GDT
A gas-discharge tube provides a normally high-impedance path that switches into conduction after breakdown. In a coordinated protection circuit, it can divert a surge. Its sparkover voltage, current capability and reset behavior are specific to the selected device and test conditions.
After the gas becomes conductive, the voltage across the GDT drops into its arc regime. Bourns uses approximately 10 V as an explanatory example, not a rating for every GDT. The current must fall sufficiently for the device to reset, and sustained follow current can prevent safe extinction. An AC zero crossing alone is not a universal guarantee of turn-off.
In plain terms
Think of a pressure-triggered bypass, but not a magic drain: once it opens, the circuit must limit the flow and allow it to stop. If the source keeps feeding it, the conducting path may not reset safely.
Why designers use it
- A selected GDT can divert a specified surge by switching to a low-voltage conducting state.
- Low off-state leakage can be useful in a suitably designed protection network.
- Bourns describes coordinated GDT/MOV protection. A component surge rating is not a guarantee that equipment survives a direct lightning strike.
Best for
- Lightning protection
- Telecom lines
- Antenna feeds
Key specifications
- Sparkover: DC and impulse ratings differ
- Arc voltage: Lower than sparkover voltage
- Surge current: Check the specified impulse waveform
- Follow current: Must extinguish safely after the surge
When not to use it
- When its sparkover behavior or residual voltage cannot protect the downstream circuit.
- As a stand-alone promise of lightning protection or mains safety. Coordination, installation and applicable requirements must be verified.
Common mistakes
- Putting a GDT directly across an AC supply without resolving follow current and the complete protection design. Bourns warns that a GDT alone is not an adequate MOV replacement on AC mains.
- Comparing surge ratings without the waveform, number of impulses and test conditions.
- Assuming every surge leaves a protective component undamaged or suitable for continued service.
Where you will find it
- Bourns describes hybrid protection that combines GDT and MOV behavior.
- Its example explains why a GDT's low conducting-state voltage limits the power dissipated in that device compared with a higher-voltage clamp carrying the same current. Complete equipment protection still requires coordination and validation.
A short history
Bourns describes GDT and MOV protection as different but complementary approaches. Its hybrid examples coordinate their behavior, including leakage and follow-current considerations. A component-level explanation is not a complete equipment-safety design.