Spark Gap in EMP Generator

I recently got into eletronics, and there are some concepts I am having a hard time understanding. I was looking around youtube and saw this video of a small EMP generator, I understand the basics of it, the magnetic field inducts a current thus frying electronics, but what I'm interested in is why there is a spark gap in the schematics. What does it do ?

• Voltage is induced and this voltage may circulate a current. – Andy aka Jul 13 '17 at 7:38

Presumably the high voltage converter is rectified and put through a multiplier, otherwise this circuit wouldn't work. The capacitors in the multiplier are charged up by the DC and when the voltage becomes high enough to jump the spark gap, all of the energy stored in the capacitor(s) is dumped into the coil, thus producing a powerful EMP. You can think of the setup as building up pressure until it reaches a certain point, and suddenly releases all of that pressure all at once. You can see how that would be much more powerful than simply passing the current through the coil.

• So that means the purpose of the capacitors are to release all the energy in a short time and the spark gap acts as a trigger for when it reaches a certain voltage? – Pedro Jul 13 '17 at 11:55
• Pretty much, yes. – DerStrom8 Jul 13 '17 at 15:06

The spark gap provides abrupt onset of energy movement; depending on how the spark/plasma extinguishes, you may also get abrupt ending of energy movement.

The abrupt changes in energy into the "coil" implement abrupt changes in magnetic fields.

Abrupt changes in magnetic fields are one form of EMP.

• so could the spark gap be replaced with a capacitor? – Pedro Jul 13 '17 at 3:44
• No. The capacitor will pass charges, using its displacement current, for ANY change in voltage. The spark gap has a "holdoff" voltage, nominally 3,000 volts/millimeter depending on air humidity and sharpness of any non-planar metallic edges. A cap does not provide "holdoff" behavior. – analogsystemsrf Jul 13 '17 at 3:49
• Unlike a capacitor where you don't want the electrons to pass from one plate to another, in a spark gap you want that to happen at a known breakdown voltage. (As analogsystemsrf said, 3kv/mm). If you would put a capacitor instead then you would store a lot of charges in one place with a breakdown voltage that is much less than 3kv/mm, and when the electrons finally break through and form a closed circuit the capacitor would start smoking and possibly start burning. A spark gap is essentially two connectors spaced from each other. – Harry Svensson Jul 13 '17 at 4:08