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Air Flash Unit

High Speed Photography - Air Gap Flash Unit

The problem with high speed photography is getting a standard camera to react fast enough to capture a short lived moment. There are several ways to work around the problem, one of them being high speed flash units. The idea here is that instead of using an exceptionally fast camera with amazing light sensitivity, one instead uses an exceptionally fast blitz along with garden variety camera. The camera is set up to take a long exposure with large aperture, in complete darkness. Sometime during the long exposure, the event you wish to capture takes place, and simultaneously triggers the blitz via a sensor. The blitz exposes the scene to an intense, but very short flash of light, which exposes the image. Since the flash duration is ultra short, only events which took place when the blitz was triggered are visible in the final image.

Update 10th Jan. 2013
See my high speed photos here!

The difficulty is creating a flash which is both bright enough to expose the image, and short enough to not cause blurring. A solution to this problem is to discharge a high voltage capacitor in a relatively high pressure (ie. 1 atm) gas. This creates a short-lived, but bright flash of light suitable for high speed photography. For anyone who's experimented with flyback transformers, the final setup will be easy to create.

completed air flash unit  Popping a balloon
The blitz here was triggered using the microphone trigger seen at the bottom right.

The air flash unit consists of a circuit for charging a high voltage capacitor, a circuit for triggering a discharge on demand, a high voltage capacitor and the air flash tube itself. All of these components can be made using scrap materials. The way the unit works is by first charging the HV capacitor to full potential. The flash tube contains two wires which are separated just enough to not spark over at max potential. There exists a third wire in the flash tube, which is isolated from the other two. This wire is connected to a trigger/pulse transformer, and when the blitz unit is activated, this pulse transformer ionizes the gap between the two capacitor leads. This causes the gap to breakdown and a spark to form, thereby creating a flash of light. If my setup is comparable to the one presented in Scientific American, the duration of peak intensity is only 300ns, which equates an exposure time of 1/3000000 of a second.

Air flash tube DIY  Air flash tube DIY

I constructed my flash tube using an olive jar and a glass test tube for vanilla sticks. The glass test tube is the most essential part, as the spark must travel along it's surface. By allowing the spark to surface track, it quenches much faster than an open air spark, shortening the blitz duration. At the same time a third wire must be brought in through the test tube, to ionize the gap between the capacitor leads on the outside. I soldered a piece of copper pipe to the trigger lead, so it had a more constant distance from the walls. The spacing between the capacitor leads is about 30mm. The greater the distance the greater the voltage needed to break the gap down, and the brighter the flash will be. 1mm of distance does not correspond to 1kV here, since the spark is tracking along a surface. I found at just 20kV, my capacitor would breakdown the 30mm gap when triggered. The gap would breakdown without external influence at less than 25kV. The voltage/distance you choose here will depend on the HV capacitor you use, and the capabilities of the flyback transformer you have. Remember that most flybacks are only designed to handle 30kV at the most, so stay below this if you want to get the maximum lifetime from your setup. When positioning the leads on the test tube, I found special care had to be taken in order to achieve high reliability. The HV electrode should be wrapped around the tube in a circular fashion, this reduces corona jets which would others pre-ionize the gap. The ground electrode and trigger electrode should be spaced apart, this is so the trigger pulse ionizes the glass between them. If the trigger electrode is placed directly under the ground electrode, it will be difficult to trigger a spark. The spacing between the trigger and HV electrode aren't critical. In the images above the ground and HV electrodes were swapped, otherwise they can be used as a guide.


Pulse capacitor  capacitor side view

The high voltage capacitor is a critical part of the air flash unit. In addition to a relatively large capacity at such voltages, it also needs to be pulse rated so the spark forms and decays quickly. In an attempt to save myself 80USD, I built my own high voltage capacitor. It uses seven rolled capacitors, of roughly 2nF each. They are built using nothing but duct-tape, tin foil and overhead transparencies. To keep them isolated from their surroundings, I placed them in the PP-pipe seen above. Four sheets of transparency were used between layers, giving it an approximately 40kV voltage rating. Instructions on constructing the capacitors can be found here.

schematic  Circuit and layout

The high voltage charging circuit is a current-mode controller which limits the primary current to about 5A peak. By adjusting the "High Voltage Adjust" potentiometer, you can vary the primary current from zero to max. In effect, this varies the output voltage. The supply voltage for the capacitor charger must be 18V if using a UC3842. This is because the chip has built in UVLO, and the IRFP450 can't have more than 20V on it's gate. If you use the UC3845, you can get away with 12V supply voltage for the capacitor charger. The flyback transformer is of the standard type found inside of old CRT monitors or TVs. It must be rectified to charge the HV capacitor. Since the power requirements are so low, I found it sufficient to use the internal primary. If using a low supply voltage, such as 18V, an external primary may be required. See my flyback driver pages for more details. The HV capacitor needs to be at least 15nF and 35kV, more capacitance gives more spark energy, and thus a brighter flash. This capacitor will be the most expensive component if purchased.

The trigger circuit works by discharging a small capacitor through a pulse/trigger transformer. This creates a tiny spark, similar to touching a door handle charged with static electricity. By placing a conductive object inside of the flash tube, and triggering the pulse transformer, the outside of the tube is ionized. This breaks down the air and causes the HV capacitor to discharge. The trigger transformer I used was a CD45 purchased from Information Unlimited. The supply voltage of 125V was acquired using a voltage multiplier on the 40V AC used to power the capacitor charging circuit. 40VAC was acquired using mains transformer found in an old stereo. To trigger the pulse transformer, simply short the two trigger leads together. If using a transistor to do this, keep in mind that the voltage on the collector will be about 25V when open circuit.

Completed unit blitzing camera

Links

High Speed Air-gap Flash - Hobby Robotics
August 1974 issue of Scientific American
Air-gap Flash - Wikipedia
Johannes Eriksson High Speed Photography




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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.