What Is a Gas Discharge Tube (GDT)?
A gas discharge tube (GDT) is an overvoltage-protection element that becomes conductive on high-voltage spikes by ionizing the gas inside it and dumps the excess energy to ground.
How Does It Work?
Normally the tube is an insulator. When the voltage exceeds the threshold, the gas inside ionizes and, behaving almost like a short circuit, sends a large current to ground. Once the surge passes the gas returns to being an insulator.
Its Strong Point
GDTs can dump very high-energy surges (like lightning); this makes them the first protection layer of telecom and power lines. Their capacitance is low, suiting high-frequency lines.
| Element | Speed | Energy |
|---|---|---|
| GDT | Slow | Very high |
| Varistor | Medium | High |
| TVS | Very fast | Low |
Layered Protection
A GDT alone is slow; in practice the GDT dumps the coarse energy, then a varistor and TVS clean the faster but smaller residual surge. This staged structure is called "coordinated protection".
Where Is It Used?
Phone/DSL lines, antenna inputs, energy meters, base stations and outdoor line inputs needing lightning protection.
Common Mistakes
- Using it alone: being slow, it doesn't protect a sensitive circuit fast enough on its own.
- Follow current: a GDT triggered on an AC line can keep drawing current after the surge.
- Wrong threshold: a suitable firing value above the working voltage must be chosen.
Frequently Asked Questions
Why isn't a GDT enough alone?+
Does it withstand lightning?+
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