The BONSAI coil


This page describes my third coil model. I named it the "BONSAI COIL" because it is an extreme small coil with a secondary diameter of only 3cm.
The bonsai coil showed a very good performance compared to the bigger models.
The technical specs
Circuit diagram
Construction details, plans and photos
Primary Capacitor
Primary Coil
Secondary Coil
Main sparkgap
Operating coil photos

The technical specs

The following information is about the technical specifications of my coil
Power consumption 400VA
Transformer 7.5kV/50mA (NST)
Primary windings 9.5
Primary capacity 2.2 nF
Secondary diameter 29.5mm
Secondary windings 950, 0.15mm copper wire
Secondary winding height 145mm
Resonance frequency about 1070kHz
Toroid diameter 120mm
Toroid thickness 25mm
RF Ground no real RF ground is used... ;-)

The coil uses a 7.5kV/50mA neon sign transformer (NST). This is a transformer type not sealed with tar. The primary coil is a conical type coil, using 2.8mm diameter copper wire.

Circuit diagram

The coil circuit is very simple. A variac feeds the NST. I must admit, that I did not build a filter unit for NST protection with this coil and also no safety gaps are used :-) (Maybe I'll do later). So it is the simplest setup you can imagine.
As sparkgap, I use the old model 1 gap.
I did not use an RF ground for the first tests I did at home. There are some interesting effects I will describe later on.

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.
Primary Capacitor
Primary Coil
Secondary Coil
Main sparkgap

Primary Capacitor

The primary capacitor is built as MMC type. 2 strings of 10 22nF/1600 Philips MKP/MMKP 376 capacitors have been built, but only 1 string is used resulting in 2.2nF capacity.

Primary Coil

The primary coil assembly is completely made of polystyrene, 5mm thick. There are 9.5 turns of 2.8mm copper wire mounted in a conical form with an angle of 20 degrees. Everything is glued together, no screws are used.
The coil is wound using the same technique used for model2. The baseplate is 16cm x 16cm.

Secondary coil

The secondary coil is wound on a PVC tube (walls about 1mm thin) with 29mm diameter. I used 0.15mm copper wire now and wound and wound about 950 windings. With my winding machine it took less than 5 minutes to complete the secondary (it can wind 4 turns per second).

This picture shows the isolation for the secondary base made from a film box. The secondary diameter perfectly fits into this. This part is glued with silicone for isolation.

Main sparkgap

Bonsai uses the static sparkgap (Richard Quick design) that was initially built for model 1. It shows quite good performance at this very low level of power. See the description here for more details.

Operating coil photos

The coil has been tested and performed surprisingly well for my impression. I would have expected less performance because of higher losses due to the quite high frequency of about 1MHz. Sparklength is about 15cm or more (measured strikes to a metal next to the coil)
The NST is far too large with 50mA, but I do not have a smaller one.
Up to now the coil has been operated without a traditional RF ground inside my apartment. But as the photos below show, the vice draws small arcs to ground.
The explanation for this effect is, that the vice provides a capacitive coupling to ground. This means, I actually have an RF ground, which is capacitively coupled.
I now use an aluminium plate of about 20 x 50cm lying on ground, connected to the secondary base. This eliminates the arcing to ground.
I once forgot to connect this plate and had an open secondary base, i.e. really NO ground at all. This resulted in a breakthrough between base of secondary and primary inner turn. This breakthrough even went through the isolation I build around the lower 3 cm of the secondary.

The "ground"

Instead of the RF ground, this vice for table mounting is used and connected to the secondary base. A metal rod can be connected to this vice to attract streamers. The white cable connects the brass rod to the secondary base.

Running

You can see on this picture, that the vice draws arcs to the floor when the coil is running. This is a very intersting effect. Even the cable draws arcs through the isolation (but no burnt holes!).

Running coil

You can see the streamers running to the brass rod.

Firing BONSAI

These streamers over 15cm long!.
© 2001 by Herbert Mehlhose