High voltage pulse generator (>30KV)

Introduction

This circuit was inspired by the reversed engineered : Electronic gas lighter. 

The idea was to build a circuit that can generate sparks with a length of several centimeters through air. This means we need an output voltage of several tens of kilovolts. The easiest way to generate this, is using a flyback transformer of an old black & white or color television. 

You can find lots of circuits on the internet in which a fly-back transformer is used in a self-oscillating configuration with a power transistor in order to generate a powerful continuous high voltage output. Quite some power is needed to continuously pump enough current into the primary winding of the fly-back transformer. 

In the following design, a pulsating high voltage output is generated instead of a continuous output, so power consumption is limited. The energy is first transferred to a capacitor, where it builds up over time, as if we are slowly filling a bucket. When the bucket is full enough, we suddenly empty it, in a very short time, into the output. So with limited power (low voltage and low current), we are able to build up enough energy to generate a high voltage spike of tens of kilovolts. After the bucket is emptied, it is slowly filled again and the process repeats itself.

Circuit

Description

The 5V CCFL driver, which is normally used as a backlight inverter for CCFL lamps, generates an AC output voltage of about 1KV. Most of the common CCFL drivers use a Royer oscillator with 2 transistors and a transformer with center-tapped primary winding and a separate feedback winding. The Royer oscillator operation is based on core-saturation to generate sinusoidal currents in both halves of the primary winding of the transformer.

The output of the CCFL is rectified with high voltage diode D1, that can withstand reverse voltages up to 1KV, so we obtain a positive output voltage. This positive output charges C1, that is grounded by the primary winding of T1. Because the CCFL can only deliver a low current, the voltage over C1 increases slowly.

R1, R2, R3 form a potential divider that divides the voltage over C1 by 3. The gas discharge tube G1 (type CG90) will break down when a voltage of about 90V is present over its terminals. The right side of G1 is at ground potential, because C2 is initially discharged. So when the voltage over R3 is about 90V, the gas discharge tube will break down and will trigger the gate of the silicon controlled rectifier (SCR) THY1 via C2. C2 creates a differentiator with R4, so when G1 breaks down, a small voltage spike appears on the gate of THY1, that will start to conduct and continuous conducting even after the gate is not driven anymore.

So the SCR is triggered when there is 90V over R3. This means that the voltage over R1+R2+R3 = 3 × 90V = 270V and that C1 is charged up to 270V when the SCR starts conducting. When the SCR conducts, it connects C1 parallel to the primary winding of T1, so the complete charge is dumped into this winding. This generates a voltage spike on the secondary of the fly-back transformer T1. When C1 is fully discharged and the current has dropped below the hold current of THY1, THY1 will stop conducting and C1 can re-charge again. This process will repeat itself, generating high voltage pulses at the output of T1.

The frequency of the pulses mainly depends on : 

1. the size of C1

2. the output current that the CCFL driver can deliver

3. the breakdown voltage of G1 or divider ratio of R1, R2, R3

It is possible to replace G1 with a neon bulb. Neon bulbs typically have a breakdown voltage of about 55 to 150V, depending on the type.

Pictures

Dead bug circuit assembly

Increased C1 to 200nF and mounted 2 high voltage transformers (from old black & white television) on 1 ferrite.
The setup is far from ideal, but was just for a quick demonstration.

Video

Using a high voltage coil and put a ferrite with a primary coil inside the coil.
Getting about 30KV.

Increased C1 to 200nF and mounted 2 high voltage transformers (from old black & white television) on 1 ferrite.
Getting about 75KV.