Add gas discharge tubes to your circuit-protection device roster

GDTs are circuit-board-compatible overvoltage-protection devices and are far removed from the “spark gap” of monster movies.

Circuit protection is like insurance: you really don’t need it until you unexpectedly do, and then you’re very glad you have it. This protection falls into two broad categories: protection against overcurrent (the common thermal fuse is the first thought in many cases) and against overvoltage using devices such as metal oxide varistors (MOVs), gas discharge tubes (GDTs), and surge arrestors (Figure 1).

Figure 1. Circuit protection is a multilayered process, with different devices providing protection against different modes and occurrences of overcurrent and overvoltage events. (Image: Bourns, Inc.)

In the big-picture overview, an ideal protection device operates in two dramatically different two modes:

When not activated, it is electrically invisible, adding little or no resistance, capacitance, or inductance to the line it is protecting.

When activated by an overcurrent or overvoltage event, the device quickly transitions to a mode where it attenuates or diverts the excess current or voltage. Depending on the device, it may then revert to its quiescent or watchful state when the fault event is over.

This protection function can be divided into two groups, called crowbars and clamps, to provide protection against different fault conditions. A crowbar puts a short circuit across the output of the supply, thus diverting the supply’s output current to the ground, and so forcing the output voltage to zero (or close to zero) volts. In contrast, a clamp prevents the voltage from exceeding a preset level.

Looking at GDTs

GDTs do not necessarily come to mind when it comes to circuit protection. When did I first encounter these devices?

Many years ago, while a relative “newbie” in a real job in the industry, I was working on the sensor front end for a data-acquisition system. When I asked the project team leader — who was also an experienced mentor for junior staffers such as me—which circuit-protection device I should use on the relatively long and electrically exposed sensor leads, his answer was simple: “Use all of them — including the gas discharge tube!”

At first, I wasn’t sure if he was joking or not. I assumed that these spark-gap GDTs would be large, imposing, and dramatic, like the ones embedded in our collective memories from movie classics such as “Frankenstein” (1931) and which served somehow (it’s not clear how) to help capture and channel energy from lightning (Figure 2).

Figure 2. (left) In the classic 1931 movie *Frankenstein*, Colin Clive (Dr. Frankenstein) and Dwight Frye (Fritz) bring the monster to life using a spark-gap gas-discharge tube to capture and control lightning-sourced electrical power; (right) the monster (Boris Karloff) stands next to the discharge device which provided the “spark” for his life. (Images: The Guardian-UK, IMDB)

Spark gaps — a precursor of GDTs — were used by early experimenters before the concept of electronics as we know it today existed, and even basic electricity was a barely understood phenomenon just being investigated by scientists and experimenters. These include Benjamin Franklin and his kite experiment (1752), Humphrey Davy’s use of spark arcing for electrolysis to decompose substances to their elements (early 1800s), and Michael Faraday’s work (also early 1880s).

I wondered if these spark-based GDTs I was told to look at were actually the buggy whips of circuit protection, still in use but only for ancient or replacement situations. I knew that spark-gap devices were still used on high-voltage/current grid power installations — but did anyone actually still use them, with that “glowing tube” appearance, in new designs or on circuit boards?

However, after doing a little research, I found that GDTs were still very viable for new applications, despite their age and the antique status of their basic operating principle. They have continuously evolved and been modernized to meet the needs of today’s circuits and systems.

GDTs versus spark plugs

While GDTs are fully enclosed and used to crowbar an overvoltage to ground, the spark plug of an internal combustion engine uses an exposed flashover of 10- to 15-kV to ignite the gasoline-air mixture in the engine. There is a big difference between these sparks confirmed within a GDT and a spark used to ignite gasoline in internal combustion engines.

For the circuit-protection operation, the GDT sparks are the result of random and unpredictable overvoltage events against which protection is needed. In contrast, for the spark plug function, the sparks are deliberately triggered with precise timing and are used to initiate a mini-explosion in the cylinder to drive the piston. The GDT function is an excellent example of engineers using a basic physics principle but in very different and even somewhat contradictory applications.

Today’s GDTs are almost indistinguishable from other tiny components on a board, yet can provide wide-ranging protection. They are offered in two-electrode versions there are also three-electrode versions) and are easily placed between a line or conductor to be protected (usually an AC power line, I/O port, or other “exposed” conductor) and system ground.

GDT operation

DTs provide near-ideal functionality by diverting overvoltages to the ground. In normal operating conditions, the gas inside the device acts like an insulator, and the GDT does not conduct current; in fact, it is as close to “invisible” to the circuit as a non-ideal component can be, with a multi-gigaohm impedance when not activated and just a few picofarads (pF) of parasitic capacitance. The gas is usually a noble gas such as neon, argon, or xenon.

However, when the voltage across the terminals exceeds the device’s sparkover voltage, the gas in the GDT becomes fully ionized and no longer functions as an insulator. Instead, conduction across the device terminals occurs within a fraction of a microsecond (Figure 3).

Figure 3. When the overvoltage limit is exceeded, the GDT gas ionizes, and the device goes from a near-infinite impedance to a highly conductive path and does so in under a microsecond. (Image: Bourns, Inc.)

When the surge event subsides and the system voltage returns to normal levels, the GDT will return to its high-impedance (off) state. As an added benefit, GDTs are non-polarized (bi-directional), nor do they wear out with repeated sparks over events, unlike some other voltage-protection devices.

The GDT’s crowbar effect effectively limits the overvoltage to a low level and shunts the associated current flow or surge away from downstream components and circuitry. GDTs are bidirectional and nonpolarized, which enhances their applicability in real-world scenarios.

The usual nominal figure cited for spark over voltage in dry air is 30 kilovolts/centimeter. By adjusting electrode spacing, shape, and other parameters, GDTs can be constructed with flashover voltages ranging from under 100 volts to thousands of volts.

GDT improvements continue

For example, the recently introduced GDT28H Series next-generation of high-current GDTs from Bourns, Inc. provides significant response time and other improvements in protection from voltage transients caused by lightning and other AC power-line disturbances. Their high surge-current handling rating provides an enhanced level of voltage limiting during fast-rising events while maintaining a compact size.

These two-electrode high-voltage gas discharge tubes offer high insulation resistance over a wide temperature range and are available over a DC sparkover-voltage range of 1 kilovolt (kV) to 3.3 kV, with a 5-kiloamp (kA) surge-current rate. Unlike the dramatic spark gap of movies, these GDTs are fully enclosed devices, and all family members are housed in an 8 × 6 millimeter (diameter × length) through-hole, the axial-leaded cylindrical package having a near-trivial capacitance of under 1.5 pF (Figure 4).

Figure 4. (left) The schematic symbol of a two-electrode GDT with the “dot” indicating the device is filled with some sort of gas; (right) the GDT28H is a modern series of GDT devices in small, cylindrical housings. (Image: Bourns, Inc.)

Among the targeted applications are power supplies, lighting, HVAC, and products which must adhere to the mandated safety standard IEC 62368-1:2018. This widely used standard applies to electrical and electronic equipment in audio, video, information, and communication technology, and business and office machines with a rated voltage not exceeding 600 V.

The UL-approved GDT28H series is especially suitable for use in AC isolation situations. It achieves this performance through a combination of its extended operating voltage range, high insulation resistance, and heightened surge rating. All members of this family feature a 5-kA nominal 8/20 microsecond (µsec) impulse discharge rating. The impulse spark over voltage is 2500 V (maximum) at 100 V/µsec and 2750 V at 1 kV/µsec.

The gas-discharge tube is a dramatic reminder that some classic devices that rely on fundamental principles do not necessarily become obsolete. They can be re-invented, improved, and fully characterized to effectively serve in critical roles for modern systems and their physical requirements. They are available from many vendors, including Bourns, Littelfuse, Pulse Electronics, TDK, and TE Connectivity, among others.