Anyone understand this fluorescent light phenomenon that can explain it? - Page 1
Forum section: electronics / general

author: hermit
author_rank: Frequent Contributor
timestamp: September 13, 2018, 07:45:47 pm
content: I have to twin tube fixtures in the basement that both operate off of the same wall switch. In the summer when humidity rises one or both of them may have trouble coming on. I've learned that if I leave the cover off of one that has most trouble coming on I can reach up and touch the tubes and then both fixtures will light up. Sometimes one or both will come on if I flip the switch a few times. That is less puzzling than touching the tubes in one and having them light in both fixtures.

author: Kirkhaan
author_rank: Frequent Contributor
timestamp: September 13, 2018, 07:52:33 pm
content: Are the tubes driven from an electronic or magnetic ballast?And what’s the typical ambient temperature there during summertime?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 13, 2018, 07:56:48 pm
content: The ballasts are failing. I replaced several with GE 74472. New on ebay for $10 in quantity.

author: hermit
author_rank: Frequent Contributor
timestamp: September 13, 2018, 08:00:25 pm
content: They are old units and the 'most reliable one' I think I replaced the ballast some years back. I'll have to take them apart to see what the ballast is. I was more interested in the why they behave this way. Touching the tubes I can sorta come up with an explanation for but for touching the one affecting the other is more of a mystery in terms of me coming up with a theory.They don't have the little can starters. Does that mean they are electronic?

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2018, 08:04:40 pm
content: Probably the fixture has lost the ground, the ballast relies on the capacitance of the tube to ground for starting. If you check the case of the fixture is firmly grounded, and the tubes are dust free and oil free, then you need a new ballast, or you have T8 tubes in a T12 ballasted fitting, and you have a weak ballast and thus need a new T8 ballast.

author: hermit
author_rank: Frequent Contributor
timestamp: September 13, 2018, 08:27:26 pm
content: Thanks. That makes sense. This house was built about the time grounding was starting to become standard. The wall boxes were grounded but didn't actually have grounded outlets. I think the electrician probably didn't even believe in it yet because if I test hot to the grounded outlet box I usually get about half voltage so somewhere along the line there is a less that 'good connection' but I've never made a serious effort to find it.

author: richnormand
author_rank: Unknown
timestamp: September 13, 2018, 08:46:00 pm
content: Similar issue here.I live in a house built in the late 60s and have recessed fluorecents in the basement with the old style transfo ballasts.Never an issue during winter when the furnace is active and the humidity in the house is low. In the summer however a few fluo will not start unless I poke my finger to touch the tube.Been doing that for about 10 to 15 years now and the problem does not seem to get worse.The lamp housing is properly grounded.Solution: I have wrapped a 1cm aluminium foil strip around the tubes about 1/3 near the connector end and have the other end touch the housing.Been going well for three years now for several of themAs the tubes are naturally dying I now tend to replace them with straight 110V LED tubes conversion, rewire one side of the tube pin socket and remove the ballast from the circuit. Many friends have bad experiences with the direct replacements LED conversion tubes that allow the unmodified ballast in place I found out.

author: amyk
author_rank: Super Contributor
timestamp: September 14, 2018, 01:34:28 am
content: Quote from: richnormand on September 13, 2018, 08:46:00 pmI've read a lot about this and even some reasonable-sounding explanations about electric fields, but haven't come across any really good explanation of why grounding the tubes makes them easier to light; I know the frequencies of the older magnetic ballast type are just the mains, so nowhere near high enough to cause RF effects with capacitance and such.

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2018, 02:32:45 am
content: +1 for check the grounding.

author: james_s
author_rank: Super Contributor
timestamp: September 14, 2018, 03:50:01 am
content: Quote from: amyk on September 14, 2018, 01:34:28 amIt is capacitive. The current required to get the gas to start to ionize is very tiny, and as soon as it starts, it quickly breaks down and arcs between the cathodes. The effect of that small capacitance at 60Hz is very small, but it's enough to aid starting.

author: TerraHertz
author_rank: Super Contributor
timestamp: September 16, 2018, 04:02:18 am
content: How fluoro tubes start is a bit complicated.Initially the glass tube contains a low pressure unionized gas. The gas atoms have all their electrons, so don't get accelerated by electric fields.There's a parameter in a gas called 'mean free path'. This is how far atoms can travel before hitting another atom (not counting the walls.) It's important because this determines how well an electric field couples to charged atoms. They accelerate in the field, but the energy (velocity) they can reach is limited by how far they can travel in the field before they hit something and lose a lot of their energy. (For the moment, ignore that moving charged particles themselves alter the electric field.)This applies to free electrons too. In a uniform field they accelerate till they hit something, and share their kinetic energy with the other thing (gas atom etc.)Air at STP is a good insulator because it takes very high E field to accelerate ions or electrons enough with their very short man free paths, to achieve an ionization cascade. Very low pressure gas is easily ionized since the mean free paths are long (mm, inches, meters, light years ha ha) and even low E fields can impart enough kinetic energy to ions, to cause every impact to produce more ions.There are two types of (pre-inverter) lighting fluoro ballasts, one uses a starter, the other doesn't. But both try to do the same thing - get an arc started in the tube, then limit current to a suitable level.This requires:1. Get the filaments hot enough to emit electrons.2. Have enough electric field around the filament surface to make electrons leave the metal surface.3. Have enough field strength in the gas around the filaments, for the electron's mean free path to allow them to pick up enough kinetic energy, that when they hit a neutral gas atom they will ionize that atom. (ie knock off one or more electrons.)If that all happens the tube will have an ionization cascade in the vicinity of the filaments, and the resulting charged particles (electrons and atoms, ie plasma, which is very conductive) carry an intensified charge gradient down the tube to the other end (or meet a similar gradient coming the other way.) Once the entire tube is full of plasma there's very little electrical resistance between the ends, ie not much electric field can exist anymore. The tube will carry whatever current the ballast allows. (This ignores a whole lot of more complex plasma behaviors, like double walls, Birkeland currents, magnetic effects, velocity waves, etc, but fluoro tubes are optimized to mostly avoid such effects.)So the early phase of tube startup depends on field strength around the filaments. Early battens had grounded metal spring contacts that earthed the metal end caps on the tubes - which are close to the filaments. This helped. Same as someone mentioned wrapping a grounded foil strip around the tube. Another technique is to have a grounded strip on or close to the tube, whole length of the tube. This has also been a commercial technique. Cost cutting removed all these.Remember that while the internal gas is neutral, the electric field inside the tube is solely determined by the charges on objects - filaments, end caps, batten body. The glass doesn't act 'capacitively' (much) because there are no free charges to accumulate on the inside of the glass, restricting the field endpoints to the inside and outside of the glass.Once the gas is ionized, it's a completely different system. The field inside the tube is entirely dependent on the filaments and current, because there are free charges that flow to neutralize external fields.Tube 'aging' is due to diffusion of extra gas into the tube, decreasing mean free paths. This makes the tube require higher electric fields to start, and also ultimately makes it unable to sustain a plasma.Humidity has an influence because water molecules adhered to the outside of the glass form a somewhat conductive film, also in contact with the metal end caps. But it's not grounded, so can float to some average. This tends to reduce the local intensity of electric fields around the filaments while the gas is still neutral. Hence making it more difficult to achieve the initial ionization cascade. Unless you touch that film, which changes the average voltage since you added your body capacitance to free space even if you are not earthed.Even in dry air, putting your finger on or near the tube ends can assist starting, for the same reasons.

author: james_s
author_rank: Super Contributor
timestamp: September 16, 2018, 05:05:48 am
content: There is one place I've seen the foil strip along the length of a modern fluorescent lamp, on the U-shaped tubes that were once common in some types of recessed troffers and various trendy 1970s fixtures. I don't remember the last time I saw one in use but the lamps are still available albeit expensive. I suspect the foil is needed because the cathodes are not line of sight to one another.

author: SeanB
author_rank: Super Contributor
timestamp: September 16, 2018, 06:56:06 pm
content: Just remember there is a big difference between tube diameter wise. T12 tubes, 1.5in in diameter, have a much lower pressure in them, and a much larger dose of mercury, along with a gas fill of mostly neon. This means a lower striking voltage, but the cathodes suffer slightly more from ion bombardment ( thus the black bands on the ends of old tubes, caused by metal being sputtered off the filaments and depositing on the walls of the glass as they are cool), while the T8 tube ( 1in in diameter) has a slighlty higher gas pressure, a gas blend of neon and argon, a lot less mercury in there to begin with, and lower sputtering of the electrodes. This means a higher starting voltage, but running almost the same till it starts to get mercury starved with age. The newer T5 tubes ( 5/8in in diameter) also have less mercury, but are such a high starting voltage that they need both preheated cathodes and high voltage starting, and so do not work with magnetic ballasts, but need an electronic programmed start one.U tubes were exclusively T12, and the starting strip was needed to get reliable starting with a low voltage resonant ballast, which uses a winding on the ballast to heat each cathode and a ferroresonant winding ( with a series 1.1uF capacitor inside the tar) that would apply 600VAC to start the tubes, and limit the current when lit. 230VAC they could get away with choke ballasts and bimetal switch starters, because the inductive kick would start the tubes, even with no starting strip on the tube.However these days it is almost universal to retrofit with LED, because of the lower running cost, and I am doing that slowly as the tubes age out, and I run out of spare tubes. The prices are dropping with time, I see them for $5 now, only twice the price of the regular tube, whereas last year they were $20 each.

author: hermit
author_rank: Frequent Contributor
timestamp: September 16, 2018, 11:12:45 pm
content: I appreciate all of the information and it makes sense. One thing that hasn't been addressed is by what mechanism does the second light 'strike' when I 'stroke' the first? That is, why does my manually intervening in the first room also bring on the light in the adjacent room? What is going on in the wires at that point?

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 17, 2018, 02:17:42 am
content: Discharge lamps that aren't lit have a slight photoelectric effect. Test that theory by shining a flashlight on the bulbs and see if it helps starting. (Way back in the day, I had an early CFL that would have difficulty starting until I shine a flashlight on it...)

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2018, 03:35:53 am
content: True but in this case I suspect it's the fact that a standard twin lamp rapid start ballast has the lamps wired in series. The yellow wires from the ballast are an isolated secondary that provides power to the cathodes in the middle of the series pair but the lamps are fed in series so one cannot start without the other.

author: amyk
author_rank: Super Contributor
timestamp: October 25, 2018, 01:31:48 am
content: I was researching something else and stumbled upon this which reminded me of this thread:http://lampes-et-tubes.info/rs/rs081.php?l=eIt turns out that the "touch a discharge tube to cause it to strike" phenomenon was noticed by someone who applied it to create touch-sensitive buttons for elevators; the patents have a lot of detail:https://patentimages.storage.googleapis.com/a8/62/ac/f6d5689a40b133/US2525767.pdfhttps://patentimages.storage.googleapis.com/4a/b8/ec/945060912095a6/US2525768.pdfhttps://patentimages.storage.googleapis.com/73/24/32/0d3a29c0d5bc81/US2525769.pdfThe buttons combine a switch, indicator, and memory device --- an interesting obscure technology.