Spark gap

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Spark gaps of a marx generator firing.

A spark gap is two (or sometimes more) electrodes that are designed to have a spark jump between them. The spark gap is a very useful component in high voltage circuits, with many applications. This article summarises the main applications including Tesla coil power conversion.

Contents

Spark gaps for voltage measurement

The breakdown voltage of a spark gap (that is the voltage at which the spark jumps) is affected by many things. Perhaps the most important factors are:

  1. Distance between the electrodes (See Paschen's law)
  2. Shape of the electrodes (sphere, point, plane, rod, etc)
  3. What gas they are in (and the temperature/pressure of it)
  4. Whether the voltage is DC, AC, or pulsed.

If you know these factors, then you can estimate the voltage. This is a cheap and easy way to measure the output from home-made HV generators. Tables are available for various electrode shapes, but as a rough guideline, the breakdown voltage between two electrodes in air at standard temperature and pressure, with DC voltage, is about 30 kV per cm.

Spark gaps for protection

Standard Safety Gap

You might well have guessed from the above that you can design a spark gap to break down at a given voltage. Once it has broken down, it acts as a short circuit. So, spark gaps can be used to protect HV components that could be damaged by overvoltage. You will see them used for this purpose on overhead power lines, inside TV sets, and in Tesla coil primary circuits, amongst other places. When used in a Tesla coil it is called the safety gap.

A spark gap connected directly across a high voltage capacitor can often cause more problems than it solves. When the gap breaks down it shorts the capacitor causing an extremely high current. This is why a Tesla coil safety gap should always be connected across the main gap, and never across the capacitor.

In the rendering to the right, the center electrode is connected to ground, each opposing electrode is connected to the device to be protected. This way, if either phase rises higher than it was designed, the overvoltage shorts to ground and not across something more important-- for example, your dielectric!

Spark gaps for high frequency power conversion

When a spark gap is connected into a LC circuit (capacitor and inductor in series) and provision is made for charging the capacitor, strange and wonderful things happen. The capacitor charges until the voltage is high enough to breakdown the spark gap. Then the circuit oscillates at its natural resonant frequency until all the charge has been spent. The spark gap then quenches (see Quenching), the capacitor recharges, and the cycle repeats.

This was discovered around the turn of the century by Nikola Tesla and was in fact the world's first switched-mode power supply. The high frequency output from it can be used for all sorts of fun things, such as induction heating, or if you add a secondary coil that resonates at the same frequency, you have a Spark Gap Tesla Coil

The main problem in a HF oscillator spark gap is to keep the electrodes cool when hundreds to thousands of high-current discharges are happening every second. If the electrodes get too hot, the gap will fail to quench and the system will stop oscillating (and usually die a flaming death). There are a number of different kinds of spark gaps optimised for use in HF oscillators, and the rest of this article will deal with these. They have largely been made obsolete by solid-state oscillators based on the MOSFET and IGBT, but are still important in amateur Tesla coil building.


Static gap

Any spark gap in which the electrodes are immobile. When the voltage difference between them is great enough, the gas between them is ionized, breaks down, and the spark gap fires; essentially becoming a short. When the voltage becomes low enough that it cannot sustain the plasma, the gap shuts off; essentially an open circuit.

To set your gap for an NST coil, connect only the gap to the NST and set it so that it just arcs across. Your gap is now set, you may make the gap smaller, but to prevent damage to the NST, do not widen the gap any more than this initial setting.

Static Gap

Multiple series gap

A spark gap made from several smaller gaps in series. Has improved quenching properties since several small arcs are easier to cool than one large arc. Multiple gaps are often cooled by having large electrodes with heatsink fins, or by blasting air through the gaps (see above)

Series Gap

Air-blast gap

A spark gap where the electrodes are cooled, and ionized gases swept away, by a blast of high pressure air. A decent air blast gap can be made from brass plumbing fittings, PVC pipe, and a vacuum cleaner motor/fan assembly. A simplified version would be the static or series gap shown above, with air blown across the gap(s).

Sucker Gap

Same as the air blast, but air is drawn, rather than blown across the gap. This gap is generally used with a shop vac. There is no difference in performance between the sucker and the blast gap.

Rotary gap

A spark gap where the discharge passes between stationary electrodes, and "flying" electrodes mounted on a wheel that spins at high speed. It has some important advantages over a static gap: The break rate (number of discharges per second) is set accurately by the speed of the wheel, and the spinning causes a strong draught of air that keeps the electrodes cool.

Rotary Gap

Rotary gaps can be synchronous or asynchronous. The synchronous gap has the rotation of the wheel synchronised with the frequency of the power supply. This ensures that the capacitor will be charged to the same voltage every time, when you charge it from an AC supply. The synchronisation is achieved by driving the break wheel directly from a synchronous motor.

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