The new Model 2 coil

On this page you find informations about the new 2kVA Model 2 coil I am currently building. This page will change when more photos are available. Also, topics will be added to the construction, theory and design pages when available.
The technical specs
Construction details, plans and photos
Primary Capacitor
Primary Coil
Secondary Coil
Main sparkgap
Power control unit
Filter unit
Coil operation
Jacobs Ladder photos

The technical specs

The following shows the technical specifications of coil Model 2
Power consumption 2kVA
Transformer 15,4kV/130mA
Primary windings 12
Primary capacity 24 nF
Secondary diameter 200mm
Secondary windings 750, 1mm copper wire
Secondary winding height 800mm
Resonance frequency 160 kHz
Toroid diameter 75cm / about 29 inch
Toroid inner diameter 16cm
RF Ground still open

The coil is built using a 15,4kV/130mA transformer). I purchased this transformer used (but looks like new) from the manufacturer. The transformer is normally used for ozone production.
The primary coil is a flat type coil, built with 8mm copper tubing.
The primary capacitor is build as stack of plates. I have built 6 capacitors, each of them with 35nF-36nF.
The secondary coil is wound on a PVC tube with 1mm copper wire. The toroid is made from aluminium flexible tubing with 160mm diameter, which can easily be formed to a toroid.
The main sparkgap is a synchronous rotary type.

Construction details, plans and photos

This chapter shows information and pictures from the construction of my tesla coil. It is sorted by the different components. The pictures are not supplied in a larger format. This is only available for the photos of the opeating coil.
Primary Capacitor
Primary Coil
Secondary Coil
Main sparkgap
Power control unit
Filter unit

Primary Capacitor

I have finished my 6 "old fashioned" capacitors using PE sheets and aluminium foil in a plate stack construction filled with oil now.
Each cap has 4 sheets LDPE at 0,2mm thickness as dielectricum. 3 caps in series result in 12nF and should resist the voltage. Two of these series caps in parallel makes up the 24nF I need. The capacitors are built from Pertinax. 2 Plates of pertinax are used to press the stack. When the stack is ready, the Pertinax plates themselves are used as case at the same time by glueing other pertinax plates on the sides and the bottom with epoxy. The top is closed with Polcycarbonate, in which also the contacts are screwed in. The caps are filled with transformer oil.
With one cap, leakage problems occur, and it looses maybe a drop per week or less. I fixed it with epoxy twice, but still it seems not to be really good.

PE foil cutting

The PE foil is cut from a large roll bought from a do it yourself market.

Stack materials

This shows the cut PE foils and the aluminium plates ready for mounting in the stack. All in all, 6 stacks will be built.

CAP stack

This is the stack containing 58 aluminium plates, separeated by 4 sheets PE, together 0.66 mm thick.

CAP stack again

The same stack from top view, as the aluminium can be seen better.

CAP stacks pressed

This picture shows two of the cap stacks pressed by 10mm pertinax plates. Each stack is mounted by 6 screws. The green color is from PVC pipe put on the screws for isolation purpose. The stack on the right side already has a mounted contact strip made from alumium foil.

complete cap

This is a complete cap. The case is glued completely with epoxy. The top plate is made from polycarbonate. This thing on top is a self made vacuum pump.

Cap Bank

This is the complete bank made up of 6 caps of the above type. Their values are 35.5 / 35.9 / 35.8 / 35.3 / 34.6 / 34.8 nF, which is a quite good tolerance.
As I learned, that the MMC type capacitors now seem to be state of the art, I will also build an MMC cap with the same value. Unfortunately, I already had bought Cornell Dubilier, 940C, 3kVDC, 47nF types. My plan was to connect them in 7 strings, 15 caps each. Maybe 2 weeks later after my caps arrived, the discussion about good and bad MMCs started on PUPMAN. Bad luck for me.
I did use some of the caps for test on my reworked small coil. More info on this and the pics of blown caps will follow later - but the effect was the same as discussed. They even do not work for small coils!
Currently I'm looking for Panasonic caps or at least 942C Types.

Primary Coil

The primary coil base plate is made of chipboard. But I learned from my small coil construction. Therefore the supports of the primary are now made of 10mm white LDPE. Each of the 6 supports consists of one single piece, no more screws used, except thoses for mounting the supports on the base plate, which are totally isolated inside.

Primary baseplate

The primary coil baseplate is made from 19mm chipboard with white surface. The base plate has 74cm diameter and there will be 6 supports for the copper tubing.

Primary supports

These are the 6 LDPE supports for the primary coil windings. They are cut in an L-form to be able to add a strike protection ring.

Winding the primary

This picture shows, how the primary copper tubing is wound together with the supports. The supports are threaded from the outside to the inside of the coil, until everything is adjusted fine.

Mounted primary

The primary coil, completely mounted, except strike ring. The supports are screwed to the baseplate from the bootom side. There is absolutely no conductive material near the primary windings.

Secondary base

The base of the secondary coil sitting in the centre of the primary.

The strike ring

The strike ring is not built as a closed ring, as this could cause losses due to induced currents. On the left bottom you can see the clip for connection to the primary.

Secondary coil

The secondary coil is wound on a PVC tube (walls about 5mm thick) with 200mm diameter. I used 1mm copper wire and wound 750 windings.

The PVC tube

This picture shows the PVC tube for the secondary coil. The PVC tube is for use as drain pipe. I dried this pipe in the oven for some time and coated it with PU varnish.

Winding the secondary

I have built a simple motor driven construction. This simplifies work, and, more important, the epoxy coat could dry while the coil was still rotating. This ensures a nice surface of equal thickness.

Finished secondary

This picture shows the finished secondary with its epoxy coating standing outside in the sun.

Detail of the windings

As you can see, the windings are completely covered by the epoxy resin coating I made. The surface is almost as smooth as glass.


My transformer is a 15.4kV/130mA transformer. I bought this from Conrad Transformatoren.

The transformer

This picture shows my transformer. It has a weight of 60kg. The cylinder on the left is the winding, where you can see the 2 spots, which are the HV contacts. The core of this transformer has a gap of 5mm.
During measurements and experiments with my transformer I found, that the air gap did lower the inductance of the primary transformer winding enormously. The transformer pulled about 10A with no secondary load. I use an arc welder as current limiter, as this transformer is not current limited itself. I found that with no load, half of the primary voltage was lost over the arc welder, resulting in only 110V primary, i.e. half secondary output. Using a jacobs ladder the performance was the same as with a small NST 8kV/80mA. When pulling secondary current, primary voltage even dropped to 20V.
So I dismantled the unit and removed the gap. Thanks to Finn Hammer who encouraged me to do this.
After this modification, the open load current reduced to 1.5A. The Jacobs Ladder test now was improved dramatically and I'm quite confident, that the transformer will now be well suitable for TC use.

Main sparkgap

I decided to use a rotary sparkgap in my new coil system. Thanks a lot to Kurt Schraner who helped me and who convinced me to build a SRSG type. I have finally found a lathe operator who made me aluminium-ring after my specs for the new sparkgap. Also, I have now a 450W (0.6HP) motor, which I changed to salient pole operation.

The rotary disk

This picture shows the rotary disk mounted on the motor axis. There are no electrodes mounted yet.

Rotary disk

This picture shows a view from top onto the rotary disk mounted on the motor axis.

The open motor

This picture shows the stator of the open motor.

The rotor

This picture shows the rotor before the salient pole modification.

The modified rotor

This picture shows the rotor after the salient pole modification. You can see one of the flats I have made with a file. The other flat is exactly 180 degrees in the opposite.

The complete SRSG

This picture shows the completed SRSG from a top view.

Detail view

This picture shows a more detailed view of the fixed electrode construction. You also can see the safety gap.

Fixed electrodes

The support of the fixed electrodes is made from teflon.

Safety Gap

This picture shows the safety gap. The electrodes are steel balls from old computer mouses.

Power control unit

I have built a new power control unit for the model 2. The variac is able to handle up to 25 A.

The front plate

This picture shows the front plate of my new power control unit for my coil. I aquired a steel housing from a local electric store for almost free, because I did not want to build it from wood again.

The open case

This picture shows the inner cabling of my new power unit.
The white box in the upper right contains the Solid State Relay control box.

The connectors

Connectors for SRSG, HV transformer, ballast and input.


This picture is taken while cabling the unit.

Control circuit

This picture shows a circuit used for controlling the solid state relay. It also contains a protection, that allows powering up the HV transformer only when the SRSG motor is running.
The picture picture below shows the overall circuit diagram of the power control unit.

On the left, there is a main relais controlled by the key switch and the emergency button in series. The current limiter limits the high current pulse of the variac during start. After switching on the unit, the variac is under power. The SSR is a solid state relay which will switch the coil. The SSR control box provides the control voltage for the relay.

The next picture picture shows the SSR control circuit diagram.

On the left, there is a lamp connected. This is fed by the voltage, that is accros the 0.47 ohm resistor you find in the main circuit. That means, the lamp only lights, if the rotary motor draws current. To avoid destruction of the lamp during motor start, the 2 transistors on the left provide a delay after starting the motor, before the lamp is switched on via a small relay.
The lamp is mounted in front of an LDR connected to a schmitt-trigger circuit. This provides voltage between pins 21 and 22 only, if the rotary motor is drawing current. The result is, that the coil only can be switched on, if the rotary is running.

Filter unit

The filter unit I have built is made as RC type only to avoid possible additional resonances introduced due to the additional inductivities in an RLC type. The resistors are 2 high power ceramic resistors mounted on 2 cooling ribs. The Capacitor is made from 14 Caps, each 47nF from Cornell Dubilier 940C. As they are not good for MMC use (see my MMC experiences...), I at least try to use some of them for that purpose.

Filter unit

This picture shows the filter unit. The white box contains the 14 capacitors in series. It's not closed yet for debugging purpose. The assembly is mounted on a PP plate.
During my first light, an arc over from the leads of the resistors to the aluminium cooling ribs happened. Even isolating them with hot glue as a intermediate fix did not help. I cannot explain this, as the only voltage between these points can be the voltage resulting from resistance and current flow. I will have to do some rework on the design.

Coil operation

These two pictures show the setup for my first light test. The small reworked coil can be seen next to the new 2kVA model 2. Thanks to my mother, who allowed me to do the tests in her garden (you see her on the right picture).
Here are 2 pictures from my first light taken on Friday, May 11th. I first tuned the coil and powered it up with the new static gap which I use for my reworked small coil. Then I started with the SRSG.
I hope that I will be able to improve the performance in the next time.
The pictures are taken with my new digital camera. I set the exposure time to 2 seconds. Please click on the pics for a larger view.

First shot

This picture was taken while powering up for a very short time only (less than 1 second).

Another shot

This picture was taken while powering up for a little longer (about 2 seconds).
There were some effects, that I found when running the coil.
You can see, the sparks only break out from one area on the toriod. This is not because of some needle positioned there. The reason is, that the ducting I bought did not have equal diameter, although it was the same type. One of the 3 segments has a diameter about 5 mm larger. The sparks break out from where I hotglued the segments together.
I tried to smooth this with some aluminium tape, but this did not help as expected.
Another effect was found, because the small coil standing next to the big coil, as you can see on the pictures of the setup. A cable of the small coil hang just a few millimeters above ground. When running the big coil, there was an arc from this cable to ground, which even burnt through the isolation. I'm not sure how to explain that, it must have to do with the field of the large coil inducing voltages in the small coils cabling. I will need to do some tests.

Jacobs Ladder photos

Here you find some photos of the jacobs ladder I have built when playing around with my transformer. Click on the photos to get a larger view.
© 2000 by Herbert Mehlhose