electrode Version
A 3-electrode version works similarly to a 2-electrode version, except it offers protection for two circuits to a common ground at once. If any circuit triggers ionization, both circuits will be protected quickly and at the same time (as opposed to two separate 2-electrode GDTs, which are not guaranteed to trigger at the same time). This is important for transformers.
4. Important Parameters
4.1. Sparkover Voltage
The sparkover voltage (a.k.a. DC sparkover voltage) is the primary characteristic defining a GDT. It is the voltage at which ionization occurs and the GDT begins to conduct under the presence of a slowly increasing voltage (key term here being slowly increasing voltage, if the voltage is a fast transient, you have to look at the Section 4.2 instead). The sparkover voltage is dependent on the electrode spacing, electrode geometry, the gas composition and gas pressure. It is typically between 70V-1kV, with 75V, 90V, 150V, 200V, 230V, 250V, 300V, 350V, 400V, 500V, 600V, 1000V, 3000V being popular values. The rate a which the voltage is increasing for this specification is usually 100V/s.
4.2. Impulse Sparkover Voltage
The impulse sparkover voltage is like the sparkover voltage, except that it’s defined as the voltage at which ionization occurs when the voltage is increasing rapidly (i.e. an impulse). Normally, a voltage for a 100V/us and 1000V/us transient is given. The impulse sparkover voltage can be hundred of volts higher than the sparkover voltage, as table below shows:
DC Sparkover @ 100V/s
Impulse Sparkover, 100V/us
Impulse Sparkover, 1000V/us
4.3. Impulse Discharge Current
The impulse discharge current is the amount of current the GDT can withstand for a short amount of time under a fault condition. GDTs typically have the highest impulse discharge current rating out of all transient voltage suppressor components (which includes TVS diodes, Zener diodes, e.t.c). Typically rated as per the 8/20us specification. A GDT will normally have a impulse discharge current between \(1-50kA\).
4.4. Number of Electrodes
The number of electrodes (a.k.a. number of poles) is the number of conductive electrodes entering the gas tube, and hence typically also the number of terminals available on the component for electrical connection. GDTs commonly have two or three electrodes.
Most GDTs have a capacitance less than 2pF.
The Bourns 2031-xxT-SM family is a range of miniature SMD GDTs. There are 3 devices in the family, with initial DC sparkover voltages of 150V, 230V and 420V respectively.
DigiKey Gas Discharge Tube Arrestors section: https://www.digikey.com/en/products/filter/circuit-protection/142
RS Components Gas Discharge Tubes section (NZ region): https://nz.rs-online.com/web/c/passive-components/surge-protection-components/gas-discharge-tubes/
[bib-bourns-gdt-white-paper] Tim Ardley (2008). First Principles of a Gas Discharge Tube (GDT) Primary Protector, Rev. 2. Retrieved 2021-07-02, from https://www.mouser.com/pdfdocs/bourns_gdt_white_paper.pdf
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