Electrically determining xenon tube polarity - Page 1
Forum section: electronics / projects

author: The Lightning Stalker
author_rank: Newbie
timestamp: December 23, 2018, 07:55:06 am
content: Say I have a xenon tube tucked up in an inaccessible place where I cannot see it or safely remove it. Does anyone know if xenon tubes like the kind found in camera flashes display a polarity dependent nonlinear characteristic that can be seen for example on an oscilloscope? thanks

author: soldar
author_rank: Super Contributor
timestamp: December 23, 2018, 10:24:04 am
content: A "polarity dependent nonlinear characteristic"? The short answer is "no".The flash tube will be powered by a DC, polarized capacitor.What are you trying to do?Have a look athttps://en.wikipedia.org/wiki/Flashtube

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 23, 2018, 10:39:34 am
content: Afaik the differences on electrodes are to do with rates of erosion over time, so seems unlikely you could measure it remotely. Worst case it just doesn't last as long

author: The Lightning Stalker
author_rank: Newbie
timestamp: December 23, 2018, 10:41:48 am
content: Sorry I should have made clear that the idea is to determine the polarity of said tube electrically when visual identification is not possible.

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2018, 11:19:38 am
content: Quote from: The Lightning Stalker on December 23, 2018, 10:41:48 amYes we understand that. Sorry so far no one knows of a way to do it.I think it may trigger at a lower voltage when given the correct polarity, but I could be wrong.

author: coppercone2
author_rank: Super Contributor
timestamp: December 23, 2018, 11:37:59 am
content: if there is errosion could the plasma sustain itself for a different period of time since its chemical composition is different?

author: floobydust
author_rank: Super Contributor
timestamp: December 23, 2018, 04:49:23 pm
content: Xenon strobe tubes DO have polarity, along with (phasing) their trigger coils.Copypasta fromhttp://repairfaq.cis.upenn.edu/sam/strbfaq.htm#strbgsc4, see alsohttp://donklipstein.com/donflash.html.1. Any red markings or "+" markings indicate the anode.2. With no markings but electrodes of unequal size, the larger one is the cathode.3. If both ends look identical but the trigger electrode is closer to one electrode, or more coupled to one electrode, then that electrode is the cathode.4. If the tube looks symmetric except for having a getter at only one end, it is probably preferable to make the getter end the cathode, especially if any getter material exists on the electrode itself. Any vaporized getter metal forms positive ions easily, and will be attracted to the cathode. Metal vapor released around the anode is more likely to condense all over the tube and discolor it.5. If both ends of the flashtube look alike except for one electrode being shinier and with rounded edges, then the shiny electrode with rounded edges is the anode, and the steel-gray (tungsten) sharply cylindrical one is the cathode.There is also info there about polarizing the trigger electrode pulse properly.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 23, 2018, 05:01:13 pm
content: Be careful: He says Xenon Tube. It could be a flash or a light source.@The Lightning Stalker: Can you please specify if that's a flash or a light tube? that would help others to find an answer for you.

author: The Lightning Stalker
author_rank: Newbie
timestamp: December 23, 2018, 05:45:02 pm
content: Quote from: Twoflower on December 23, 2018, 05:01:13 pmIt's a flash tube.

author: floobydust
author_rank: Super Contributor
timestamp: December 23, 2018, 05:49:24 pm
content: see if you can post a pic of the tube

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2018, 09:06:52 pm
content: Quote from: floobydust on December 23, 2018, 04:49:23 pmSo, absolutely nothing applicable.The trigger, I wonder if it's sensible which one it's closer to?Maybe not. I recall for example, the horseshoe style ones have a metallized strip all along the inner path, so you wouldn't observe a lower breakdown impedance to one electrode or the other.Tim

author: floobydust
author_rank: Super Contributor
timestamp: December 23, 2018, 10:09:02 pm
content: Horseshoe and ring tubes most have aquadaq or a spiral wire on them as the trigger electrode running along the entire length. But not so on the smaller straight (camera flash) xenon tubes, they have just a ring by the fat sintered cathode. Little camera flashes use the reflector for the trigger electrode as well.Some tubes have symetrical electrodes but a red mark, I'm not sure why.For trigger-sensitivity I think the ionization should be the same polarity as along the main discharge path? It's got something to do with the streamer. I just remember having the trigger-coil backwards-phase, makes a huge difference.I looked and Xenon arc-discharge (not strobe) lamps are polarized and run with DC. I see smaller car HID lamps are non-polarized with HF AC.pics fromhttps://www.xenonflashtubes.com/but no datasheets. Nothing seems consistent with the polarity.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 23, 2018, 10:45:38 pm
content: If the trigger transformer is integrated into the assembly as is usually the case, you can check which end the primary coil connects to - usually the negative.

author: Globe Collector
author_rank: Contributor
timestamp: December 23, 2018, 11:33:44 pm
content: Was/Is it connected to a capacitor?Is the capacitor a polarized electrolytic of some "oil filled" non-polarized thing?You might determine the polarity from any possible capacitors or other circuitry is is/was connected to.Generally, if it is something I am pulling apart, but wish to ultimately restore function to, I will draw the circuit first so I can see what went through the head of the designer...often it is not what you would traditionally expect.Example, "Gigastrobe" units...don't use capacitors, least not here in "240v, 50Hz land, they simply have a large diode and a low value inductor and the xenon tube all in series right across the mains. When the tube is fired, it conducts for one half cycle and extinguishes at the next zero-cross. (Which is pretty rough on the building's switchboard and distribution transformer). At very large discharge currents xenon discharges exhibit a POSITIVE thermal resistance coefficient, so there is something to prevent a catastrophic failure. (In Russia they had 100Kw + xenon tubes that connected directly across 415v phases, these were about 8' long or more).So, can you elucidate more about this inacessable xenon tube.Why is is so inaccessable?Is it in a piece of existing gear whose function must be restored?Is it in a new design?Is it being re-purposed from a failed piece of gear into a new design?Is it in a capacitor-discharge type flash/strobe circuit?Is it in a non-capacitor circuit like the one described above?

author: The Lightning Stalker
author_rank: Newbie
timestamp: December 24, 2018, 12:09:29 pm
content: Since sometimes it is not possible to remove a xenon flash tube without causing damage, I went ahead and tested some tubes to see if they exhibit any polarity dependent behavior. Milliamp current waveforms were measured for each tube and then the tube was reconnected the other way around. On average I noticed that these samples have a tendency to begin conduction more often in a specific direction. For these tubes at least, it was apparent to me that they conduct more easily in reverse. This may not be the case for every tube but at least for me it is a satisfying result.

author: coppercone2
author_rank: Super Contributor
timestamp: December 24, 2018, 01:36:06 pm
content: maybe it has to do with surface porosity and asparities that form

author: The Lightning Stalker
author_rank: Newbie
timestamp: December 27, 2018, 06:03:54 pm
content: Since this thread is basically done thanks to everyone for your help.

author: Fred_47
author_rank: Contributor
timestamp: December 27, 2018, 10:15:17 pm
content: Quote from: The Lightning Stalker on December 24, 2018, 12:09:29 pmSo, just to be clear, which end was +?

author: The Lightning Stalker
author_rank: Newbie
timestamp: December 30, 2018, 03:06:08 am
content: Quote from: Fred_47 on December 27, 2018, 10:15:17 pmIt may be some peculiarities with my test conditions but the tubes behave differently when connected in reverse and forward using an AC signal.