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I am putting a GDT in parallel with TVS diode, and I want to know if I need to have clamping voltage of TVS diode slightly higher than DC breakover voltage of the gas discharge tube OR have clamping voltage and DC breakover voltage the same.

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A GDT is slow compared to a TVS hence the TVS should be chosen to activate first when the surge is rapid. That’s the aim and to achieve that, you must have a sufficiently high clamping voltage across the TVS so that the GDT will (eventually) trigger a few micro seconds later. This means that the TVS clamping voltage must be greater than the GDTs slow spark over voltage. By how much is down to the designer.

If the surge is slow, then the GDT will activate before the TVS and, under these circumstances, the TVS will never be called into action because the GDT will have sparked over and fold-back limited the surge to a few tens of volts. The TVS is there to handle the fast edge and then the GDT takes over to handle the bulk of the surge energy.

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  • \$\begingroup\$ "TVS should be chosen to activate first when the surge is rapid. That’s the aim and to achieve that, you must have a sufficiently high clamping voltage across the TVS so that the GDT will (eventually) trigger a few micro seconds later." From your statement, it seems that the TVS diode should have higher clamping voltage than Impulse Spark Over Voltage. You then mention that the "TVS clamping voltage must be greater than the GDTs slow sparkover voltage" Slow sparkover voltage is DC spark over. Confused on if the TVS clamping voltage be higher than DC Sparkover or Impulse Sparkover Voltage. \$\endgroup\$
    – Kasap23
    Nov 17, 2019 at 0:26
  • \$\begingroup\$ @Kasap23 The TVS should activate above the DC spark over voltage else there’s no point having a GTD. And, it must clamp at a voltage level that protects the circuitry that follows. \$\endgroup\$
    – Andy aka
    Nov 18, 2019 at 1:20
  • \$\begingroup\$ Thanks for responding. If we are worried about a slow surge then I believe that I can look at the DC Sparkover voltage but what if there is a fast waveform around 8microsecond. Would the DC Sparkover voltage still apply? I would need to focus on Impulse Sparkover voltage since it is a fast waveform at 8 microsecond. I understand that I need to clamp at a voltage level that protects the circuitry that follows. Please tell me if my logic makes sense. \$\endgroup\$
    – Kasap23
    Nov 18, 2019 at 1:33
  • \$\begingroup\$ You are over thinking things. The TVS handles the surge front then the GDT always takes over to handle the bulk of the remainder of the surge after a period of time that is equal to or less than a time period defined by the slow sparkover impulse dv by dt. \$\endgroup\$
    – Andy aka
    Nov 19, 2019 at 19:00
  • \$\begingroup\$ @Kasap23 are we done with this Q and A now? \$\endgroup\$
    – Andy aka
    Jun 2, 2020 at 9:15

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