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Tesla Coil Version 2

Specifications:

Input transformer:

- HV AC Transformer v2 (See research area)

Output 6kV at 50mA AC, Input 24v 12A 288W, frequency around 40Khz.

Coil Specs:

I have again used the excellent TeslaMap program to calculate the dimensions of this coil. I also manually calculated some of the components, so as to confirm the programs outputs. It's just easier to use a program now isn't it!

- The measured resonant frequency of the secondary coil with topload was 112.5Khz

- The measured Secondary Coil Inductance was 80mH

- Secondary self capacitance was calculated to be 11pF, with the topload being 20pF

There are many Radio Handbooks that describe how to perform these measurements and calculations if you are un-familiar with them.

A good guide on how to measure various parts of a Tesla Coil can be found Here.

Parts views:

For the second attempt at making a Tesla Coil, I decided to use a rotary spark gap instead of the traditional spark gap or multiple spark gap. The rotary gap I designed is based around a small 12V dc motor that is controlled via a simple variable voltage that sits off-board. Allowing me to adjust the BPS (breaks per second of the circuit) and hence tune the Tesla Coils resonance more effeciently than a spark gap setup.

Rotary spark gap spec:

- 12cm Perspex disc with 4 flying electrodes. Electrodes are ZBP steel bolts and nuts with domed end nuts. (These have lasted fairly well despite people saying that tungsten is the only way to go, there cheap and readily available too). The outer ring around the perspex disc is aluminium to allow a solid contact with all 4 electrodes.

- 2 stationary electrodes on brass L brackets with ZBP steel bolts and nuts with domed end nuts. Setup to be as close as possible to the flying electrodes, but do not touch when rotating.

- Motor brackets are perspex that have been bent into L shapes. The motor must be insulated from any conductive parts!

- Motor Spindle sits in the middle and is a brass/steel screw thread and nut with rubber washers either side. The spindle locks to the motor shaft with a grub screw.

- Protection spark gap is a spark gap across the input terminals and is set at 8mm

- The 2 L brackets are joined by a small peice of alumium. The entire spark gap setup is mounted on a piece of MDF that is raised up on 4 plastic joining blocks. The whole lot is bolted through onto the base PVU board. The PVU board sits on 4 rubber feet.

- Input power from the HV transformer is applied to the 2 end ceramic insulating terminals.

The Capacitor Bank:

The capacitor bank is of the MMC (multi mini caps) type. This is basically the same setup I used in V1, just reconfigured for more voltage and less capacitance. The capacitors I have used are RMC 0.01uF 2-3Kv capacitors. These have intergral spark gaps to prevent overcharging (which has pros and cons!). There are also a string of different caps at the end, these are 2nF 2Kv.

Total capacitance = 14.6nF at 11Kv

Secondary Coil:

The secondary coil was constructed from a 6", 5mm thick cardboard tube the I scored from a local office supplies shop. The tube was first varnished and then cut to 67cm long, 60cm of which will be covered by the windings. The last 7cm will be used to gradually spiral out from the coil winds to the topload. This also creates some space between the topload and the winding to prevent arcing. The plastic tube ends were kept and re-fitted to the finished coil.

The coil winding took a long time. Winding was done using my previously made coil winder from TC V1, that was extended to accomodate the larger coil. Wire used was about 450 meters of 30SWG Enameled wire. This involved using 2 reels of wire and carefully joining the two together in the middle of the coil. The soldered join was covered in superglue and varnish. Total winds on the secondary is about 1700 turns.

The finished wound coil was then coated in 4 coats of PU varnish, allowing drying time in between.

Primary Coil:

The primary coil is constructed from 10 meters of 8mm copper gas tubing. The form is made from perspex and has been angled to a 40 degree incline. The copper tubing was then bent into a spiral shape and laid in place on the form. Grooves were made under each point that the tubing contacted the perspex, so that they would sit neatly in the form. A hole was drilled 10mm down from each groove and a cable tie was put around each mounting point. The form uprights are held in place by plastic mounting blocks and then bolted to the MDF board.

A strike ring was added to the top of the perspex uprights and is grounded to the earth. The ring is just over 3/4 closed with a small gap. The gap is there so as not interfere with the magnetic field envolope.

Secondary Topload:

The secondary toploard is constructed from 11cm aluminium ducting. The ducting was bent in the correct size toroid and held in place by stitching copper wire between the the 2 ends of the aluminium ducting. The topload was then covered with aluminium tape to smooth out the ripples. A circle of cardboard was cut and covered with aluminium foil to the size of the inner topload circumference. The plate is held to the topload by more aluminium tape. This plate allows the topload to be mounted a small balsa wood cross that was mounted to the top of the secondary coil. The top of the secondary wire is then taped to the bottom of the topload.

Method of tuning:

Tuning is best done with a sine wave signal generator (output in mV or V, doesn't matter) and a scope or spectrum analyzer connected to a small antenna. First determine the secondary resonant freq by feeding a sine signal to the secondary (with topload on), varying the frequency, and checking at which frequency you'll get the biggest spike on your spectrum analyzer or the highest amplitude on your scope. Then put the secondary in place, short out the spark gap and unconnect the transformer, and feed the same sine signal to the primary HV circuit. Adjust the tapping of the primary coil and possibly the height of your secondary until you see two maximum big spikes to the left and right of the secondary self resonant freq on your spectrum analyzer, but no peak at that self res.freq. Once that's done, your coil is optimally tuned. (From http://users.tkk.fi/~jwagner/tesla/tc-plans.htm)

Finished Setup:

All connections to and from each board are made with multicore copper earthing cable. An earth ground point is made from the bottom of the secondary and strike ring to a copper pipe that has been driven into the ground.

All transformers and driving circuits are situated on the bottom shelf. Power is supplied off-board on a flying lead, so that the user is not in harms way. ;-)

Rotary spark gap is located on the middle shelf, along with the capacitor bank.