Tesla coil 2

In early 2013 I started design work on another Tesla coil. Due to a lack of motivation I paused the project for about two years and finally finished it in 2015 after rebuilding most of the bottom layer. This tesla coil peaks at about 1,5kVA exceeding a discharge length of one meter making it the most powerful Tesla coil I've built so far.

Specifications

  Value Unit
Primary LC-circuit    
Resonance frequency 241,0 kHz
Capacity 50,0 nF
Inductance 8,7 uH
Secondary LC-circuit    
Resonance frequency 227.2 kHz
Capacity 22,3 pF
Inductance 22,0 mH
Secondary coil    
Radius 55 mm
Height 500 mm
Ratio 4,5  
Wire diameter 0,5 mm
Windings 1000  
Parasit. capacity 8,5 pF
Topload    
Radius 100 mm
Capacity 13,9 pF
Primary coil    
Radius [inner winding] 90 mm
Radius [outer winding] 144 mm
Wire diameter 6 mm
Distance btwn. windings 6 mm
Windings 5  
HV-Transformer    
Voltage(RMS) <6,3 kV
Current ~250 mA
Power ~1,5 kVA

The primary frequency is exactly matched to the secondary freqence by tapping the primary coil as needed. (less inductance is usually not really a problem).

Construction

The unit is powered by a 6,3kV 120mA neon sign transformer. The short circuit current was boosted to about 280mA by removing the magnetic shunt. The transformer must not be operated continously without it's current limitation as the windings (especially the primary) start to heat up significantly after a few minutes of operation. However as I don't run my tesla coils any longer then a minute it's not really a problem for me.

I chose a MMC design for the tank cap. I used a total of 48 WIMA FKP-1 220nF / 1.25kV- pulse capcitors. These caps were specificly designed to operate under heavy pulse conditions and deliver very high currents. I used a bleeder-string consisting out of 1M resistors to ensure that all caps are charge evenly. The bleeder-string also helps to discharge the caps after the transformer has been switched off.

The primary coil was made from 6mm copper tubing and is mounted on four PVC holders. The secondary is wound on a 110mm / 50cm PVC tube It's attached to the base using a plug connector so it can me removed at any time without needing any kind of tool.

In previous tesla coil designs I used a contact out of aluminum tape which was placed inside the secondary to form a circle. That's obviously a pretty bad idea as it represents a winding which happens to be located just in the middle of the primary's magnetic field. Regarding the fact that it's only one winding this leads to a pretty high current which is induced in that winding. So when I first fired this coil up parts of the aluminum tape were vapourized while carbonating my secondary. I had to do a fair bit of scratching and sanding to clean up the mess.

 

The topload is made from a 25cm stailess steel sphere. I placed a small steel plate inside the sphere which is pulled against a PS plate which is mounted inside the secondary using a threaded rod. This allows a perfect fit to the secondary keeping the distance to the topload as small as possible.

The components are mounted on two laminated beech wood sheets, each 40x40cm in size. The two layers are connected using M12 threaded steel rods. I used four pieces of aluminium profile to hide the threaded rods and make everything look pretty.

Every part carrying high voltage was isolated from the wood base using 5mm Polysytrol (PS) sheets. This prevents creepage currents caused by any moisture in the wooden sheets which might lead to performance losses.

I used a safety gap and two safety chokes to protect the transformer. Any overvoltage which might be caused by resonance effects in the primary circuit or by a streamer striking the primary gets shorted out the safety gap even if the main gap fails to ignite. The two chokes damp any HF-kickbacks which might lead to corona in the transformer's secondary winding and destroy it. I used ferrite rods which were placed in the center of each choke to boost the inductance and therefore get a higher inductive resistance.

The layout is not perfect - the wire links could be a bit shorter, however I don't think it's really worth buying a new sheet of wood. The coil is connected to 230V~ wall power (single phase). The control elements are mounted on a aluminum sheet. The high voltage is activated using a remote control.

Sparks

Strike to a grounded electrode:

Streamer:

 

Floating streamer after turning off the high voltage, the topload is at a distance of approx. 20cm

 

2016 J. Weigelt - www.janelo.net