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Copyright © 1994-2008
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DISCLAIMER
The power supplies for electronic flash and strobe equipment operate at extremely lethal voltage and current levels. The energy storage capacitors in even the smallest disposable camera flash operating from a 1.5 V AA battery can be deadly under the wrong conditions. Line powered strobes have added danger of high power at high voltage AND are often non-isolated (no power transformer. Do not attempt to troubleshoot, repair, or modify such equipment without understanding and following ALL of the relevant safety guidelines for high voltage and/or line connected electrical and electronic systems.
We will not be responsible for damage to equipment, your ego, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury or worse that may result from the use of this material.
PART I provides a basic description of the characteristics and principles of operation of electronic flash and related devices based on the xenon flashlamp. Especially important if you intent to be working inside this equipment is the SAFETY information. It is all too easy to electrocute yourself on the energy storage capacitors or line powered circuits.
PART II deals with troubleshooting and repair with emphasis on the kinds of electronic flash units found in photographic equipment - from tiny disposable cameras to high power studio 'speed lights'.
PART III provides information on the design of small to medium size electronic flashes and repeating strobes including basic design guidelines, shortening or lengthening flash duration, power supply component selection. There is a detailed discussion on retrofitting an old camera to use a modern electronic flash. And, there are a variety of circuits for repeating flashes, trigger circuits, inverters, and more.
PART IV provides over a dozen complete (well, very nearly complete, anyhow) schematics for electronic flash units from disposable cameras, external (Hot shoe or side mounted) strobes, higher performance line powered units, as well as repeating stroboscopes and even a timing light.
Note: Links to all the diagrams and photographs referenced from this document can be found in Sam's Strobe FAQ Files.
The single largest collection of hobbyist type xenon flash and strobe information can probably be found at the Don Klipstein's Lighting Technology Web Site which is a valuable resource for information relating to lighting technology in general and also includes additional articles dealing with strobe principles and design. Don's Xenon Flash and Strobe Page also includes a guaranteed late model version of Sam's Strobe FAQ.
A large collection of lighting and strobe related schematics and links can be found Tomi Engdahl's Lights and Electronics Page.
There are many other documents at the Sci.Electronics.Repair (S.E.R) FAQ Web site or one of its mirror sites which may be of use in the design, testing, and repair of strobe equipment. The Main Table of Contents (ToC) provides links to a variety of information on troubleshooting and repair of many types of equipment, general electronics, an assortment of schematics, over 1,000 technology links, and much more. Most of these documents are nicely formatted, indexed, and cross-referenced. (Silicon Sam's Technology Resource, which may be present at this site and others, usually contains slightly more recent versions of many of these same documents but most of those under the S.E.R FAQ Main ToC are easier to use and the actual content differences are likely to be minor.)
All modern electronic flash units (often called photographic strobes) are based on the same principles of operation whether of the subminiature variety in a disposable pocket camera, high quality 35 mm camera, compact separate hot shoe mounted unit, or the high power high performance unit found in a photo studio 'speed light'. All of these use the triggered discharge of an energy storage capacitor through a special flashlamp filled with xenon gas at low pressure to produce a very short burst of high intensity white light.
The typical electronic flash consists of four parts: (1) power supply, (2) energy storage capacitor, (3) a means of generating a trigger pulse, and (4) flashlamp, as shown below:
HV+ o------/\/\-------------+----------------+
Current | Anode _|_
DC Power Limiting C1 | | | |
Supply Resistor Energy _|_+ FL1 | ||
(300 V Storage --- Xenon | ||---- Trigger Pulse
typical) Capacitor | - Flashlamp | ||
| | | |
| Cathode'-|-'
HV Ret o-----------------------+----------------+
An electronic flash works as follows:
While details differ, everything from the flashing lights at your local disco to the flashlamps in monster pulsed lasers operate on essentially the same principles.
The energy of each flash is roughly equal to 1/2*C*V2 in watt-seconds (W-s) where V is the value of the energy storage capacitor's voltage and C is its capacitance. Not quite all of the energy in the capacitor is used but it is very close. The energy storage capacitor for pocket cameras is typically 100 to 400 uF at 330 V (charged to 300 V) with a typical flash energy of 10 W-s. For high power strobes, 1000s of uF at higher voltages are common with maximum flash energies of 100 W-s or more. Another important difference is in the cycle time. For pocket cameras it may be several seconds - or much longer as the batteries run down. For a studio 'speed light', fractional second cycle times are common.
Typical flash duration is a millisecond or less resulting in crystal clear stop action photographs of most moving subjects. However to capture really high speed motion like a the splash of a water droplet or a speeding rifle bullet flashes down to 1 millionth of a second or less are needed. These can be provided by specially designed strobe equipment but still based on principles very similar to those used in a pocket flash.
On cheap cameras (and probably some expensive ones as well) physical contacts on the shutter close the trigger circuit precisely when the shutter is wide open. Better designs use an SCR or other electronic switch so that no high voltage appears at the shutter contacts (or hot shoe connector of the flash unit) and contact deterioration due to high voltage sparking is avoided.
Note that for cameras with focal plane shutters, the maximum shutter speed setting that can be used (X-Sync) is typically limited to between 1/60 and 1/120 of a second. The reason is that for higher shutter speeds, the entire picture is not exposed simultaneously by the moving curtains of the focal plane mechanism. Rather, a slit with a width determined the by the effective shutter speed moves in front of the film plane. For example, with a shutter speed setting of 1/1000 of a second, a horizontally moving slit would need to be about 1/10 of an inch wide for a total travel time of 1/60 of a second to cover the entire 1.5 inch wide 35 mm frame. Since the flash duration is extremely short and much much less than the focal plane curtain travel time, only the film behind the slit would be exposed by an electronic flash. For shutter speed settings longer than the travel time, the entire frame is uncovered when the flash is triggered.
For complete schematics of both battery and AC line powered equipment, see the sections starting with: Schematics for Pocket Camera and Externally Mounted Compact Flash Units.
Red-eye reduction provides a means of providing a flash twice in rapid succession. The idea is that the pupils of the subjects' eyes close somewhat due to the first flash resulting in less red-eye - imaging of the inside of the eyeball - in the actual photograph.
This may be done by using the main flash but many cameras use a small, bright incandescent bulb to 'blind' the eyes when the shutter is pressed to meter, then it goes off and the flash preserves the 'closed' pupils. This approach works. Using the main flash would require sub-second recycle time which is not a problem if an energy conserving flash is used (see the section: Vivitar Auto/Thyristor 292 Energy Conserving Automatic Flash. However, it would add significant additional expense otherwise (as is the case with most cameras with built in electronic flash). A separate little bulb is effective and much cheaper.
Failure of red-eye reduction or the automatic exposure control circuits will probably require a schematic to troubleshoot unless tests for bad connections or shorted or open components identify specific problems. However, some of these use fairly simple circuits with mostly standard components and can be traced without too much difficulty. For red-eye in particular, It is also possible for that extra incandescent light bulb to be burnt out but good luck replacing it!
Remotely triggered 'fill flashes' use a photocell or photodiode to fire an SCR (or light activated SCR) which emulates the camera shutter switch closure for the flash unit being controlled. There is little to go wrong with these devices.
Cameras Underwater Flash Technical Primer has a summary of flash fundamentals is also be worth reading before delving deeper into this technology.
Automatic electronic flash units provide an optical feedback mechanism to sense the amount of light actually reaching the subject. The flash is then aborted in mid stride once the proper exposure has been made. This means that the flash duration will differ depending on exposure - typically from 1 ms at full power to 20 us or less at close range.
Inexpensive automatic flash units just short across the flashlamp with an SCR or second internal 'quench' tube (an internal small xenon tube that looks like an oversize neon indicator lamp) triggered by a photosensor. See the sections starting with: "Vivitar Auto 253 electronic flash circuit". With these units, the same amount of energy is used regardless of how much light is actually required and thus low and high intensity flashes drain the battery by the same amount - and require the same cycle time. The excess energy is wasted. Note that it is not the distance to the subject that matters but the amount of total light energy reflected back to the sensor. The travel time of the light has nothing to do with controlling exposure. True energy conserving flash units use only as much energy as needed and the batteries last much longer since most flash photographs do not require maximum power. Furthermore, when using low power flashes, the cycle time is effectively zero since the main energy storage capacitor does not discharge significantly. Therefore, multiple shots can be taken in rapid succession. See the section: Vivitar Auto/Thyristor 292 Energy Conserving Automatic Flash.
Many energy conserving flash units use a clever approach to avoid having to interrupt the 100 AMPS or more that may be flowing through the flashlamp. Like the non-energy conserving type, they bypass current around the flashlamp at the instant that the flash is to be terminated. But rather than dumping the energy to ground and wasting all of it, the current is diverted into a small capacitor. The voltage across the flashlamp drops to a low value just long enough for the flashlamp to revert to a non-conducting state. Only a small amount of energy is lost (that which goes into the bypass capacitor). More sophisticated units use something like a Gate TurnOff Thyristor (GTO) or high power Insulated Gate Bipolar Transistor (IGBT) to actually interrupt the flash discharge at the proper instant. These save virtually 100 percent of the energy and the circuitry is actually simpler, but the cost and availability of GTOs or IGBTs with the required peak surge rating of 100s of AMPS are a consideration in their design.
Although the high voltage inverter and actual flash tiggering circuitry is usually easy to trace, failure of the automatic exposure control circuit itself will probably require a schematic to troubleshoot unless tests for bad connections or shorted or open components identify specific problems. However, some of these use fairly simple circuits with mostly standard components and can be traced without too much difficulty though the compactness of modern flash units makes this somewhat more of a challenge. The most likely failures are still in the power circuits, not the control.
There are two potential hazards in dealing with the innards of electronic flash and other xenon strobe equipment:
High voltage with high energy storage is an instantly deadly combination. Treat all of these capacitors - even those in tiny pocket cameras with respect. Always confirm that they are discharged before even thinking about touching anything. On larger systems especially, install a shorting jumper after discharging just to be sure - capacitors have been known to recover a portion of their original charge without additional power input. Better to kill the power supply than yourself if you forget to remove it when powering up.
Reading and following these recommendations and heeding the warnings is especially important when working with high power strobes.
See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment.
A working electronic flash or strobe may discharge its capacitors fairly quickly when it is shut off but most DO NOT do this. Furthermore, do not assume that triggering the flash fully discharges either the power supply filter or main energy storage capacitors fully - especially if it is a sophisticated automatic unit.
The main filter capacitors in the low voltage power supply may have bleeder resistors to drain their charge relatively quickly - but resistors can fail. Don't depend on them. For battery powered equipment in particular, efforts may have been made NOT to bleed the energy storage capacitor to conserve on battery power should another shot be desired at a future time. Some units even keep the flash fully charged when supposedly turned off!
The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).
Then check with a voltmeter to be double sure. Better yet, monitor while discharging.
Obviously, make sure that you are well insulated!
For the power supply filter capacitors or main energy storage capacitors, which might be 400 uF at 350 V, a 2 K ohm 25 W resistor would be suitable. RC=.8 second. 5RC=4 seconds. A lower wattage resistor (compared to that calculated from V^^2 / R) can be used since the total energy stored in the capacitor is not that great (but still potentially lethal).
The discharge tool and circuit described in the next two sections can be used to provide a visual indication of polarity and charge for TV, monitor, SMPS, power supply filter capacitors and small electronic flash energy storage capacitors, and microwave oven high voltage capacitors.
Reasons to use a resistor and not a screwdriver to discharge capacitors:
A suitable discharge tool for each of these applications can be made as quite easily. The capacitor discharge indicator circuit described below can be built into this tool to provide a visual display of polarity and charge (not really needed for CRTs as the discharge time constant is virtually instantaneous even with a multi-M ohm resistor.
Again, always double check with a reliable voltmeter or by shorting with an insulated screwdriver!
A visual indication of charge and polarity is provided from maximum input down to a few volts.
The total discharge time is approximately 1 second per 100 uF of capacitance (5RC with R = 2 K ohms).
Safe capability of this circuit with values shown is about 500 V and 1000 uF maximum. Adjust the component values for your particular application.
The following schematic is available as a PDF in Capacitor Discharge Indicator Circuit or in ASCII, below.
(Probe)
o-------+
In 1 |
/
\ 2 K, 25 W Unmarked diodes are 1N400X (where X is 1-7)
/ or other general purpose silicon rectifiers.
\
|
+-------+--------+
__|__ __|__ |
_\_/_ _/_\_ /
| | \ 100 ohms
__|__ __|__ /
_\_/_ _/_\_ |
| | +----------+
__|__ __|__ __|__ __|__ Any general purpose LED type
_\_/_ _/_\_ _\_/_ LED _/_\_ LED without an internal resistor.
| | | + | - Use different colors to indicate
__|__ __|__ +----------+ polarity if desired.
_\_/_ _/_\_ |
In 2 | | |
o-------+-------+--------+
(GND Clip)
The two sets of 4 diodes will maintain a nearly constant voltage drop of about
2.8-3 V across the LED+resistor as long as the input is greater than around
20 V. Note: this means that the brightness of the LED is NOT an indication
of the value of the voltage on the capacitor until it drops below about 20
volts. The brightness will then decrease until it cuts off totally at around
3 volts.
Safety note: always confirm discharge with a voltmeter before touching any high voltage capacitors!
If the rotation rate of the wheel is such that one spoke or slot goes by a given position in exactly 1/24th of a second, the wheel will appear stationary since successive images will be identical. If it is moving a bit faster than this it will appear to be moving forward slowly. However, if it is going a bit slower, then it will be appear to be turning backwards slowly. The shorter the exposure with respect to the total frame time, the sharper will be the apparent effect. The number of slots per second of perceived motion will be equal to the difference in frame rate and number of slots per second passing a given point. So, a roulette wheel rotating such that 23 slots are passing by per second captured on a 24 frame per second camera will appear to be moving backwards at 1 slot per second.
The same applies to the use of a strobe light to freeze repetitive motion like the rotation of a shaft. It is all a matter of the relative speed of the sampling (the movie, video or strobe) with respect to an object which is periodic like a roulette or wagon wheel.
You can perform a simple experiment: run an electric fan under a fluorescent lamp (one with an ordinary magnetic ballest). The light from such a lamp is not continuous but pulses 120 times per second. Watch for stationary or slowly rotating blade patterns as the fan speeds up and slows down. See if you can compute the speed of the fan from this behavior.
Just for funsies, I decided to see how much torture I could inflict on the flashlamp and energy storage capacitor from one of those little Kodak cameras. The tube was 1.2" long, in a metalized plastic reflector, with a thin metal backing to hold it in. The capacitor was 120 uf, 330 V. I hooked it up to my inverter (12 V->300 V at high current) and fired 'er up! Pop, pop, pop, pop, pop, pop, pop, (turn up trigger oscillator frequency) popopopopopopopopopopopop! It was firing about 30 or 40 times a second; it appeared as it was constantly on! I turned it down to about 15 flashes a second, and let it run. First thing I noticed was that wonderful scent of melting acrylic. Then, I noticed that the tube was kind of skewed in the reflector. The plastic was in full smoke-mode by this point. Still, the tube kept firing! (Let's see: 5 W-s times 15 flashes per second is 75 W average power, not bad for an itty bitty tube --- sam).
I left it on a bit more, and the plastic really started the smoke-signals! I noticed that one electrode was glowing cherry red. Even after all this torture, it kept going! The smoke was getting too much, so I hit the 'off' on my inverter. A few more gouts of smoke, and the little fire I created was extinguished. I let it cool down and then I examined the damage. The reflector was totaled; the tube had all but melted clean through. When I touched it, the little metal plate popped off.
On closer examination, the tube appeared to be in good shape. I couldn't see any visible damage to either the electrodes, or the glass seals. A quick test reveals that the tube still functions. As a side note, the storage capacitor got quite hot; probably around 35 degrees C. All in all, an interesting test, I must say. The next will involve connecting up a normal NE2 neon bulb and observing the results of high voltage and high current on it. I suspect it will be quite spectacular, so I'm taking precautions - It will be performed in a proper enclosure, so if the neon decides to really go 'pop', it won't do any damage.
I used to design stage-effects, and played some time with strobes. Built a number, from 750 W-s at high rates to 22,500 W-s single flash. Philips makes xenon lamps, designed for photographic use - they are not flashtubes but burn continually - so using them as flashtube shortens their life span (assuming you increase power). They are expensive, from $250 for the smallest to $1,100 for the biggest.
For caps for the smaller (20 to 100 W-s) strobes, I used a huge array of MKT motor-caps. 10 uF at 630V is cheap, a few dollars, and building an array of these is not too hard. These caps are screw-mount, and you can just fill a board with them, and switch them in parallel. Keeps the ESR low, which is a requirement.
The monsters used larger caps, 680 uF each. My boss often visited executory sales and bought components and machinery. These caps (he had a number of crates full of them) sat on the shelves for a year before I decided to do something with them. Beautiful Siemens stuff. Very low ESR, large cap. Nevertheless, I did say "huge array" which was exactly that. Two boards (one for each set of lamps) filled with them, each board 1 by 1.5 meters, which dictated the size of the case. As I remember these caps were about 5 centimeters diameter, and something like 12 centimeters high, so - guessing - you could stack around 280 of them on a board, which sounds right.
All problems I had related to heat. The 750 and 1500 W-s models had a habit of melting their main wire. On typical stages one uses a lot of extension-wires, and its power consumption could be high enough to heat the extension to the point the insulation came dripping off, without blowing a fuse. Had to lower the amount of energy per flash at higher rates. The protective window in front was another problem area. Poly-carbonate covers work fine, but a single fingerprint absorbs enough IR to melt a hole in the cover. Glass won't melt, but shatters if dirty. Don't allow anything near it. Colored paper will catch fire within seconds, at max rate. Always use it to flash at a wall, never let the public look into such a bright flash.
The biggest used four larger tubes, flashing two by two, but charge-times were too long to make it usable as a real strobe. It was used to flash the ceiling of a large stadium. I considered it to be useless. I never solved the problems it had, like eating its eight, expensive, diodes (>$40 each) for lunch. It sucked dips in the mains, big enough to cause digital equipment to fail. Imagine all the effects of the audio-boys resetting after each flash. I kept it running for a few months, but when the edges of the window caught fire I scrapped it. I modified one off the triggers for the small strobe (750 and 1500 w-S) to allow multiple units.
I found a note in the same binder with a capacitor-free design for a strobe. The smallest Xenon tube made by Philips has a burning voltage low enough to start it on 220V. With a suitable choke and a diode in series it will burn - after ignition - for the rest of the half-cycle. The diode makes sure it dies when the polarity reverses. (Residual ionization will make it re-ignite without the diode) The choke will keep it from aggravating the utility-companies. I wonder if anyone knows a trick to enhance ionization. Fully ionized it has a burning-voltage of 50 Volts, but even after the 4 kV pulse it needs 200 Volts to get started. Only tricks I know are the 'normal' starter-pulse, microwave pulses, radioactivity and laser-pulses. Only the first one is acceptable with audiences around. It probably won't work with 110 V mains.
(From: Tomi H. Engdahl (then@cc.hut.fi).)
A friend of mine has around a 3 kW disco stroboscope. (It is really 3 kW as it blows a 230 V, 10 A fuse at full power. And you can guess that it is quite bright!) That stroboscope seems to be taking quite heavy (few tens of amperes) of current for one half wave when it flashes. The firing angle is controlled by the internal brightness control (dimmer).
What it looked inside on quick glance it seemed to have a heavy thyristor, rectifier, heavy line filter, one coil in series with the tube, the tube itself, and the triggering electronics.
(From: Sam.)
On 110 VAC you're out of luck if no caps are allowed and you don't want to use a stepup transformer. In any case, what you end up with is more of an arc lamp than a flashlamp since the current is limited to a few amps as opposed to 10s or 100s of A for even a tiny strobe.
WARNING: Defibrillators are at least as good at stopping beating hearts as restarting misbehaving ones. The charge in their energy storage capacitor (typically 300 to 400 Joules) is enough to kill a half dozen healthy adults instantly. The operating voltage (up to 5 kV) doesn't respect common wire insulation and can jump 1/4" or more in air. There are no second chances.
(From: Steve Roberts (osteven@akrobiz.com).)
Older defibrillators are now showing up as inexpensive surplus because their ancient edmark waveform is being replaced with newer computer controlled biphasic waveforms.
So what do you get in a typical edmark waveform defibrillator:
Notes: The relay is usually a 5 kV 50 A DPDT which has a short across one set of contacts to protect the patient. The other set of contacts goes to the capacitor common leads and to the patient via the paddles. So, presto! - apply 12 volts to the relay and you get up to 360 joules dumped into the victim or patient via the inductor to control the waveform. A patient's chest is assumed to be about 50 ohms impedance via the conductive cream to the paddles so the test circuit monitors what happens when the second smaller relay dumps the cap into the 50 ohm air cooled test resistor. The cap is also dumped during power-down.
I can't overstress the absolute need for safety when handling a 33 uF 5 kV capacitor. Newer defibrillators have a MOSFET H-bridge for bipolar switching and only go to two kV with smaller caps.
(From: Don Klipstein (don@misty.com).)
I actually busted some smaller flashtubes in a cup of vegetable oil to get an idea of the xenon pressure! In the Radio Shack U-shaped tubes, the pressure is about 80 Torr. In a smaller cheap linear tube with the electrodes 19 mm. apart, the pressure is about 180 Torr. In at least one version of the tiny tubes often used in cheap and disposable cameras, the pressure is about 450 to 500 Torr. Most other small camera flashtubes are 100 to 300 Torr.
In many medium and large flashtubes, the pressure seems to be around 80 Torr, except one I have seems to have a little less - maybe 60 Torr. This was for three different professional photoflash tubes, a larger version of the popular U-shaped strobe tube, and a large photocopier flashtube. I estimated the pressure in these by passing a few milliamps through them from a neon sign transformer (operated at reduced voltage) and commparing the appearance of the discharge to that in tubes of known pressue.
Larger photographic flashtubes - mostly around 80 to 200 Torr:
EG&G, Electro-Optics Division 35 Congress St Salem MA uses a standard pressure of 450 Torr in their superduper linear flashtubes, but won't hesitate to use a custom pressure at customer's request.
"I had an interesting idea. Get a small (2 watt) fluorescent tube, wrap several turns of wire round it, connect ends of tube to xenon strobe outputs, connect wire to HV trigger connection, switch it on. As far as I can tell, this would make one hell of a bright strobe! Any ideas???"Depending on the voltage and size of the tube, you may not need trigger - it will break down at lower voltage than xenon tube. For your 2 W tube, this is a certainty.
It will also only likely work for one or at most a few flashes unless you use a much smaller capacitor. What happens is the filaments disintegrate. Fluorescent tubes are NOT designed for the high peak current of a strobe type circuit with a large energy storage capacitor. Go much beyond their normal ratings of a few hundred mA and they will fail.
Been there, done that. I once powered an 8 foot fluorescent tube from a six volt lantern battery pulse circuit and stepup transformer - and even such a large tube was destroyed after a few flashes. But the flashes WERE pretty bright. :-)
(From: Don Klipstein (don@misty.com).)
It is not as bright as xenon. Also, triggering characteristics will change as strobe duty changes the condition of the electrodes, also as temperature changes and mercury vapor concentration changes. I've tried it - the fluorescent tube changes too much with temperature and past history of strobe use.
One more thing: A strobing fluorescent tube is more conductive than a xenon tube, which means more of the energy stored in the energy storage capacitor is used to heat the capacitor, and less is dissipated in the tube. But enough is dissipated into the tube to beat up the electrodes!
(From: Craig Douglas (blackspear@hotmail.com).)
You can very successfully flash banks of fluorescent lamps by running a continuous low current through the lamps and increasing the current (the lamps appear off, but are actually still lit). This works well in the electric sign industry.
The same principle is used to flash standard incandescent bulbs, without the bulbs blowing continually. It is the spike in initial power that blows the filament, running a low current continually through to keep the filament warm minimizes stress on the filament which stops it from blowing.
(From: Sam.)
It should be noted that this will not increase lamp life significantly in normal operation but may extend it by a few percent.
Mouser stocks a few xenon flashlamps and trigger transformers suitable for both small and medium power strobes.
A flashtube, trigger coil, and a more complete camera flash assembly are listed in their catalog.
Some strobe kits, flashtubes, reflectors, flashtube-reflector combos, a trigger coil, a quench tube (!!), two different inverter transformers, and two complete strobe schematics, one of which is a 12 volt strobe using one of these transformers.
The also sell small flashtubes by the bushel :-) about 1.2 inches long (~30 mm) by .15 inch (~3.5 mm) diameter. These cost 49 cents each, or 100 for $25. So, if you are planning on building your own New Year's Times Square celebration sphere, these may be ideal! These were offered in 1996 and may no longer be available but should be worth an inquiry.
High power capacitors (like 450 uf at 500 volts) and other strobe parts may be had though the list of strobe service centers at Lumedyne.
However, it should be remembered that they are repair centers and do not normally sell parts at retail. I have ordered a capacitor like the one mentioned above from one them at a cost of $26.00 plus $3.50 S&H.
(What this means is that (1) their prices may be quite high and (2) they may not be eager to sell to the public. --- sam)
The original Strobotac flashtubes were made by EG&G Optoelectronics. They used a FX6-A. I believe they now supply a FX7-A as a replacement. You can reach them at 1-800-950-3441.
You might also try Quad Tech, which still manufactures the General Radio 1531AB, and other General Radio stroboscopes. They can supply spare parts. You can reach them at 1-800-253-1230.
The going rate for a typical cheap flash camera is generally $.50 to $1 at a garage sale or flea market. While these may in fact still work, they often use 110 size film so you won't feel too badly about gutting them for the flash unit or its parts.
Although in principle the capacitor may deform after a long period of non-use, I have yet to see any real trouble having picked up over 2 dozen cameras and strobes from these sources. None of these have had any actual defective components (though a couple had bad connections or broken wires). My last acquisition was a completely functional variable rate stroboscope for $2.
(From: Scott Johnston (sj@thor.iac.net).)
Complete working strobe circuits are available for *free* at photo developing places (not K-mart, but the expensive places that actually do the developing in-house). When they develop film from those cheap weekend disposable cameras (you know, the kind that are made out of plastic and cardboard?), they rip out the film and throw away the camera housing. The disposables with flashes have a complete xenon strobe circuit (triggered by a tiny little switch on wire leads) powered by a single AA (1.5v) alkaline battery. Recently, I called the local photo developer, asked if they could save some of the kind with flashes, and a few days later I picked up a pile of 12 complete flash units, with almost unused AA batteries in all of them! Really fun, although I discovered quickly that the capacitors in those things don't have bleeder resistors...
(From: Alfred C. Erpel (aerpel@ptd.net).)
I was picking up my Halloween party photos from the 1 Hour Photo place at my local drug store and I noticed a trash box full of thrown out single use cameras, empty 35mm spools and plastic containers. I asked if I could have the entire box.
When I got home I found I had 27 cameras with usable flash units and most of the AA batteries were still good. My wife got the plastic containers for her Girl Scouts crafts. The inside of used Kodak film canisters contains a nifty spool which may make a useful bobbin for some types of coils.
Watch out for residual charge on the flash capacitor when you disassemble these! Also, observe the mechanics carefully because, although I'm not certain about this yet, it seems that some of the cameras are designed to purposely disable the flash circuitry by mechanically breaking an existing connection when the board is removed. Obviously this could be restored if you see where it is.
No doubt some places won't give you their trash (afraid of the potential for liability), but it can't hurt to ask.
Information is available for driving flashlamps (and other topics) in the Perkin Elmer (formerly EG&G) Technical Library. However, much of the product and technical info that used to be on the EG&G Web site is no longer present. but this material is available on the Perkin Elmer CDROM, which includes complete product specifications and technical papers. The CDROM is accessed using your normal Web browser. Some flashlamp info is also available at Polytec PI France - Department Electro-Optique - EG<G.
General technical information on flashlamps and arc lamps may be accessed via their Download Page.
Some very complete technical notes on driving and triggering of flashlamps has been published by ILC Technology (now part of Perkin Elmer). Some of these include:
These were originally published around 1986 so there may be newer versions. As far as I know, they are not currently on-line but should be available in print by contacting ILC.
A variety of failures are possible with electronic flash units. Much of the circuitry is similar for battery/AC adapter and line powered units but the power supplies in particular do differ substantially.
Most common problems are likely to be failures of the power supply, bad connections, dried up or deformed energy storage or other electrolytic capacitor(s), and physical damage to the to the flashtube or other components.
Symptoms: unit is totally dead, intermittent, or has excessively long cycle time.
Test and/or replace batteries. Determine if batteries are being charged. Check continuity of power switch or interlock and inspect for corroded battery contacts and bad connections or cold solder joints on the circuit board.
Symptoms: unit is totally dead or loads down power source when switched on (or at all times with some compact cameras). No high pitched audible whine when charging the capacitor. Regulator failure may result in excess voltage on the flashtube and spontaneous triggering or failure of the energy storage capacitor or other components.
Test main chopper transistor for shorts and opens. This is the most likely failure. There is no easy way to test the transformer and the other components rarely fail. Check for bad connections.
Symptoms: unit is totally dead, operates poorly, catches fire, or blows up. Spontaneous triggering may be the result of a regulator failure or running on a too high line voltage (if the unit survives).
Test outlet with a lamp or circuit tester. Check line voltage setting on flash unit (if it is not too late!).
Symptoms: unit is totally dead or fuse blows. Excessive cycle time.
Test fuse. If blown check for shorted components like rectifiers and capacitors in the power supply. If fuse is ok, test continuity of line cord, power switch, and other input components and wiring. Check rectifiers for opens and the capacitors for opens or reduced value.
Symptoms: reduced light output and unusually short cycle time may indicate a dried up capacitor. Heavy loading of power source with low frequency or weak audible whine may indicate a shorted capacitor. Excessively long cycle time may mean that the capacitor has too much leakage or needs to be reformed.
Test for shorts and value. Substitute another capacitor of similar or smaller uF rating and at least equal voltage rating if available.
Cycling the unit at full power several times should reform a capacitor that has deteriorated due to lack of use. If the flash intensity and cycle time do not return to normal after a dozen or so full intensity flashes, the capacitor may need to be replaced or there may be some other problem with the power supply.
Symptoms: energy storage capacitor charges as indicated by the audible inverter whine changing frequency increasing in pitch until ready light comes on (if it does) but pressing shutter release or manual test button has no effect. Spontaneous triggering may be a result of a component breaking down or an intermittent short circuit.
Test for voltage on the trigger capacitor and continuity of the trigger transformer windings. Confirm that the energy storage capacitor is indeed fully charged with a voltmeter.
Symptoms: flash works normally but no indication from ready light. Or, ready light on all the time or prematurely.
Test for voltage on the LED or neon bulb and work backwards to its voltage supply - either the trigger or energy storage capacitor or inverter trans- former. In the latter case (where load detection is used instead of simple voltage monitoring) there may be AC across the lamp so a DC measurement may be deceptive.)
Symptoms: manual test button will fire flash but shutter release has no effect.
Test for shutter contact closure, clean hot shoe contacts (if relevant), inspect and test for bad connections, test or swap cable, clean shutter contacts (right, good luck). Try an alternate way of triggering the flash like a cable instead of a the hot shoe.
Symptoms: energy storage and trigger capacitors charges to proper voltage but the manual test button does not fire the flash even though you can hear the tick that indicates that the trigger circuit is discharging.
Some xenon tubes have "getters", which are silver or dark silver coatings of a highly reactive metal, deposited on the inner surface of the flashtube at one or sometimes both ends. Less frequently, a getter may be found on a metal surface such as one of the electrodes inside the tube, but not on the tubing inner surface. The getter "gets" any traces of air or water vapor in the flashtube. If a flashtube with a getter is broken or leaky, the getter will be corroded into a powdery gray-white form. If you know there is a getter and it is corroded badly, the flashtube is no good. Please note that unrelated glass discoloration or staining that resembles corroded getters can occur in a heavily used or moderately abused flashtube that still works.
Inspect the flashtube for physical damage. Substitute another similar or somewhat larger (but not smaller) flashtube. A neon bulb can be put across the trigger transformer output and ground to see if it flashes when you press the manual test button shutter release. This won't determine if the trigger voltage is high enough but will provide an indication that most of the trigger circuitry is operating.
For rechargeable units, try charging for the recommended time (24 hours if you don't know what it is). Then, check the battery voltage. If it does not indicate full charge (roughly 1.2 x n for NiCds, 2 x n for lead-acid where n is the number of cells), then the battery is likely expired and will need to be replaced.
Even for testing, don't just remove the bad rechargeable batteries - replace them. They may be required to provide filtering for the power supply even when running off the AC line or adapter.
For units with disposable batteries, of course try a fresh set but first thoroughly clean the battery contacts.
See the sections on batteries.
The energy storage capacitor will tend to 'deform' resulting in high leakage and reduced capacity after long non-use. However, you should still be able to hear the high pitched whine of the inverter.
Where the unit shows no sign of life on batteries or AC, check for dirty switch contacts and bad internal connections. Electrolytic capacitors in the power supply and inverter may have deteriorated as well.
If the unit simply takes a long time to charge, cycling it a dozen times should restore an energy storage capacitor that is has deformed but is salvageable. This is probably safe for the energy storage capacitor as the power source is current limited. However, there is no way of telling if continuous operation with the excessive load of the leaky energy storage capacitor will overheat power supply or inverter components.
When a flashlamp fails, it may do so quietly or with a bang.
Generally, only laser pump flashlamps or similar ones with a lot of flash energy for their size will likely die spectacularly. When lower power flashlamps such as those used in small to medium size photographic strobes crack, they tend to stay in one piece or sometimes break apart surprisingly quietly.
Line voltage transformers: Most AC line powered flash units don't have any transformer so this isn't general a problem. For those that do (higher speed or other special types of strobes), it shouldn't be difficult to match up the secondary voltage and find a standard replacement that will be acceptable. These may be cobbled together from the power transformers for vacuum tube equipment (yes, they can still be found), small isolation transformers with multiple windings, and possibly the addition of some additional lower voltage windings in buck or boost phase to adjust the output voltage.
For safety reasons, I don't recommend attempting to repair transformers connected to the AC line, though this may be a possibility if all else fails.
Inverter transformers in battery powered flash units:
There is virtually no chance of successfully repairing any of these. The secondary winding uses wire so fine that it's almost impossible to even handle it. With a decent coil winding machine, a new spool of #45 or so wire, proper insulating tape (these are wound in 10 to 20 separate layers), and a few days of patience, it can be done but doesn't rank up there on my "fun things to do list". :) Furthermore, it's almost certain the core got destroyed in attempts to get at the windings. Thus, replacement is the only viable option.
There is NO chance of getting one of these from an electronics distributor as they are all custom. Since it's almost a certainty that the original manufacturer will have little interest in selling you a new one, salvage from other flash units is the best hope. These can be $1 garage sale specials (other 35 mm, 126, or similar cameras), disposable camera flashes, or shoe mounted units, depending on the physical size and energy (guide number) rating of your broken flash. The main problem will be the number of turns on the primary. If you can match those up by adding or removing turns to your replacement, there is a good chance it will work since they all seem to have roughly the same number of secondary turns (probably around 1,600 to 2,000). Even if the primary is buried, you can still add turns on top of the secondary in the appropriate direction to adjust the total net turns. Once its running, adding or removing an additional turn or two may be needed to tweak the output voltage.
Another option is to transplant the entire inverter if one can be found that operates on the same input (battery) voltage. I've done this successfully. without problems. See "Repair Brief #100: Minox ME1 Electronic Flash for Minox B Camera - Dead" in the document: Sam's Repair Briefs - Complete: 1 to 100.
Trigger transformers: Fortunately, these are fairly standard. Just match up the input voltage and select one that has an adequate output voltage for your strobe - 4 to 5 kV for most small strobes should work. The only remaining thing that needs to be determined is the wiring polarity. While the strobe may work with either polarity of the trigger pulse, one may result in reliable operation. Electronics distributors like DigiKey and Mouser should have a suitable replacement if a garage sale or disposable camera isn't handy.
Among the features that may be found there are:
Don's site is constantly evolving so more interesting articles will likely appear in the future.
See the section: Flashlamp and Arc Lamp Manufacturers and References for links to specifications as well as externsive technical information and application/design notes.
Specifications for the 1300 series linear flashlamps can also be found in the chapter on solid state lasers in Sam's Laser FAQ.
Most small flashlamps will operate on about 300 V (some as low as 250 - or less). If the flashlamp voltage is too low, the tube may not fire reliably or at all. If the flashlamp voltage is too high, spontaneous firing or damage and/or shortened flashlamp life due to excessive current may be the result. For power, you will need one of the following:
WARNING: If left charging for longer than needed to get the ready light to come on, the actual voltage on the energy storage capacitor may approach 400 V with some of these cameras! Take even more care.
An SCR can be substituted for physical switch contacts where electronic control of the trigger is desired. For the battery powered unit, there is no issue of line isolation and the cathode of the SCR can be tied directly to the ground of your logic circuits. However, with the line operated strobe, isolation is essential for safety - use capacitor or transformer coupling, or an optoisolator.
Some of the highest speed photographs using the light source to control exposure have been taken with spark gaps operating at many kV resulting in flash durations as low as fractions of microseconds. Even higher speed photography is possible using electronic image tubes. The first instants of conventional or nuclear detonations have been captured using this type of technology.
For more information on high speed photography, see the classic works by Harold E. ("Doc") Edgerton. The following are just some general comments:
Several design parameters influence flash intensity, duration, and maximum repeat rate. However, the relationships are not linear as a flashlamp is a gas discharge device with complex nonlinear resistance characteristics. It is necessary to consult the flashlamp manufacturer's data sheets to do any detailed design.
The guidelines above will adequately handle typical small to medium size strobes - perhaps to 50 W-s or so depending on the extent to which the flashlamp maximum energy specifications exceed the power input you are using and the characteristics of other circuit components.
For higher power strobes, it is essential that appropriate flashlamps are used with photoflash rated capacitors. A series inductor - matched to the flashlamp, capacitor, and voltage - is critical to preserving the life of some flashlamps (perhaps beyond one flash!) and achieving maximum flash intensity. The flashlamp manufacturer's datasheets are probably the best source of this information. Also see the section: Super High Power (Laser Pump) Strobe Circuit.
The series inductor is often needed for laser pumping applications and other applications where the quantity of energy and/or the peak current are particularly great for the size of the flashtube.
For additional design information, see the section: Flashlamp and Arc Lamp Manufacturers and References as well as the chapters on solid state lasers in Sam's Laser FAQ.
Where you are designing a strobe requiring a specific pulse shape and/or duration, it is desirable to have a way of measuring its output. If you have an oscilloscope (almost any will do), the following can't be beat for simplicity and cost - total component complement is a green LED (hooked up backwards to act as a photodiode) and a 3.3K Ohm resistor! I assume a red or other color LED would work just as well but haven't tried one.
Even better would be to use a "proper" silicon photodiode. There are probably several kicking around in your junk box from computer mice or from a VCR (beginning/end of tape or "tape in" sensor). Or you can buy an inexpensive ($2) photodiode from an electronics distributor.
The following schematic is available as a PDF in Strobe Light Output Test Circuit or in ASCII, below.
PD1
+5 to +15 VDC o--------|<|--------+----------o Scope Input
Green LED |
or Silicon /
Photodiode \ R1
(Note polarity) / 3.3K
\
|
Common o-------------------+------------o Scope Ground
Adjust the value of R1 and/or the location of PD1 relative to the flashlamp so that that the voltage across PD1 doesn't go below 2 or 3 V on the peak of the light pulse. Or equivalently, that the output voltage doesn't approach the power supply voltage.
The response of this circuit is quite decent, easily showing the shape of the light pulse in strobes with pulse durations in the 10s of microseconds. Voltage is proportional to light intensity as long as it doesn't approach the value of the power supply voltage (so there is still some bias on the LED or photodiode).
Note that even with the power supply removed and the inputs shorted together, there is a photovoltaic response to light, but it quickly saturates and may give a false indication of the shape of the light pulse.
(From: Don Klipstein (don@misty.com).)
(The following is from: Kevin Horton (khorton@tech.iupui.edu))
This is *always* the kicker. I have devoting heavy amounts of time into figuring out how these transformers work. They are very, very special. *nothing* else will work in their place, or if it does, it'll be woefully inefficient. They are usually .4" or so cubed, but may be larger. The gap on the core seems to be pretty critical- it limits the overall current that the circuit will draw. In one particular strobe I disassembled, they had a 100 pf cap coming from the output of the HV winding directly tied to the base of the drive transistor! I finally figured out why: it controlled the frequency vs voltage of the oscillator, hence giving it more current as it was completing a charging cycle!
I've disassembled many of these small transformers. Unlike most ferrite transformers, these are usually held together by dipping them in wax, rather than varnish. Some transformers have the primaries wound on the core, while others have it on the outside. I haven't figured out exactly why this is. However, one one transformer I took apart, the feedback and drive windings were wound on the core; bifilar. The feedback was 11 turns, while the primary was 10. Both were #24. On top of that was thousands of turns of #40 or so wire.
It seems that the small sizes play a part in the efficiency of these transformers; since the magnetic field is contained in such a small core area, the losses are small.
Depending on the circuit, the required voltage rating may be anywhere from the peak of the AC waveform to the maximum output voltage of the circuit (both with a safety factor). Using the highest value will always be safe and not that expensive for modest size capacitors.
For the capacitance value:
However, for anything with more stages or stages arranged where some of the capacitors are effectively in series with the output, analysis can become interesting (translation: I am not about to attempt it here!).
In these cases, the impedance of the capacitors at the line frequency (60 Hz in the U.S.) will affect the power available before the output drops and/or has excessive ripple.
My very rough rule of thumb just treats the impedance of the capacitors like a series resistance. Then, I would select the capacitor value so that this resistance is small compared to the needs of the circuit.
A 1 uF capacitor has an impedance (magnitude) of about 2.65K ohms at 60 Hz. A 10 uF cap is 265 ohms. The 22 uF capacitors in the tripler described in the section: Higher Power Photoflash with SCR Trigger have an impedance of about 120 ohms. Consider a load of 100 W at 350 VDC (average - which would be a high power strobe indeed). The load resistance is then: R = V*V/P = 1.22 K. Since this is large compared to the capacitor impedance - even if all capacitors are assumed to be in series - I wouldn't expect very much improvement with the use of larger capacitors.
Keep in mind that this is an edge of the envelope calculation so a factor of 2 (or 20) either way is possible (and likely!).
The ESR (Effective Series Resistance) of smaller electrolytic capacitors is also higher. This may result in excessive heat dissipation in the capacitor. There is also a 'ripple current' rating for capacitors which should not be exceeded. However, if your capacitors are from Radio Shack, this particular specification is probably not available :-).
A surge limiting resistor on the line input should be provided to limit the peak current through the diodes and capacitors.
Once a particular circuit has been constructed, test it under a dummy load which simulates the expected average power. If the output voltage drops excessively and/or there is too much ripple, try increasing the capacitor uF values (not all of them may need to be changed.) Check the waveform on each capacitor with a scope (you MUST use an isolation transformer for this!). The voltage must NEVER go negative for an electrolytic capacitor. Feel the capacitors for evidence of excessive heating.
Also see the section: Voltage Doubler Design Considerations.
"I have a problem. I am using a standard voltage doubler (2 diodes, 2 capacitors) in a strobe circuit. The doubler consists of two 4.7 uF 450 V caps and two 1N4005 diodes. The timing circuit is a neon-bulb relaxation oscillator that triggers an SCR, which in turn dumps a .1 uF cap into a trigger coil to fire the flashtube. The flashtube gets a 47 uF cap discharged through it, which equals about 2.5 watt-seconds.(From: J. M. Woodgate (jmwa@thenet.co.uk).)The problem is that the 4.7 uF doubler capacitors overheat and fail! The doubler voltage is 325 volts with no load, so a 450 V rating should be adequate. Should I be using more capacitance for these?"
Well, you really haven't given enough information. The problem is likely to be that you are exceeding the ripple current rating of the caps. I guess that you are running the neon and SCR from the doubler and your neon takes a thump of current when it fires, even if you have set the duty cycle down so that the average current is low. Higher value capacitors usually do have a higher ripple-current rating. But first you need to find how much ripple-current you are producing, and this depends on the cap. value (at least to some extent). Very roughly, the ripple current is pi times the DC current, and you should look at the load current waveform with a scope and take the maximum value as the DC value, to be sure of not over-running the capacitors.
When constructing systems where the size of the flashhead is critical and/or where the flashhead(s) are to be remotely located relative to the controller, one might be tempted to keep all the electronics with the controller.
While the energy storage capacitor(s) can be centrally located, this is not recommended for the trigger transformers as it is the risetime or dV/dt of the high voltage pulse that ionizes the gas and any length of wire - even if it is adequately insulated - will add capacitance - a few dozen pF may prevent reliable triggering.
Generally, the best approach is to locate the trigger transformer, its capacitor and associated charging resistors, and the trigger SCR (if used) in the flashhead.
This determines the minimum size of the wiring between the energy storage capacitor and flashhead(s) since its resistance should be low compared to this - say .1 ohm or less. For very long runs, the wiring inductance may also be a factor.
Care must be taken if using multiconductor cables to assure that cross coupling from the high discharge current pulse(s) doesn't result in false triggering where multiple flashheads or other circuits are involved that aren't supposed to be activated simultaneously.
The typical trigger capacitor is only about .022 uF. The turns ratio of the trigger transformer is often about 13-plus, and has been known to exceed 20. Divide .022 uF by the square of the turns ratio, and this is the maximum capacitive load which will probably get a majority of the normal trigger pulse voltage. So you may want the stray capacitance of the high voltage trigger line to maybe not exceed maybe 150 pF or possibly as low as 60 pF! And that is optimistically! If you run the trigger wire along other wires or close to grounded or even remotely coupled-to-ground conductive surfaces, you can get this much capacitance in just a few feet (or maybe a meter) of wire. In the unlikely event you can run the trigger wire through the middle of the air with nothing nearby, you may get OK results with a few times this distance.
If the trigger capacitor is larger, you can get more distance without loading down the peak trigger voltage too much - but there may still be bugs! If you try increasing the trigger capacitor, you risk blowing up/out parts of the trigger circuit. And just to sometimes put the odds against you for use of a long trigger line - the stray capacitance can slow down the risetime of the trigger pulse. Sometimes fast risetime is essential for the trigger pulse to get enough peak current through the tube to make the xenon conductive enough to flash at the main voltage. Sometimes not and you may get away with things...
The compact inverters in pocket and disposable cameras and externally mounted flash units will charge an energy storage capacitor to about 300 to 320 VDC. What if you need more? Yes, it is possible to wire several of these in series.
The trigger circuit can be one of those associated with any of the inverters or a totally separate unit.
(Replies from: Andreas Nowatzyk (agn@acm.org) and Sam).
(From: Local Echo (localecho@aol.com).)
"Yesterday, I hooked a small photoflash tube desoldered from a discarded photoflash unit to a power supply of 1.2 kV (Also using parts from photoflash units (I used 4 small Fuji disposables), mainly the inverters- the trigger capacitors were placed where the normal discharge capacitors were.). I noticed that at this voltage it would sometimes trigger on it's own, so I carefully adjusted things to conditions just before this happens. What's interesting is that I was able to trigger the tube with the trigger electrode (An alligator clip connected to a trigger transformer) up to 5 cm away from the center of the tube's wall. I was even able to trigger one with only 235 V across it even when the trigger pulse was generated from a separate circuit with no return path. (Though, the separate one was not able to trigger the tube at distances greater than .5 mm away from the tube's wall.) Also, it seems that the orientation of the trigger electrode makes a huge difference (Such as being parallel or perpendicular to the discharge path). Why is this? (I have a few ideas, but I don't wish to bias any answers.)"
(From: Sam.)
Interesting.... Realize you are running that tube at roughly 4 times its normal energy so don't be surprised if it explodes.>>
(From: Local Echo.)
"Yes, that was one thing I was worried about so I made sure the capacitance connected to the tubes was very low (is only .0025 uF). The arc is bright, but well confined."
(From: Andreas Nowatzyk.)
The energy is = 1/2 * C * U^2, so a much smaller capacitor at a higher voltage can have less discharge energy.
(From: Sam.)
The closer the tube voltage is to its breakdown voltage, the less trigger energy you need. So, at 1.2kV, you are on the hairy edge.
(From: LocalEcho.)
"Are the dynamics different from that of a tube not on the borderline (Such as commutation time, for instance)?"(From: Andreas Nowatzyk.)
Yes, pulse duration will be shorter. I experimented with flash tubes from disposable cameras for the purpose of generating short light pulses. Under normal operating conditions (electrolytic capacitor at 300V), the typical pulse duration was about 5msec. With 0.05 uF at 4 kV (low inductance, HV capacitor), pulse duration is down to 0.9 us.
Unfortunately, there is a downside to short pulses: they create a shock wave inside the tube that erodes it slowly. Micro-cracks are formed and eventually the envelope shatters. The tubes from disposable cameras lasted 20 minutes on average when pulsed at 25 flashes a second (with proper cooling).
This can be avoided with a pulse forming network, typically consisting of a series inductor and a diode to avoid ringing. However, that causes much longer pulses, say 250 us.
(From: Local Echo.)
"One other neat thing a friend noticed is that the trigger transformer isn't necessary at this point. We were able to trigger it by bridging a wire from one of the electrodes (In this case, the cathode) to the center of the tube. Eventually, the tube would become unresponsive (after about 20 or so discharges). So, still operating on the assumption that it's a capacitive effect, we used the anode connection instead. Instead of the multiple or fragmented arcs we normally saw, the discharge was very uniform (and occupied the entire volume of the tube). This seldom happened after that except an occasional arc of non-uniform density. This entire sequence can be repeated over and over. Also, the plasma seems to avoid the general area of the trigger electrode at times."(From: Andreas.)
Since the trigger pulse is capacitively coupled into the tube, instabilities about the trigger arise from charge that is deposited on the surface of the fused silica envelope. Hence a larger pulse with fast rise times is desirable to minimize jitter and get a reliable trigger. Because of this problem, flash tubes for strobe applications use internal trigger electrodes and large tubes - say for pumping a laser - are triggered via a trigger pulse superimposed on the discharge voltage.
(From: Sam.)
Since the idea is to ionize the xenon gas in the tube, it orientation would be critical especially as you move further away.>>
(From: Local Echo.)
"Which orientation would be best? (It seems to be perpendicular to the arc, but I could somehow be introducing bias.) Speaking of bias in a different sense, in certain schematics there are provisions for biasing the trigger electrode. I wish I knew more about it."
(From: Andreas.)
The trigger electrode is more effective if the capacitance between it and the plasma discharge path is maximized. Larger tubes tend to have trigger electrodes that consist of a small wire that is wrapped along most of the envelope. This provides a path for an initial discharge that is "fueled" by discharging the linear capacitor that is formed between the electrode, the silica wall and the discharge column. A DC bias generally doesn't do anything because *clean* fused silica is a good isolator. Things change when the tube is dirty or very hot.
(From: Sam.)
The fact that it isn't connected to a return is quite reasonable - with those sharp pulses, there is enough stray capacitance between the tube electrodes and your trigger circuit to create an adequate return.>>
(From: Local Echo.)
"I thought about that and I'm assuming that is correct. Although the trigger electrode was about 5 cm away from the tube's wall, the transformer is located about 4 cm from even that. Is the capacitance still significant?"
(From: Andreas.)
Hard to tell. Under these conditions, you should take the phase of the moon into account. Basically, any electrostatic disturbance can cause the discharge, as well as natural radioactivity: A tube that is run very close to its breakdown voltage becomes a crude Geiger-counter and can trigger by any change in the electrostatic environment. Note that the breakdown voltage will change a lot depending on the tube temperature, the charge that has accumulated on the envelope from prior discharges and erosion of sharp features of the electrodes of a new tube. For example, a new tube from a disposable camera can trigger a 2 kV. After running it for a while, you may find that it needs 5 kV.
But what about providing fixed, but selectable, flash energies? Over a range of perhaps 2:1 to 4:1 in flash energy, the input voltage to the flashlamp can be used to control flash energy. This range would be between the minimum voltage specification for the flashlamp and the self-triggering spec. However, this range can be extended by simply having a small fixed capacitor (call it C1) to maintain the required voltage for reliable triggering of the flashlamp backed by a much larger capacitor (or capacitor bank, call it C2) whose voltage can be varied to control the flash energy. The two capacitors are separated by a high current diode. Once triggered, C1 provides the initial discharge current to the lamp. As the voltage drops, current starts flowing from C2 and continues to do so until the voltage on the flashlamp drops below the maintaining voltage (usually about 50 V) with no inductance in the circuit. This simple approach can work over a very wide range of repeatable flash energies. There are only two catches:
Without redesigning the inverter circuit for higher power and using a larger flashtube, the only variable you have to play with is the size of the energy storage capacitor:
Since power dissipation is still limited by the inverter, the flashtube should not overheat. The only concern is that the trigger capacitor has enough time to charge up - check its time constant and reduce its charging resistor if necessary to assure that the voltage on the trigger capacitor is high enough (close to what it would be for the unmodified circuit).
What you DON'T want to do is use a higher voltage on the input. That would almost certainly blow the inverter transistor (either immediately or from overheating) and/or the transformer, energy storage capacitor, or flashtube.
Where reducing the size of the energy storage capacitor is not adequate, here are some guidelines for more extensive redesign:
(From: Don Klipstein (don@misty.com).)
A 15 watt fluorescent lamp choke ballast will probably work for this. This goes in series with the power feed to the capacitor, not in series with the flashtube. CAUTION: This inductor may cause a voltage overshoot of the energy storage capacitor - probably to your favor if the capacitor can take the extra voltage.
By my quick calculations, such a choke is order of 1 to 2 Henries of inductance so you could use an actual inductor if you have one handy. You won't beat the price though - a 15 W ballast is about $3.
Use two capacitors, with the inductor between their positive terminals, if the power feed requires a capacitor load. The first capacitor can be the larger value original energy storage capacitor. The second capacitor will be the low uF value one used for flashing, and will need to withstand extra voltage.
Caution: While it may be possible to totally eliminate (or greatly reduce size of) the series resistor in if you use an inductor, there is a chance of a meltdown if for some reason the arc didn't quench as might happen if the flashtube overheated - an inductor eventually looks like a short circuit to the power line while a resistor still has resistance :-). Make sure you have it fused!
Due to the drop in efficiency, trying to use this approach to create a continuous-appearing light source isn't worthwhile. It is easy to reduce the energy to a level that is safe to repeat 60 times a second, but a usual cheap glass flashtube will not be especially bright - almost certainly dimmer than a halogen lamp consuming the same amount of power. There may also be a slight "flicker" effect from the discharge being "sparklike" instead of uniformly filling the tube, and taking a different path through the tube on each flash.
All other factors being equal, flash duration is roughly proportional to the size - uF rating - of the energy storage capacitor.
Where lower flash energy is acceptable and/or the strobe can be moved closer to the subject and/or faster film can be used, the normal energy storage capacitor in your electronic flash can be replaced with one that is smaller. Flash duration and energy will then be reduced in proportion to the ratio of the capacitor's uF ratings.
Using a higher voltage will enable the uF rating of the capacitor to be decreased and still achieve the same total light output - the required uF (and flash duration) goes down as 1 / (V * V). Of course, since the same energy is involved, the physical size of the capacitor doesn't change much. There is no free lunch :-).
For example, the typical small electronic flash unit uses a capacitor voltage of about 300 V. Designing a strobe with a 3 kV energy storage capacitor will permit its uF rating and flash duration to be cut by a factor of 100!
High voltage flashtubes and capacitors must be used but the basic principles of operation of these strobes are unchanged. Power to charge the capacitors can be provided by a line operated transformer or high frequency inverter either directly using a rectifier or doubler, or diode-capacitor voltage multiplier. For ideas, see the chapters on helium neon lasers in Sam's Laser FAQ as the operating voltage requirements for HeNe lasers are similar. Where fast cycle time is not critical or your required flash energy is modest, one of the sample circuits may be acceptable.
Those pictures of bullets in flight were likely made with air spark gap light sources with 10s of kV on the energy storage capacitors resulting in flash durations in the microsecond range.
In some cases, simply adding an inductor in series with the flash tube can provide some increase in flash duration. However, where you want 20 ms instead of less than 1 ms, this is not going to work. If the inductor is too small it won't do much of anything. Once it starts to have an effect, the effect will be to simply cut off the flash.
What should work better (and I have not tried this) is to add a high current constant current driver between the capacitor and the tube. For example, assuming a small flash with say 500 uF at 300 V results in roughly a 200 A peak current assuming a 1 ms flash duration. This is an equivalent resistance of about 1.5 ohms! To extend the flash duration to 20 ms requires dropping the current to 10 A.
One way to do this is with a constant current series regulator set for 10A:
The following schematic is available as a PDF in Lengthening the Duration of a Flash or in ASCII, below.
+-------+ R1
+300 o---+----|- |-|---------/\/\------+-----------+
| +-------+ 1 | |
| FL1 | |
Main | +-----------+ |
Energy + _|_ C1 | Constant | +_|_ C2
Storage --- 500uF | Current | --- 10uF
Capacitor | 350V | Regulator | - | 350V
| +-----------+ |
| | |
| | |
Return o---+--------------------------------+-----------+
The use of a high frequency switchmode (buck) converter will almost certainly
be necessary unless you have some really HUGE transistors floating around in
your junk box. The problem isn't the voltage or current rating - a common
BUT12A would meet these requirements - but rather maximum power and SOA (Safe
Operating Region). Peak power dissipation in a linear regulator would be
about 2,500 W!
C1 provides a sink for the flash tube current until the regulator can start up. It may be necessary to play with their values to achieve reliable operation. An alternative is to bias the transistor from a separate power source prior to triggering.
Also see the section: Driving Continuous Output Xenon Arc Lamps.
Details are left as an exercise for the student :-).
Another alternative that may be adequate for some applications is to split the single high intensity short duration flash normally provided by a standard strobe circuit into multiple lower energy flashes spread over a longer time.
As a starting point, consider the schematic of an energy conserving flash like the unit described in the section: Vivitar Auto/Thyristor 292 Energy Conserving Automatic Flash. Bypass the light sensor to control flash intensity directly. I guestimate that on a single charge of the energy storage capacitor, you should be able to get 10 or so short flashes spread over, say