This article is very old, from around 2006. It was originally published on my old website, flylab.ovh.org, now defunct. It’s unlikely to have much educational value. I’ve republished it mainly for sentimental reasons.
I’d like to describe the High Voltage Generator (HVGen 2.0) that I built using two ignition coils salvaged from a car junkyard. You can conduct some interesting experiments and demonstrations using this device.
Construction
The HVGen consists of three main parts: a signal generator, a high voltage block, and a power supply.
List of required components
| Part | Description |
|---|---|
| Two car ignition coils | The coils should be identical, but it’s not strictly necessary. However, they must have exactly the same primary winding resistance. |
| Power MOSFET transistor | It should have the drain-source resistance (RDSon) as low as possible, high power, and UDSmax of more than 600 V. I’ve used IRF740. Buy more than one because you may easily burn them. |
| CMOS 4011 | |
| 2x standard low power diode | For example, popular 1N4148. |
| Resistors | 2x 4.7 kΩ and 1x 10 kΩ. |
| Capacitors | 1x 68 nF, 2x 100 nF, electrolytic 6800 μF 50 V and 470 μF 16 V. |
| Rectifier bridge 20 A | Or high current diodes. |
| Power supply transformer 24 V 100 W | I used an isolation transformer 230 V -> 24 V. |
| Stabilized power supply 12 V | Needed to supply the signal generator. |
| Other stuff | Prototyping boards, wires, insulating tape, suitable cabinet etc. |
Schematic
The ignition coils are connected in parallel with opposing phases, so the voltages between the HV terminals add up. The rectified 24 V AC results in an approximately 30 V DC supply.
Signal generator
This circuit can be assembled on a universal PCB.
Potentiometers and R2 and R4 should be of the linear taper type (characteristic A).
They are used to adjust the frequency of the pulses and their width.
To protect the generator from electromagnetic interference, it should be placed at least 20 cm away from the ignition coils and the transistor, or be enclosed in a metal grounded box.
It’s supplied from its own 5 V supply.
I changed the supply to 12 V because the MOSFET’s gate voltage needs to be at least 8 V to open completely.
I also experimented with another generator, based on NE555.
Keying transistor
The power MOSFET transistor should have low drain-source resistance (RDSon), high power, and UDSmax of more than 600 V. The back EMF may destroy it if it is a low-voltage one. I’ve used a very good IRF740 with a large radiator. After a few minutes of operation, it gets fairly hot. If the RDSon is low, the transistor won’t get very hot.
Ignition coils
You need two car ignition coils with a primary winding resistance of 1.5-4 Ω, just like the ones shown in the photos. They must have exactly the same primary winding resistance. Otherwise, one coil will take the majority of the current and may burn out.
Assembly
First, you should check the signal generator to ensure it works properly. Connect it to the power supply, then to a small speaker or headphones. There should be a buzzing sound. You may also use an oscilloscope to observe the shape of the pulses.
The cables connecting the high power supply, the transistor, and the coils should be as short as possible and made of thick wire, Φ1 mm in my case. As for the I-shaped chassis, I made it from thick Novotext sheets (in Poland, the material is called Tekstolit).
HV connections should be made with specialized HV cable (for example, salvaged from old TVs or car ignition systems). In my own construction, I used ordinary cable because the connections are short and distanced from other components. On the top, there are two screw terminals. Insulate these connections properly, as an arc may appear on the surface of the plastic otherwise.
Experiments
Here are some experiments I performed using this device. Of course, they would look more impressive in the dark. Be careful, as you might accidentally touch a HV element when working in the dark.
Electric spark
The simplest experiment: You need two thick wires with sharpened ends. Attach them to the HV terminals, so the ends are spaced 2 cm apart. When you start the device, you’ll see a continuous spark between them. Try to determine the maximum distance between the wires at which the spark still appears. In my case, it was 4 cm.
Jacob’s ladder
A Jacob’s ladder is a device that consists of two V-shaped electrodes and a HV power supply. The electric arc starts between the electrodes at the bottom, and due to the fact that hot air rises, it travels upwards along the diverging rods until they are too far apart for the voltage provided by the power source. Then, the arc extinguishes, and the process starts again. It is very eye-catching, which is why it was used in Frankenstein movies.
My version of this experiment consists of two 50 cm copper wires. It is 44 cm high. The distance between the electrodes at the bottom is 2 cm, and at the top, it’s 3.5 cm. A low-quality animation shows the working device.
Kirlian photography
I’ve experimented with a simple kirlian camera. I placed a sheet of thermal paper between a coin and an aluminum sheet, which were connected to the HV terminals. However, I didn’t obtain any good results.
Sparks inside the light bulb
A very interesting effect is to wind some copper wire around a light bulb and connect one end to the generator’s terminal and the base to another. This creates nice sparks inside the bulb. I used a 200 W light bulb and a Φ0.2 mm wire. However, the device worked for only a few seconds before the bulb broke.
