How to build a simple solid state Tesla Coil

WARNING!!!
   Information provided here is only for people who are experienced in electrics / electronics and know what they are doing. Although in carefully designed experiment conditions, this circuit would not cause any harm, it can severely shock you, cause things to catch fire, destroy nearby electronic equipments, produce ozone and other reactive gases that can irritate your lungs, and even kill you at extreme carelessness.
   So that, this information is  'only'  for people; again; who are experienced enough and already in search of something like this. I do not take any responsibility about the dangers mentioned above. This circuitry is not a toy, only in right hands it should be constructed and operated.
   If you fully understood the dangers and take responsibility then you can continue ...
  
    How to build a simple solid state Tesla Coil
   While I was in search of something that could satisfy my desire to build up a working Tesla Coil, I met with the many copies of a simple circuit. All of these were the same except some of them offered different values about resistors. When I had the chance, (in fact gained a small flyback transformer) soldered the parts together, applied the voltage and had 2cm. long violet sparks at the first test drive.
   Although the circuit was so simple thus seemed should work with no problems, I realized one of my transistors became very hot quickly while the other was somehow wormed. To investigate the problem, I measured the collector currents of transistors separately and found that one of them was conducting about %85 of the the total current. According to my digital avometer, the hFE (current gain) value of the more conducting transistor was 180 and the other one's was a little different: 160.
   This little difference (only %12) made the circuit asymmetric and decreased the performance. Just to learn how much an individual transistor could drive the transformer, I disconnected one of them each time and found that only one transistor was enough to operate the circuit. In fact connecting the second transistor was making the circuit unstable and decrease voltage output.
   So I decided to start with a single transistor circuit and optimize it. Dual transistor circuit required identical transistors and also identical primary and feedback windings that have the same resistance (capacitance / inductance) values. 2N3055 or equivalent power transistors are not so cheap to buy a lot and not many of the sellers would allow you to select identical pairs with your multimeter at their shop. Since it's impractical to measure winding inductances, you'll appreciate why I offer you a single transistor solid state Tesla Coil as a starting point:

   Be sure that you really understood the basic operating principle of this circuit. If there was no feedback winding, the transistor would conduct a steady current determined by the values of R1 and R2. But now the circuit is dynamic: It will oscillate on a natural frequency that puts your transformer in a resonance activity resulting in enormous voltage gain. Of course without any tuning attempt you can't expect the system running on the right resonance frequency. But it is not so important that your transformer would already be running in it's limits in the given voltage range. Such a tuning is more critical when you deal with a real Tesla Coil. If you want to learn what 'Resonance' means in this case, you can find very detailed explanations over the net or in electronics books about this phenomena.
   The heart of the circuit is a ferrite-core flyback transformer that you can find in an old black-white TV or a malfunctioned monochrome monitor. Even though color TV and monitors have stronger flybacks, their secondaries contain additional windings and built in diodes usually whole coated with plastic that makes them hard to deal with and just because the diodes cause inconsistency on output voltage and voltage multipliers formed with capacitors and these diodes create dangerous voltage + current + frequency combinations I do not recommend this ones at the beginning. Find the simplest, single wound secondary transformer. Check out if it's secondary is ok with an analog multimeter. If there is a resistance (about a few hundred ohms) then you may think it is not burned (yet). Here is what it might look like:

   Second step is the removal of the primary windings. An old but sharp enough knife would do this so quick; just cut them and clean the winding area.

   Now you only have the secondary, although it looks innocent, is capable to produce thousands of volts if you wind 5-6 turns of wire on core and then connect and disconnect it to the power supply rapidly to watch the secondary convert your high current / low voltage input to low current / high voltage output. There's no need to remind you not to touch secondary terminals while doing this. Otherwise you'll get nasty shocks on your fingertips as a natural warning!

   What the transistor will do is just to make this open-close loops automatically and continuous. When you apply the power, circuit will start to operate on a frequency determined by the winding inductance, capacitance, and ohmic resistance of primary, secondary's load status, composition and size of the ferrite core, length and position of the connection cables, junction capacity of the transistor, and many more as you can think of !
   To form low inductance primary; wind 5 turns of enameled 18AWG wire (1mm diameter) on the core. Secure it with tape or glue at your choice.

   To form feedback winding use 22AWG wire (0.64mm diameter) and wind 2 turns (or 3-4 turns depending on your transistor's Hfe) then secure it.

   Now the mechanism is ready. Connect your resistors and transistor to this transformer system as in the circuit diagram. Do not forget to mount your transistor to a heatsink, otherwise it may be damaged after a very short period of operating time.
   The value for R1 is for 24-32 volts operation. You may replace it with a lower value like 110 ohms for lower voltages. But 240 ohms will operate the circuit without any problems. As the input voltage rises the power consumption on R2 (27 ohms) increases. Although it seems won't become hot as much as R1 according to ohms law, current inducted on feedback winding uses R2 to reach the ground and this should be added to the calculation. So check it's temperature and use a higher wattage one if necessary.
   Be sure that your power supply is capable of giving the 2-3 amps current as circuit will take. Set two cables in clearance of about 1mm at their tips to observe high voltage output from the secondary leads. This is an initial gap length to be sure. Please use a push button type switch since it would stop the circuit as you take your finger away. This is a legitimate precaution and the circuit should never be operated while it is constantly connected to the power supply. As you understood the security reasons well, apply the power by pressing the button. Now you should be able to see the sparkings. If not, and your multimeter shows that the circuit is already dragging 1-2 amps (or more) current then it's just the feedback leads you have to reverse. Now your mini monster should be working.

   Use cables with very thick isolations if you are going to pick them and play around. Even when they are very isolated looking, cables can leak electricity since they are capacitively conductive at these much high frequencies. So your fingers can be burnt without any pain. Prefer to hold them by an isolated pliers or something like that. Better, have a rubber glove. In fact, nothing (even mica isolators) behaves as an insulator in this case. Be careful! Be very careful!! Always be in a mood like you're in the unknown deep ocean of the mysteries of the electricity. Again; be careful!!!
   Do not give up if you can't make it happen at your first try. Check out everything. Get help of someone who experienced in electronics as well. Once you operated it successfully, you can go on next steps like building a plasma globe or an ion motor. How to make a simple plasma globe
   Please be careful: All of the dangers mentioned in the parent page are valid for this apparatus too. So be aware of high voltage. Please read additional notes concerning your safety given in the text below.
   Once you operated your coil it's easy and funny to make some magic with it. A simple plasma globe would amaze you enough. All you have to do is to find an ordinary electric bulb. Since these bulbs are filled with gases to prevent the filament from corrosion and make it much brighter, you can ionize the gas in the bulb and turn it to a plasma globe. Not all of the bulbs have the same gas composition so you may have to search for. Your globe won't look as mysterious as original ones but it's worth to take a look at it.



   As you see this circuit is grounded. You are grounded too and this means there is more availability for current to pass through your body. Take precautions for the high voltage cable and the conductive parts of the bulb so prevent yourself and other ones touching them. When you approach your fingers to the bulb you'll observe plasma attracting there. As you dare to touch the glass you may find tiny sparkings hitting your finger without pain but they will burn out your tissue. Intense lightning hitting a point in the glass sphere would cause that point to heat. With my tiny coil, I haven't seen this caused the glass to break but it's possible. If the glass breaks then there will be a chance that sparkings may jump from the bulb's wires to your fingers. Another danger is ozone, produced by the corona built up at the outer surface of the glass, that is harmful to your lungs. Also ultraviolet emission is very high for your eyes in this operation. So run your globe only for a short period, in a well ventilated place, avoiding to touch the bulb, and at an optimum voltage so it won't break and hurt you.
   Here are some photos of a bulb I've operated. Notice that sparkings are formed easily and longer in the bulb compared to the ones in the air.

Attracting plasma with hand
 

Another one similar
 

Plasma is visible even in strong room light
 

In the dark, plasma looks fantastic