strobe tubes

[dacflyer]

i got another currious question...
i know how strobes work etc..
but what i would like to know is.. how is it that they can keep some certain lamps lit for a few seconds?
i thought a strobe tube could only be flashed..
was at a local disco and they had one that flashed
and dimmed,,then i say it stay on for 4 seconds..the ends were cherry red almost white after the lamp went out..
can someone tell me how they can make it stay lit so long? i am suspecting they useing a switch mode supply rather than charged caps.. and i also suspect high current/hi voltage too...

[Externet]

I would say that if the flashing rate is above the retina retention, it would appear as steady on, just as television. The durability of the tube should be seriously shortened too.
Next time you visit the place, move your vision across the room while the thing is on, you may discern some flashing.
Miguel

[Chris Smith]

Strobes work by dumping a high current and voltage from a cap, hence the single flash. <p>However, like any gas lamp if you supply a current and voltage that isn’t stored in a cap, but is produced continuously, it will light the lamp, and as you witnessed, it will also over heat which is why they flash them in the first place. <p>Its no different than a neon or a fluorescent tube except that it conducts a lot more current, runs brighter, and runs hotter. <p>I would also suspect the conventional dump of the current/voltage package would also have to be significantly higher to maintain the “on” or constant flow of electrons because as it heats up, Its impedance also changes [probably negativly] and thus it would or might stall out using a constant current level that normally was designed for a single pulse.

[rshayes]

There are several types of discharge in gas filled tubes. Neon lamps use the glow discharge, which occurs at moderate currents (milliamps). If the current is not limited, the discharge voltage drops, the current increases (amperes), and the discharge becomes an arc discharge.<p>Strobe tubes use the arc discharge. An arc discharge can be continuous. It can also be maintained over distances of several inches. Continuous operation generates a lot of heat.A continuous arc lamp is usually built with a quartz envelope and use either forced air or water cooling. This get expensive. It also takes several seconds to get a continuous arc lamp operating in a stable fashion.<p>If the arc is pulsed, the heat can be reduced to the point where a glass envelope can be used with normal air cooling. Most strobe lamps have limits on how many pulses can be used in a given time. Externet is probably right, it is probably a series of closely spaced pulses that appear to be continuous. The lamp may be driven at its maximum rate, and I suspect that the lifetime is short.

[dacflyer]

nope,, i checked,,it is a steady burn...but limited to 2-4 seconds , it is not a high frequency strobe when it appears to be on..it is actually a steady burn...
i did a little more research..police helicoptors use a NIGHT SUN search lamp...this lamp is actually a XENON lamp , i also read that this lamp ,if shined on the ground , while the helicoptor is on the ground , that it is hot enough to ignite the grass , i am sort of thinking that this strobe lamp i saw might have a simmelar power supply to make it possible to stay lit for a short time....Hmmmmm

[terri]

I wonder if they are using decaying rf to excite the gas and if the the final glow near the electrodes is really only like the glow which appears at the electrodes of a regular neon bulb when it's running normally, rather than the electrodes actually being white-hot.<p>Over-exciting a regular neon bulb will cause the entire gas volume to glow, but running them at normal current results in the desired glow only near the electrodes. <p>Any gas discharge tube can be run continuously if the current is limited for safe dissipation... I think the police searchlight might be just like the conventional neon bulb/limiting resistor circuit, but scaled way up --with appropriate cooling.<p>I ran one of the little Radio Shack flashtubes continuously once, but with a wimpy potted 5KV power supply which burned out because while the ballasting resistor I used was small enough to allow the lamp to glow continuously, the current demand was too great for the power supply, which was an unlabeled surplus junk part which cost US$2.00. That's how I found out it was so wimpy.<p>According to my (probably outdated) GE flashtube manual, the FT-617-40 and the FT-623, which were the biggest flashtubes GE used to make, dissipate 1600 watts with forced air cooling. These things run at 5.5 kV as a hold-off voltage with a quartz helix and a pyrex outer envelope. The helix is 5 turns of a quartz tube coiled into a length of about 4 inches and about 2 inches in diameter.<p>Flashtubes can also be flashed in parallel with appropriate triggering circuitry, and can also no doubt be run in continuous-glow mode in parallel with appropriate ballasting (current limiting.) <p>My sources on glow tube technology and flashtube technology:<p>"GE glow lamp manual" --Miniature Lamp Department, Product Planning and Application, General Electric Company, Nela Park, East Cleveland 12, Ohio.<p>"GE Flashtube Data Manual" --Photo Lamp Department #281, Nela Park, Cleveland, OH, 44112<p>Note the address for the glow lamp manual predates the Zip Code, but my later copy of the flashtube manual has a valid zip code.

[rshayes]

An old textbook distinguishes between the glow discharge and the arc discharge by the type of cathode emission. The glow discharge uses emission caused by bombardment of the cathode by positive ions. This requires a substantial voltage drop to accellerate the ions and is a relatively inefficient process. The arc discharge uses thermionic emission, where part of the cathode is hot enough to emit electrons without an external field. This reduces the total voltage required relative to the glow discharge.<p>If the strobe tube is operated continuously, the cathode could very easily reach incandescent temperatures. The trick is to stop operation before the metal to glass seals fracture.<p>The police searchlights use an arc in Xenon gas. The electrodes are probably closely spaced to give a short arc that can be tightly focussed by an optical system. Long Xenon arcs can be used to pump lasers. Overall efficiency used to be terrible, about 1 kilowatt into the arc for a laser output of about 5 watts. The police searchlights probably use similar power levels. My best recollection is that the arc voltage was about 60 volts. The current would be about 16 amps for a 1 kilowatt arc.<p>Starting one of these arcs used to be a three stage process. A pulse of several kilovolts is needed to initially start conduction in the tube. Once the gas was conducting, an intermediate supply of several hundred volts at about 100 milliamps provided enough energy to initiate thermionic emission from the cathode. When the arc started, the voltage dropped to about 60 volts and the current increased to the final value of several amps. The arc current was limited to a maximum value by an external ballast. This sequence seems a little too elaborate for a disco strobe.<p>It is possible that the continuous operation is not intentional. It may be a result of the strobe lamp overheating and failing to deionize between pulses. This continuous load might pull the power supply down enough to extinguish the arc or an overload circuit might shut the system down in the event of continuous operation.

[dacflyer]

(quote) I wonder if they are using decaying rf to excite the gas and if the the final glow near the electrodes is really only like the glow which appears at the electrodes of a regular neon bulb when it's running normally, rather than the electrodes actually being white-hot.<p>no... the electrodes were definatly hot..they would be almost white hot then cool down till dark...<p>(quote) It is possible that the continuous operation is not intentional. It may be a result of the strobe lamp overheating and failing to deionize between pulses. This continuous load might pull the power supply down enough to extinguish the arc or an overload circuit might shut the system down in the event of continuous operation<p>no there was no failure to extinguish.. the DJ could let the strobe burn as desired...but it also seemed to have a lock out feature..like it will not let you "continious on" untill after some time untill the lamp was cooled..
i hope next time i goto germany i can find the name of this strobe lamp.. they had 8 of these attached to the celing and they was all working in unison or sequence...with a master controller of some sort... europe has the best discos i ever seen... makes the ones here so,so..

[terri]

Good point about the arc discharge in glow lamps, Stephen --there is much I have forgotten about gaseous discharge.<p>Without making a major research project out of it I did a quick search on Helicopter Searchlights, and my quick impression is that these are arc lamps. I can't see getting 60 to 90 billion candlepower out of a conventional xenon lamp, but I seem to recall that some carbon arc lamps used to flood the arc with CO2, though I'm not sure if this was to alter the light output/quality or to keep the electrodes from oxidizing, or just for cooling. So I can see a large volume of Xenon Gas in a sealed chamber for these searchlights. Do you have a clue as to the electrode material?<p>One "illuminating" site (pun intended) is "Universal Searchlights" out of Omaha NE:<p>http://www.universalsearchlight.com/Hel ... og%202.pdf<p>This is a nifty illustrated pdf sales brochure which describes their lamps, but doesn't go into principles of operation. In their main site they speak of "electronic ignition," which implies they have some kind of rf stabilization/arc starting method -- much like rf is used in stabilizing welding arcs and rf is used for commercial fluorescent lighting.<p>Dacflyer, you might check out the German "Ultrablitz" Company, which seems to be a leader in the field of large studio photographic strobe units amd might well have penetrated the entertainment lighting market and produce the disco lamps you describe.

[dacflyer]

thanks.. i will look into it.. good thing i can read german :p

[rshayes]

I would expect the cathode of a continuous arc lamp to be made of tungsten. This is practically the only material that can be heated white hot for extended periods without vaporizeing. Even carbon electrodes erode at a substantial rate in a carbon arc.<p>Pure tungsten was used as a filament in early high power vacuum tubes. It was not the most efficient emitter, an oxide coated cathode or a thoriated tungsten cathode could emit far more current at lower temperatures (red or yellow rather than white hot). Tungsten was the only material that could withstand the back bombardment from positive ions generated from residual gas at high voltages. The alternative is a special cathode structure that avoids bombarding the active cathode area with positive ions. These structures were used in thyratrons and mercury vapor rectifiers where it was not necessary to have a grid in close proximity to the emitting surface. Ignitrons used a pool of Mercury. Incandescent spots on the Mercury surface served as the cathode.<p>The photographic flash tubes may use some other material as a cathode, since long life is not needed. The total active life of the flash tube in a disposable camera is probably about 1/10 of a second. The important thing here is probably a temperature coefficient match between the cathode material and the glass envelope. In this case the cathode might be something like Kovar, and the increased rate of erosion and sputtering would be tolerated for the short period of actual lamp operation.<p>If the arc tube is rugged enough, it can be operated continuously with a DC power supply if the current is limited by a ballast or other means. A programmable lamp might use a switched DC supply rather than discharging a storage capacitor. The light level would be lower, but probably still adequate in a darkened room.<p>There is practically an entire web site describing arc lamp operation (http://members.misty.com/don/index.html). Probably more than you really wanted to know.<p>Ultrablitz has been making photographic strobe lights for at least 40 years and possibly more than 50.

[terri]

Well said! I'm impressed, Stephen! You sound like an OT like me. I built a studio light for myself a very long time ago, whence my limited expertise. I forget the joules, but it was a lot. Used to make a pretty good pop! when triggered. I used it sometimes to take pix of buildings and houses for fill-in light and at night.