Tesla coil 3

In early 2014 I started construction on yet another medium sized Tesla coil. Still being in high school at that time, I built this coil for a school project.

The specs are pretty much the same as for my first Tesla coil. I used a slightly more powerful HV-transformer though. The NST I used in this project has a specified output of 6.3kV at 80mA. The transformer had an adjustable magetic shunt. By removing it entirely I managed to boost the transformer's short circuit current to about 150mA.  

Specs

Bezeichnung Value Unit
Primary    
Resonance frequency 292 kHz
Primary capacity 36,6 nF
Primary inductivity 8,88 uH
Secondary    
Resonance frequency 325 kHz
Secondary capacity 19 pF
Secondary inductivity 21,5 mH
Secondary coil    
Radius 55 mm
Height 450 mm
Ratio 4,09  
Wire diameter 0,5 mm
Windings 900  
Capacity 8 pF
Topload    
Radius 100 mm
Capacity 11,13 pF
Primary coil    
Radius [inner] 90 mm
Radius [outer] 144 mm
Wire diameter 0,5 mm
Dist. btwn. Windings 0 m
Windings 5  
Transformer    
Voltage(eff) <6,3 kV
Current >150 mA
Power ~750 VA

The primary-LC-circuit's resonance frequency is adjusted by tapping the correct inductance from the primary coil (less inductance is usually not a problem).
This will affect the coupling, however as it's not that critical as long as it stays in a rough range of about 10-20% it should be fine.

Construction

The teslacoil is build using a 2-layer construction. Both plates are made out of beech edge-glued panels. 
The high-voltage conductors are isolated from the baseplate by 5mm Polystyrol plates as it's quite easy to obtain where I'm living (and a lot cheaper than PMMA).
According to wikipedia, Polystyrol has a conductance of about 10^−16 S/m.  This table mentiones a resistance of about 10^15 Ω/cm for PMMA which equals about 10^-13 S/m, so the Polystyrol should be even better than PMMA.

The bottom layer contains all primary-components (except the coil):

The primary coil was made from 6mm cooper tubing. It has a total of 5 3/4 turns About 3 3/4 are used to aquire the correct inductance.

The tank cap is a MMC design made out of 2x12 WIMA FKP-1 impulse capacitors. Each cap having a capacitance of 220nF at 1,25kV- the resulting cap
has a capacitance 36,66nF while withstanding 15kV DC. Good caps are essential to a relieable performance. I previously tried WIMA MKS caps which were not specificly designed for pulse operation. They were blown into pieces. Keep in mind that the tesla coil's relatively high operating frequency will have a significant effect on the dielectrica's breakdown voltage! I oversized the MMC's max. voltage to take that into account and so far it held up just fine.

The spark gap consits out of 3 pieces of 20mm cooper pipe. I mounted a small fan above the gap to help cooling the pipes and archieve faster quenching.

I used two coils and a safety gap to protect the transformer from being destroyed by high voltage spikes or HF-kickbacks.

The contol panel is made out of a few pieces of aluminum. The transformer is fused at 16A. I added a filter to prevent any HF from getting to the power grid. The fan will always run after the main-breaker is closed, the high voltage is activated using a remote switch.

The unit is designed to be kind of "portable". The secodary can be removed without any tools just by pulling it off the socket. Depending on how tight the M12-connectors between the two baseplates were tied together it's also possible to remove the top baseplate without any tools. All connections can be unplugged. Of course I managed to forget to reconnect the secondary's earth-connection:

I already cleaned up most of the mess before taking this picture. I was really lucky, the transformer did not take any hits which I'm pretty sure would've destroyed it. Luckily the secondary's ground connector was designed to "go to ground" so only the transformer's ground-connection was hit.

Operation

Due to the school's safety guidelines I had to test the unit at school. I noticed that the spark-gap wasn't adjusted properly after disassembling the unit afterwards.
I didn't do any further tests so far.


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2016 J. Weigelt - www.janelo.net