DIY High Speed Flash
This DIY project uses a high voltage system to create a very bright and short burst of light which is ideal for high speed photography. Standard flash guns will illuminate a subject for a relatively long time which will create a blur when photographing high speed events such as bullet impacts or exploding objects.
By using this homemade high speed flash, it is possible to generate a very short burst of light which is bright enough to allow you to capture high speed events with a typical SLR or DSLR camera.
How It Works
The flash of light comes from the electrical breakdown of ordinary air at atmospheric pressure. When a large enough capacitor is discharged through an air gap, the intense current will briefly generate a burst of electromagnetic radiation covering a wide spectrum. The optical part of this spectrum is seen as a white flash of light just as in lightning discharges.
The circuit uses two high voltage inverters which will convert 12V DC into about 20kV. One inverter is used to charge up a large high voltage capacitor which will be used for discharging energy as a flash of light. The second inverter circuit is used to create a brief high voltage pulse that will trigger the discharge of the main flash on demand.
The Charging and Trigger Circuit
The primary inverter consists of a Power Pulse Width Modulator circuit which is used as an ignition coil driver to power a small High Voltage Spark Coil. The output of the ignition coil is fed through a HV Diode and a small inductor so that it can charge up a bank of large high voltage pulse capacitors. The capacitor terminals are connected to a pair of electrodes that are spaced a little larger than the spark could normally jump when the capacitor is fully charged.
The second inverter circuit is used to briefly ionize the air around the two main electrodes which will allow the capacitor to discharge. Another Power Pulse Width Modulator circuit is set up to drive an ignition coil whenever it is activated by a button or sensor. The output of the ignition coil is directly connected to a wire which is placed near the main electrodes.
When the flash fires, the capacitors may not be fully discharged. It is possible to fit something known as a bleeder resistor between the capacitor terminals. This resistor will slowly disharge the capacitor when not in use. The resistance will have to be very high; something like 100M ohms should be suitable for this.
The High Speed Flash Tube
The high speed flash tube consists of two main electrodes on the outside of a small glass tube, plus a third electrode inside the glass tube so that it is isolated from the others.
The main electrodes are made by wrapping some wire around the outside of the tube. The wire used must be thick enough to withstand the high current and heat of the capacitor bank discharge. You could also use something else for the electrodes such as a pair of washers or nuts.
The cables from the electrodes need to be fixed so that they come away from the central tube and do not come close to each other, or anything else. They should be well insulated, especially once they are outside the tube. We used some silicon cable to feed directly from the capacitor to the electrodes and then added extra insulation by feeding the cable through some PVC tubing. This insulation was able to prevent any accidental discharge between the high voltage cables when in use, but should not be considered as safe for handling when live.
The third electrode is a piece of standard solid core wire which is stripped bare at the end. The length of bare wire should be about the length of the gap between the main electrodes. This wire is fitted inside the small glass tube so that the bare section is between the two main electrodes. The other end of the wire leads out of the tube and connects directly to the ignition coil. It can be quite useful to mount the high speed flash tube directly to the end of the HV coil so that it is not necessary to have another large insulated HV cable.
Using the Circuit
Before building the circuit, it is important to become familiar with the PWM Circuits being used for the ignition coil drivers. The setting of the duty control will determine the charge voltage and rate of charge. Setting this too high will overheat the the circuit or damage components.
The circuit should be powered from a 12V power source capable of delivering at least 3A. Check our notes on power supply considerations as there are tips that can help improve performance. This circuit is a high voltage circuit so the GND connection should be suitably earthed. If using a mains operated supply such as a lab bench PSU, this earth will most likely be built in.
The diagram above shows momentary push to make switches being used to charge and fire the circuit. To charge the flash, the 'charge' switch is held down for a few seconds. The exact time need will depend on number of capacitors used and the duty setting of the PWM. There is no built in method to determine if the capacitor bank is charged, so this will come down to user experience with the system. To fire the high speed flash the 'fire' switch just needs to be pressed briefly. It is possible to add a sensor instead of a push switch for activating the flash. The sensor just needs to be wired so that will pull low the INT connection of the trigger PWM when the flash needs to fire.
When using the system is is essential to have a clutter free work area, and to prepare for your photographs carefully. This will improve safety and also increase the chances of getting a good picture. The flash tube and capacitors should be fixed securely in position and mounted in such a way that someone could not accidentally touch or get too close to live parts.
To get the high speed photographs with an ordinary camera, the camera shutter needs to stay open so that an image is only exposed when the flash illuminates the subject. Most DSLR cameras will have a 'bulb' setting that allows you to keep the shutter open as long as is needed. It is of course essential that when performing DIY high speed photography like this, the room is totally dark (except during the flash) while the camera shutter is open.
The short bright flash from the tube when fired will be very bright and should never be viewed directly. It might also be helpful to wear sunglasses if the subject will be viewed when the flash is activated.
REMEMBER: This project involves high voltage, high current electrical discharges, potentially exploding glass shards, loud bangs, bright flashes, and must be operated in darkness! It is VERY dangerous! This project is here for educational purposes, we do not recommend that you attempt to replicate this project. You are entirely responsible for the way you use the information presented on this website.