- Improved -

Solid State Tesla Coil   
Robust operation at 270kHz / 600W / 230Vac

 

 


Introduction

The first solid state tesla coil design I made was functional, but not very robust,
this circuit design is an improved version of my first solid state tesla coil.
The following  improvements were made compared to the previous design:

  1. No output transformer, but inductive coupling

  2. IRF2110 half bridge IC

  3. Ultra fast 8 amp MUR840 diodes (BYV229 were too slow!)

  4. BIG 20 Amp schottky diodes:

The circuit runs at 270 kHz with a continuous input power of about 600Watts (I measure the voltage and DC current
after the bridge rectifier). It does not fail in capacitive mode operation (Foperate < Fresonance).
The MOSFETs with heatsink become quite hot, the diodes just warm.
At 600 Watts nice streamers of 12 cm come out of the screw I placed on top of the metal sphere, the noise is limited
to a nice low hissing 50Hz hum.
Statistics: I blew up only one pair of BUZ326 MOSFET's when I boosted the power above 1 kW by reducing the
number of primary windings...........

Circuit description

The tesla coil driver circuit can be divided in 4 parts:

  1. An oscillator running at 10 x f output built with a 4046 VCO.
  2. A Non-overlap drive signal generator built with a 4017 Johnson counter and 2 4002 NOR gates
  3. An IR 2110 level shifter circuit with bootstrap supply.
  4. The power ouput stage with series/parallel diodes to disable the MOSFET body diodes.

1) The Oscillator

The oscillator is built with a 4046 VCO. The 4046 also has a PLL on board, but it is not used here.
The frequency range is set at 0 - 5MHz by R5,R6 and C5.
The output frequency is proportional to the voltage on pin 9.
For the time being I use a multi-turn 10k potentiometer to set the frequency, eventually it can be replaced by a regulating
circuit which measures the mains current.

2) The non overlap drive signal generator.

The non overlap is the time that both transistors are off: to prevent cross conduction due to switch off delays
it is necessary that both T1 and T2 for a short time during each transition.
This function is built with a hef4017 and a hef4002: output0--9 of the 4017 will consecutively become high;
this results in two non overlapping 40% duty cycle drive signals on pin 1 and 13 of the hef4002.
The clock signal of the 4017 must be 10 x the desired output frequency of the half bridge.

3) The IR2110 level shift circuit

The IR2110 IC is an integrated level shift IC with 2 Ampere MOSFET drivers. The signal Hin controls the Hiside MOSFET T1,
Lin the Lowside MOSFET T2.

C5 is added as buffer capacitor for the low side MOSFET driver.
C4 is the buffer capacitor for the high side driver. C4 is charged via D9, D5 and T2 each time when T2 is conducting.
D9 must be an ultra fast high voltage diode, regular fast types run hot and fail!
D3,D4, F2 and F3 protect the drive circuit when the MOSFETs explode.

4) The MOSFET power output stage

The BUZ326 are MOSFET's not really state-of-art, but I did not have any bigger FET's at the moment.
It is essential that the MOSFET body diodes of the output stage are disabled because they have a very sluggish
reverse recovery characteristic. The body diodes are disabled by diodes D5 thru 8: Schottky diodes D5, D6 block
any reverse current while the ultra-fast, diodes D7, D8 take over this reverse current.
Without this diode construction the mosfets will be destroyed instantly when the load has a capacitive character:
one mosfet will pull out the huge reverse charge of the body diode of the other fet, at 300 kHz this results in excessive
power dissipation. The MUR840 diodes are ultra-fast types which can handle this situation.
 

Tesla coil half bridge circuit schematic: (right click to save)

Picture of the assembled circuit:

 

Tesla coil description

Tesla coil secondary:

The tesla coil secondary coil is made with 1750 wdg 0.6mm diameter copper wire coiled on a 80mm diameter PVC pipe.
The top of the coil is connected to a double steel sphere of ..mm diameter.
(I got the spheres of an old double floor lamp, but any conducting sphere or toriod will do)

Tesla coil primary:

The primary is made with 16 wdg 1.5 mm2 PVC insulated wire tightly wrapped on on a 100mm diameter PVC pipe.
The primary coil is placed around the tesla coil secondary at about 10 cm above the first winding (see picture).
This coupling method is much easier than using an output transformer with ferrite core as I did in the previous coil design.

The number of windings is determined by trial on error; at 50 windings, the power was limited to about 200 Watts,
16 windings resulted in 700 Watts. When the 16 windings are stretched a little (e.g. 3 mm space between the windings),
the maximum power was more than 1000 Watts (5 amp DC at 230 Vac).
 

Picture of the tesla coil setup:

Circuit application

First try the half bridge circuit without the primary coil at a low voltage using a DC supply (20-40Volts or so),
the DC supply current must be very low at all frequencies (<100mA).

When this works you can connect the primary and the tesla coil secondary. The low end of the tesla coil
secondary must be connected to earth (safety earth of the wall outlet).
Start at low voltage again and sweep down slowly starting from Fmax ; at a certain frequency, the
DC supply current increases significantly: this is the tesla coil resonance frequency.

-Note 1: The 4046 VCO is quite sensitive to capacitive coupling from the tesla coil so use a plastic screwdriver
to adjust the frequency.
-Note 2: There is no minimum frequency, do not sweep too far, e.g. stop at about 50kHz!

When this works, you can try higher voltages, I use a variac to increase the voltage/power gradually.
I read on other tesla coil pages that wall dimmers also work, but note that you are working with mains voltages!

Picture of the tesla coil in action, movies are available on my main home page!

Download specifications of key components at the  Component specs page

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