My first tesla coil

On this page you get informations about the first, low powered tesla coil I have built.
The technical specs
Circuit diagram
Construction details, plans and photos
Primary Capacitor
Primary Coil
Secondary Coil
Main sparkgap
Power control unit
Filter unit
Operating coil photos

The technical specs

The following information is about the technical specifications of my coil
Power consumption 400VA
Transformer 8kV/50mA (Neon)
Primary windings 10, tapped at 8,25
Primary capacity 11,7 nF
Secondary diameter 75mm
Secondary windings 972, 0,39mm copper wire
Secondary winding height 380mm (5:1 ratio)
Resonance frequency about 350 kHz
Toroid diameter 28cm
Toroid inner diameter 9cm
RF Ground 70cm copper tubing

The coil is built using a 8kV/50mA neon sign transformer (NST). The primary coil is a conical type coil, built with 8mm copper tubing. The primary capacitor is build as stack of plates. I have built 2 capacitors, which are used in parallel to obtain 11,7nF capacity. The dielectricum is 1mm, consisting of 10 layers of 0,1mm PE foil. The capacitors are set into PP boxes, sealed and filled with transformer oil. The secondary coil is wound on a PVC tube with 0,39mm copper wire. The toroid is made from aluminium flexible tubing with 90mm diameter, which can easily be formed to a toroid. The main sparkgap is made in Richard Quick style. I used 7 pieces of copper tubing, which results in 6 gaps in series. Each gap is about 0,7mm distance. Remark: I think, that the small fan I use for quenching does not provide enough airflow for that purpose. It is a small Sunon CPU cooling fan. After having finished the design, I have doubts, that the gap performs as good as it could. I will do a design with a stronger airflow to check, if I can achieve a performance improvement.

Circuit diagram

The coil circuit is fed by the power control unit. Tr1 is the neon sign transformer, which delivers 2x4kV center tapped. The elements C1a, C1b, L1a, L1b, R1a and R1b build the filter unit. S1a and S1b are protection sparkgaps for the transformer. The main tank circuit consists of Cm, L2 and spargap S2. Sparkgap S3 is a protection sparkgap for the main capacitor. L3 is the secondary coil connected to RF ground and with the toroid on top.

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 operating coil.
Primary Capacitor
Primary Coil
Secondary Coil
Main sparkgap
Power control unit
Filter unit

Primary Capacitor

My primary capacitor consists of actually 2 plate stack type capacitors used in parallel.
Capacitor components
On the right, you see the PP box in which the stack is built in. Left to the box, you see the PE sheets and the aluminium plates.
Building the stack
The layers of the capacitor unit are setup with aluminium and PE sheets. To have all layers in exact position I have constructed a base, where the layers can be assembled.
Mounted stack
The mounted capacitor unit before mounting contacts and plexiglass top and putting into the PP box and sealing and filling with oil. The plates are pressed using 5mm Pertinax with 4 screws at the edges. This material is very hard and well suited for this purpose.
Plexiglass cover
The cover for the capacitors, holding the contacts. The cover is made from plexiglass.
Mounting the cover
Mounting the cover onto the stack. Contacting is done via the aluminium moulding by replacing 2 washers. You can see the 2 gaps where the moulding will be placed in. The screws from both sides hold the other washers in place.
Complete unit
One mounted capacitor unit in the PP box before sealing and filling with oil.
Finished capacitors
Both capacitors finished. The yellow color in the boxes is the transformer oil.
I ran my first full power test on April, 21th. When opening the sparkgap to the last (6th) gap I found, that one of my capacitors had problems. Sometimes there was a short flash in the unit and the oil showed some bubbles. But the unit still remained operable. I still could use the capacitor, but I will build new ones. I don't think, that there is a real breakdown of dielectricum, but the 10mm overhead of PE seems to be too low. I will build a new capacitor.

Primary Coil

The primary coil assembly is made of chipboard. There are 10 turns of 8mm copper tubing mounted in a conical form with an angle of 20 degrees.
The primary coil assembly made from chipboard. The base plate is 50x50cm sqare and there are 6 supports for the copper tubing.
The primary coil assembly, now black painted and with isolators of silicon sheet (red). This type of design was not good. See the description of problems in the construction page.
The primary coil, completely mounted. You see the aluminium moulding on top of each of the 6 supports, which were needed because the copper tubing had too much pressure and destroyed the wood. Note, that there is not yet a strike protection ring.
Problems with the primary coil assembly.
During my tests I encountered problems with my primary coil due to insufficient isolation. Although I isolated the contact points of the copper tubing with silicon sheets and silicon glue, there occured sometimes arcing. The weak point in the design is, that I use many screws between the windings to fix the tubing. The distance between the windings turned out to be too small.

Secondary coil

The secondary coil is wound on a PVC tube (walls about 1mm thin) with 75mm diameter. I used 0,39mm copper wire and wound 972 windings.
The PVC tube for the secondary coil. The PVC tube is for use in gutters. The ends are closed with disks of white PVC. On the lower right end, you see the screw coming out of the tube.
The top of the secondary coil. The white plastic cylinder is mounted on the screw, and the top carries a flat copper plate, to which the upper end of the wire is soldered. This is the toroid contact point.
The bottom of the secondary coil, with the contact to the RF-ground.

Main sparkgap

The main sparkgap is built with a PVC tube in which segments of copper tubing are mounted. This is the type of gap described by Richard Quick.
This picture shows the components needed to build the main sparkgap. I use 7 pieces of copper tubing, mounted in a PVC tube. This is the type of sparkgap described by Richard Quick. A fan is mounted at the end to provide cooling and quenching.
The main sparkgap mounted on the circuit board. In the front you can see the small fan (maybe too small for good quenching).

Power control unit

The power control unit is build as a separate box, used to control the voltage applied to the coil. It contains a variac for this purpose. Two instruments are used to display voltage and current. A safety key is used to prevent unauthorized use. Powering on the variac always resulted in a too large peak of current. So I provided an NTC to limit the current while switching on the unit. This has one disadvantage: you must wait before turning on the unit again after switching off to allow the NTC to cool down, otherwise the currents will again be too high.
This picture shows the power control unit front plate. On the left, the variac, on the right instruments as well as switches.
This picture shows the opened power control unit seen from the back side. On the right, you see the blue variac transformer, on the left the instruments and the switch relay. On the bottom, the input and output plugs are mounted.

Filter unit

The filter unit is needed to prevent high voltage of high frequency to strike back to the power supply. As you can see in the circuit diagram above, the filter consists of 2 capacitors, 2 resistors, 2 chokes and 2 sparkgaps.

Filter unit assembly

The filter unit is assembled on a PVC plate.
This picture shows the filter unit plate. The capacitor is not mounted yet. You can see the inductivities and the resistors, and in the back, there is the sparkgap, made of plexiglass and screws.

Filter unit capacitor

The parts needed to build the filter unit capacitor. The stack is pressed with pertinax plates like the main capacitors, too. On the left, you see the aluminium sheets.
This picture shows the capacitor for the filter unit before mounting in the case and filling with oil. The contacts are fixed on a plexiglass top plate. The original case was build from plexiglass, but I had problems with that after some time. See construction page. So I built a new box of PP, photo below.
This picture shows the capacitor for the filter unit mounted in a self made polypropylen case filled with transformer oil. See construction page.
This picture shows the capacitor for the filter unit mounted in the original plexiglass case, which resulted in problems. See construction page.

Operating coil photos

This part shows pictures of the coil in operation, taken during first test phase on 21.04.2000. The photos have been taken with an aperture setting of 2.8. Exposure time was variable for several seconds by hand. Film material ASA200. Please click on the small photo to view a larger version.
1 Wait for darkness 2 First shot 3 Another shot
4 5 6
7 Free arcs 1 8 Free arcs 2 9
© 1999 by Herbert Mehlhose