Kapanadze Cousin - DALLY FREE ENERGY

[Sergh]

For Kacher, the protection from overheating is more necessary. A large radiator is required for the transistor. Overvoltage protection is only necessary when powered from relatively high voltages.

[AlienGrey]

It only overheats because the device doesn't turn fully on or off in that circuit. Use a mos fet driver with 1n4148
one to - rail and one to + rail taped at pin 2 with feed back so as not to overvoltage it.

and your looking for EPCOS Standard 'Series Metal Oxide Varistor'  but they have loads of 'stored Capacitance 
1 to 2 nF for a 100A, Clamping 395V, Varistor 240V

[Jeg]

Thank you Sergy. Personally i use the same transistor for the last 4 years without any overvoltage failures across transistor's base.

One of the most important components of katcher is this nf Range capacitor at the input right after the chock. Ruslan uses a 100nF but you can go a lot lower. The chock is nessesary there and needs to be at least 50 times higher in inductance than the thick primary and more. But do not over do it there. The reason for this araingement is as follows:
When transistor conducts, this nf cap is discharging through the primary before current starts flowing again from the battery. This delay of input battery's current happens due to this chock. So this cap in combination with the primary gives a nice voltage oscillation and just before the cap goes empty, transistor closes. This action superimposes a beautiful high voltage spike right in the middle of the Cap/prim voltage oscillation. Like in the old times 
This has nothing to do with OU of cource, but makes your Kachers work very close to the operation that they were designed for. 
     

[seychelles]

TO ALIEN GRAY.. I WILL DRAW A BLOKE DIAGRAM OF MY OWN AND SEE IF THAT WILL
SEND A SHOCK WAVE OF MONKEYS UP YOUR ASS.

[NickZ]

   Jeg:   Why do you want to build the simple Kacher circuit, when you have a much better controlable Kacher circuit already?   Although the controlable Kacher circuit is more complex, with many more components, each one is hogging up the output.
Yet it can be controlled as far as duty cycle, frequency, dead time etz...  so in that sense, it's the right way to go for the grenade set up. So, does your controllable Kacher circuit still work? And what are you intending is the purpose of building the simple Kacher?
   
   Apecore:  Good to see you back, and that you've got nice big artistic and decorative wall hanging.    I'm still wondering what we are all missing...
   Jeg: The two caps after the choke on the Ruslan simple Kacher are there to filter any AC component that may try to make it's way to the transistor. However, they in themselves do not produce a spike, but a sine wave, instead. If that is not what you are talking about then what capacitor are you mentioning that will produce the spike. 
   Sorry for the blurry image of the diagram.
   Picture of Ruslan's simple Kacher circuit. Same as what I built up and use.