2-Stage bug zapper:

let's fry those bastards

Introduction

As most people, I do not like bugs in my home, so I bought an electric bug trap with a blue lamp and an electric grid to electrocute the bugs.
I installed the thing, but it did not work at all; the gnats remained! So I checked the dish: no corpses!

Time for some improvements! I opened the bug trap, it worked as follows:
A 100nF/630V capacitor is charged to about 600 Volts by a charge pump built with two diodes and two 47 nF capacitors.
This "high" voltage is connected to the grid. When a bug passes though the grid, it will short ciruit the 100nF capacitor and should be killed by electrocution

In practice 600V proves to be too low for many bugs! I checked this with some gnats I catched: they just passed though the grid without any problem!
I measured the grid voltage, it did not even change when a gnat touches the grid! Clearly 600V is too low to penetrate the external shield of the bugs.

The solution

My first thought was: higher voltage higher voltage higher.kill.... kill.. So I increased the grid voltage with a
diode/capacitor cascade and found out that the maximum was about 4kV - 10kV (depending on weather conditions).
This was already quite an improvement, I catched some unfortunate guinea pig-gnats: all were zapped, but still some survived!

Now it is time for the ultimate solution: the problem is the limited amount of energy stored in a high voltage capacitor:
E= ½ * C * V²: so 5 kV in a 10nF capacitor will give you 125mJ only!
Eletrolytic capacitors from TV sets or photo flashers are better fit for energy storage: 100uF/300V gives 4500 mJ!

In the schematic below, the best of both capacitor types is used: A high voltage capacitor is charged
to about 1800V (which proved to be sufficient to penetrate the external shield of the bugs)
A large electrolytic capacitor from the flash unit of an old photo camera is used for energy storage.

2-stage bug zapper schematic:



This is how it works: the grid is charged to 1800V by the cascade built with D1-6 and C1-6;
the 100uF elcap is charged to about 300V.
When a bug passes through the grid, the high grid voltage will cause a discharge though the bug
The grid voltage will discharge from 1800 to about 300V, then the 100uF elcap will take over!
The elcap will discharge though the diodes D7-D8: resistors  the discharge arc will be sustained by the large current from the elcap.

The bug will be fried instantly by the 4.5 Joules from the elcap (this is enough to heat 1 cc water by 1.6 C)

This bug zapper works for anything which can pass though the grid. Even wasps (tough suckers) do not survive
By the way, the sound of the discharge is quite impressive............

Tip: do not bother to search the dish for bug-corpses; just sweep up the body parts scattered on the flour below

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