The mosquito zapper from hell: why does it arc at different places instead of at the shortest gap?

Question

A few years ago, I modified a mosquito zapper for fun, removing the original transformer and making the zapper more 'interesting'. I just found it again and I had an old question I wanted answered:

Here is the mod:

Here is a video of it in action:

https://i.stack.imgur.com/Gj7bs.jpg

What I do not understand is the arcing pattern:

The voltage is high enough to have the zapper arc by itself without having a mosquito closing the gap, but why isn't the arcing happening at the place where the two parts of the frame would be the closest to each other?

The arc is jumping from one place to another in what seems like a random pattern.

Answer 1

What I do not understand is the arcing pattern.... but why isn't the arcing happening at the place where the two parts of the frame would be the closest to each other?

It boils down to Paschen's law - basically, for some voltage levels at some particular ranges of gas pressure and type, the voltage may prefer to take an elongated route when making a spark: -

If you look at the graph for nitrogen (\$N_2\$) above you will see that a voltage of (say) 1,000 volts will have two distances for breakdown (about 5 mm and 100 mm) at a pressure of 1 torr or 1 mm of Mercury. These limits cover the distance range at which breakdown may occur.

You have also to consider that once the zapper has discharged, there will be a ramping voltage building up over a few tens or hundreds of milliseconds that might discharge at about 500 volts at a distance of 10 mm or might get all the way to around 2,000 volts and take a discharge path that is a bit less than 5 mm. Statistics are also involved i.e. there is a probability involved in an arc starting (see Impact ionization section in the linked wiki page for Paschen's law).

Also, if you have a fixed spark gap you can see several different paths taken by the arc here at wiki spark gap: -

Quote from that page: -

When a spark gap consists of only two electrodes separated by gas, the transition between the non-conducting and conducting states is governed by Paschen's law. At typical pressure and electrode distance combinations, Paschen's law says that Townsend discharge will fill the gap between the electrodes with conductive plasma whenever the ratio of the electric field strength to the pressure exceeds a constant value determined by the composition of the gas.

You should also look up "Jacob's Ladder" on this section of spark gap. it shows how a spark can seemingly climb up to electrodes that have a slight angle between them: -

It's all down to Paschen's Law.

Answer 2

I'd guess once the first arc discharge occurs, it releases a pressure wave (that's the 'pop' you hear) and causes the air density to vary in a concentric ripple pattern spreading outward from that arc.

Meanwhile, the circuitry, having been discharged, has to build up to a sufficient voltage again. This takes time.

The random pattern is the result of the "beating" between the air density spatial ripple pattern and the temporal discharge interval.

To experiment with this, modify your circuit to slow down the recharging and see how the pattern is affected.

Honestly, this is a wild guess to what's an interesting question.

Answer 3

So you're feeding the mesh plates for the mosquito racket from a stun-gun energizer that is itself spark triggered.

Ihe rising edge of each pulse will be very steep and will rapidly charge the capacitor plates formed by the two grids. after that you get echoes of the pulse reflected from the edges of the grid.

The pulses from the stun gun energizer are themselves made using a spark gap and that spark gap will be changed by each pulse, so each pulse will be different.

So the echoes from the edges of the plates will be different each time, thus the sufficient voltage peak can appear in a different place each time.