Application Notes


Selection Guide

Common Spark Gap Terms

Frequently Asked Questions (FAQ)

Frequently Asked Terms (FAQ)

  1. How is the DC breakdown voltage measured?

  2. Any adjustable voltage supply, with adequate voltage output, can be used for DC breakdown voltage testing. For more information, please refer to HED Application Note, How to Test Spark Gaps.

  3. How can I measure the pulse breakdown of a gap?

  4. The pulse breakdown of a gap is the voltage level at which the gap fires after having been dormant for at least two hours when a pulsed voltage is applied with gradually increasing amplitude. The pulse breakdown can be as much as ten times the DC breakdown for a sufficiently fast rising pulse. For more information, please refer to HED Application Note, How to Test Spark Gaps.

  5. Why is it necessary to add a radioisotope to the spark gap and is it a hazard to personnel?

  6. The radioisotope, which is added to the gap, induces a constant level of residual ionization. This background ionization reduces the statistical time lag of the avalanche buildup thus reducing the voltage variation from one breakdown to the next. The isotope used by HED is Nickel 63, which emits a low energy Beta particle (0.067MEV), which cannot penetrate the glass or ceramic envelope. Therefore it is not a hazard to personnel, as long as the gap remains intact.

  7. What is the Paschen Law that applies to spark gaps?

  8. Paschen’s law states that the DC breakdown voltage of a spark gap does not change if the product of the fill gas pressure and the gap spacing remain constant. For example, if the fill pressure is reduced by one–half and the gap spacing is increased twice as large, the DC breakdown will remain unchanged.

  9. What is meant by the “time lag” in spark gaps and why does it occur?

  10. The time which elapses between the instant of application of a pulse voltage to a spark gap and the occurrence of breakdown is known as time lag and consists of two parts: statistical time and formative time.

    The statistical time lag is caused by the need for electrons to appear in the gap during application of the voltage in order to initiate the discharge. The charged carriers of electric current originate from the cathode by photo emission and within the fill-gas by collision of gas particles with cosmic rays or radioactive decay particles when an isotope is used in the protector design. Higher levels of activity will result in lower pulse voltages.

    The formative time lag is the time required, once the discharge has been initiated for breakdown to occur. The formative time is approximated as the transit time for an ionized gas molecule to travel from anode to cathode. For more information, please refer to HED Application Note, BX Miniature Spark Gaps Description and Use.

  11. What is Corona Discharge and is it harmful to spark gaps?

  12. Corona is an electrical discharge that may occur at the anode of high voltage spark gaps when the electric field is high enough to form a conductive region, but not high enough to cause a breakdown of the gas in the spark gap. Corona discharge will originate at sharp points on the anode where the electric field is very high. The occurrence of the corona on high voltage spark gaps may be controlled by the design engineer by designing electrodes with smooth surfaces to limit the fast transients to low values of impulse ratio.

  13. What is meant by Impulse Ratio?

  14. For test purposes, when a single pulse having very fast rise time is applied to a spark gap, the conditions are entirely different from other modes of pulsing, such as high frequency repetitive pulsing. For a single pulse, the breakdown voltage may be considerably above the static breakdown voltage because of the finite time required to produce the electrons and establish the ionization process. The ratio of the peak voltage of the single pulse for breakdown to the static breakdown voltage of the spark gap is referred to as the impulse ratio.

  15. In a triggered spark gap, why is there a time delay in the breakdown of the two main electrodes and why does it occur?

  16. The time delay is the time required for the main discharge to occur after the application of the trigger pulse. The time is measured from 10% of the leading edge of the trigger pulse to 10% of the leading edge of the main gap discharge. The reason for the delay is that it takes time for the electrons and ions created by the trigger spark to be dispersed between the main electrodes to allow cumulative ionization to occur with the resultant breakdown of the gap. The time delay decreases rapidly when higher voltages are applied. For more information, please refer to HED Application Note, TX Miniature Triggered Spark Gaps Description and Use.

  17. What is the best mode of operation of a triggered spark gap when the shortest delay time and lowest jitter are the most important characteristics?

  18. The delay times are much shorter with the least amount of jitter for Mode A as compared with Mode C when the trigger spark forms to the opposite electrode than when it forms to the adjacent electrode for the same applied voltage. For more information, please refer to HED Application Note, Use of Triggered Spark Gaps.