Lately I was obsessed with making a stun gun.
I was particularly interested in fly swatter for the source of high voltage due to being easy to get and very cheap compared to using other high voltage circuitry (like from arc plasma lighter or plasma globe driver.). It cost me around USD4 for a fly swatter compared to plasma globe (USD8), or arc plasma lighter (USD40) and I also dont have to wait for weeks to get it since its just available at the store in front of my house. (all prices are converted to USD from local currency)
Prior to this I have experimented with salvaged transformers and my self-made cockroft walton multiplier and oscillators (ZVS, 555 timer, joule thief) for making a stun gun but all gave disappointing results such as very thin arc (low current) even though the arc length was quite impressive. This is because for all these designs, I utilized usual 9V batteries which does not give enough current (hence power) to the final output due to its considerably high internal resistance compare to the load circuitry resistance (which subject to its design and chemistry). To add to this, the salvaged transformers were not optimized for the operating point I used.
Based on research I found out that the only optimally designed transformer for stun gun is flyback transformer. Flyback transformer is not only the one you can salvage from old CRT monitor, but the one miniaturised as in plasma globe driver or fly swatter and sometimes appears indistinguishable from common charger transformers (flyback transformer also available in a core-type design instead of the usual shell-type design as in CRT monitor). This type of transformer was designed so its ferrite core will have low coercivity (easily magnetised and de-magnetised to reduce hysterisis loss) and low leakage inductance at high frequency and generally has the ability to step up voltage lower than 10V to several thousands volt.
Fly swatter came to my mind to solve these two problems; getting high current and efficiently transforming voltage lower than 10V to several thousands volt. The reason behind high current from fly swatter is because of the battery it utilizes (which can supply high current at particular voltage) which I reckon is a Lithium polymer rechargeable battery and the specialized step up transformer which intentionally designed for increased efficiency at high voltage and high frequency operation. To my surprise, the voltage supplied by the battery is just 4.25V.
Fly swatter usually output around 1 or 2kV direct current which is sent to the mesh . You may Google and Youtube around to find out how fly swatter looks like. At first glance it seems like a usual tennis racket where the only difference is that the mesh is supplied with high voltage to give a fatal jolt to bugs (which is the primary purpose of its invention, hence the name).
Below is the circuit I adopted (only the high voltage circuitry part) for the fly swatter stun gun;
In the video below, the white stick is the handle of the fly swatter that contains the oscillator (I have removed the wire meshes). Two protruding wires (red and white) are the high voltage direct current output which originally attached to the removed wire meshes. The white plastic former is the spark gap ( I punch a common tape plastic former with two screws which were distanced less than 1mm apart). I found that the smaller the spark gap, the fatter the arc at secondary of high voltage pulse transformer which implies that the transformer can optimally operate at high switching rate (the lesser the spark gap distance, the higher the switching rate). The black taped bulky cylinder is the high voltage pulse transformer, homemade (description in the circuit diagram above).
The length of the arc at the secondary of this transformer is around 1cm (corresponds to voltage around 9kV to 10kV). Enjoy the video!
If you are impatient in waiting the video loading, I attach the final result snapshot below;
(the Powerball (green bulky stuff) and the tape are to fix the high voltage wire terminal in places)
Until next time!