Extending the life of a fluorescent tube with a series capacitor  - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: February 02, 2015, 10:51:45 am
content: Usually with a fluorescent tube close to the end of life one of the cathodes fails first and then it goes into its usual blink blink blink cycle and is considered dead. What if when that tube is still new we put a large capacitor in series with the AC apply to the tube so that if the conduction of the two cathodes becomes asymmetrical it automatically applies at DC bias to the tube to force the weak cathode to emit more electrons and the good one to emit less. A capacitor value of several thousand microfarads would probably be suitable and you could put two fairly low voltage electrolytic caps in series back to back with a diode across each one so neither cap get reverse voltage on it. Haven't tried it yet but it might be an interesting idea.

author: MrAl
author_rank: Super Contributor
timestamp: February 02, 2015, 11:34:55 am
content: Quote from: Circlotron on February 02, 2015, 10:51:45 amHi,I guess you mean the line voltage operated kind? Do you really need a cap then (it is already AC)?A cap is normally used when the light is being driven from a DC source, like a battery. But here we see a higher frequency signal so only a small value cap is required.I think this is how the curly bulbs work too, taking the line voltage and rectifying it, then creating a higher frequency. There are schematics on the web for these which would help.

author: helius
author_rank: Super Contributor
timestamp: February 02, 2015, 11:46:44 am
content: Fluorescent lamps like the T8 type have filaments at both ends. The only reason they are there is the lamp is designed to warm start. The starter circuit runs current through the filaments to warm them up for thermionic emission. When the lamp is operating, no current flows through the filaments: they are kept warm by electron or ion bombardment.Warm start is not necessary to operate these lamps, and in many cases they can be started cold with a different kind of circuit (with higher frequency or voltage). Even if both filaments are burnt out, you can cold-start the lamp, and advanced starter circuits will do this.

author: Circlotron
author_rank: Super Contributor
timestamp: February 02, 2015, 12:18:44 pm
content: Quote from: Circlotron on February 02, 2015, 10:51:45 amI meant "if theemissionof the two cathodes becomes asymmetrical", that is to say, if one emits less than the other so that the AC supply current begin to have a DC component.Also, talking about 50 or 60Hz supply with iron cored ballast, not CFL.

author: helius
author_rank: Super Contributor
timestamp: February 02, 2015, 12:41:05 pm
content: Sorry, I totally missed what you were trying to sayI don't think that DC bias can ever be good for a fluorescent lamp: since the cathodes are kept hot by being anodes on the previous half-cycle. If you apply DC bias you will just get unequal heating, and the DC cathode will get cooler and emit even worse than before.

author: Circlotron
author_rank: Super Contributor
timestamp: February 02, 2015, 12:48:32 pm
content: Dang!

author: MrAl
author_rank: Super Contributor
timestamp: February 02, 2015, 02:58:49 pm
content: Quote from: Circlotron on February 02, 2015, 12:48:32 pmHi,I dont see how you can get a DC offset from an AC signal unless there is something that is known to cause that. with just an inductor i dont think this can happen unless something goes wrong somewhere.Just in case there was a DC offset from some mechanism, i suppose using a capacitor would help to get rid of the DC offset just as it does in other lamps with higher frequency. I dont see why that would not help.Yes, you would have to have a cap large enough to pass enough current at the required frequency.Another interesting idea might be to run the tube with an H bridge. With that you could control the polarities as you needed. More complex, yes.

author: bktemp
author_rank: Super Contributor
timestamp: February 02, 2015, 04:20:50 pm
content: Or use the most simple solution: An electronic ballast. It has a coupling C, therefore absolutely no DC offset. It runs from DC you will have no 100/120Hz flickering, it will increase the tube life and it is more efficient than a magnetic ballast.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2015, 07:29:41 pm
content: It's a valid thought -- even once the discharge is going, the filament cathodes are still doing their job. Most of the time, the ionic space charge of the plasma more than overcomes the cathode space charge that enables rectification, but if that is also going weak for whatever reason, maybe it would matter. And yes, asymmetrical emission should result in DC, which should result in asymmetrical current, which a capacitor would be able to partially oppose, which could encourage more voltage on the opposite phase to force it to conduct.Might be worth a try, but likely not worth the cost of a "motor run" style capacitor. The tube is already aging, you're not going to get much more life out of it.I once operated a well-aged CFL as a CCFL (cold cathode -- hit it with enough voltage and it sparks over, who needs heaters). The one cathode had already failed open, so at first the tungsten was dangling, and probably not receiving the discharge very well; pretty soon (a few minutes, likely at well over the nominal power rating to begin with) the vacuum seal broke, as the lead-in wires melted from the hot end down.

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2015, 08:06:40 pm
content: Lead lag ballasts run the one tube vis a capacitor, typically a 8-12uF motor run capacitor. Made to reduce flicker as they have phase shift so the one lamp is at full brightness as the other is going through a zero crossing. Tube life though for them is not different between the lamps, they typically last around the same time in operation.You will find the biggest contributor to short tube life is the manufacturer, as they reduce mercury dose the tube life decreases, and as they move from T12 to T8 and T5 ( and now even T3) lamps the lifetime decreases. If you have tubes with the correct full dose ( 12mg) of mercury, as opposed to the Eco junk ( less than 0.5mg) the low mercury lasts typically 1-2 years in most cases, while the full dose tube will do 5-8 years in typical office use.Low dose tubes are also very prone to starvation, when they have reduced light output and often will be a dim pink.

author: Circlotron
author_rank: Super Contributor
timestamp: February 02, 2015, 11:10:15 pm
content: Quote from: SeanB on February 02, 2015, 08:06:40 pmSo the low mercury tubes get replaced more often, decreasing the benefit of low mercury content. Add to that the environmental impact of manufacturing and transporting more tubes, as well as having someone travel to a location to replace them more often.

author: SeanB
author_rank: Super Contributor
timestamp: February 03, 2015, 06:36:59 pm
content: I have some old tubes dating back to the 1970's, and they were full dode tubes, and lasted 30 years in operation most days of the week in a business. Outlasted the instant start ballasts in many cases, you changed the ballast and put the tubes back. I have the same thing with modern electronic ballasts, where I simply took the electronic ballast out after it failed, and put in 2 iron core ballasts and the starters, using the ready made holes in the ballast tray and the existing ballast screws. I just had to replace some of the short wires with new from the reel I have.