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| Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Fluorescent light capacitor function and failure mode
I'm trying to fix a fluorescent kitchen light. It was blowing the
downstairs lighting circuit fuse every time the wall switch was turned on, even when the fluorescent tube was removed. The breakage seems to have been caused by me leaving something cooking on the kitchen grill for ages (never again!), causing the room to become very hot (condensation on the window, so maybe some in the light, too), and maybe some grease / carbon to settle on surfaces in the room (though I can't actually see or feel any). I'm pretty sure the light was switched on when I left it. There was no fire, thankfully. The light has a standard old-fashioned circuit with just a ballast (just an inductor?) and a "starter" (bimetallic strip thingy). There's also a big electrolytic capacitor, connected directly across the mains. So, with the fluorescent tube removed, the circuit should just be the capacitor connected across the mains. If I disconnect the capacitor from one side of the connecting block that it's wired to, switching it on at the wall no longer blows the fuse (and there's no short across the mains terminals with the capacitor connected). So it's the capacitor that's at fault, not some grease or something shorting things out. First, is the capacitor just there to smooth the voltage, to reduce flicker? Can I disconnect it and switch it all on again, to see if anything else is broken? Second, when I measure the (DC) resistance of the capacitor with a multimeter, I get 500 kOhms or so. Anybody care to guess what happened to it to cause it to start breaking down at 250V? Was it my fault, or was it just a coincidence that it started blowing fuses just as I left the grill on? TIA John |
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#3
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Fluorescent light capacitor function and failure mode
"Sam Goldwasser" wrote in message ... (John J. Lee) writes: I'm trying to fix a fluorescent kitchen light. It was blowing the downstairs lighting circuit fuse every time the wall switch was turned on, even when the fluorescent tube was removed. The breakage seems to have been caused by me leaving something cooking on the kitchen grill for ages (never again!), causing the room to become very hot (condensation on the window, so maybe some in the light, too), and maybe some grease / carbon to settle on surfaces in the room (though I can't actually see or feel any). I'm pretty sure the light was switched on when I left it. There was no fire, thankfully. The light has a standard old-fashioned circuit with just a ballast (just an inductor?) and a "starter" (bimetallic strip thingy). Should be. There's also a big electrolytic capacitor, connected directly across the mains. So, with the fluorescent tube removed, the circuit should just be the capacitor connected across the mains. Are you sure it's electrolytic? Should work without it if it's just across the AC input. There are oftem small uF caps for RFI filtering. If I disconnect the capacitor from one side of the connecting block that it's wired to, switching it on at the wall no longer blows the fuse (and there's no short across the mains terminals with the capacitor connected). So it's the capacitor that's at fault, not some grease or something shorting things out. First, is the capacitor just there to smooth the voltage, to reduce flicker? Can I disconnect it and switch it all on again, to see if anything else is broken? Second, when I measure the (DC) resistance of the capacitor with a multimeter, I get 500 kOhms or so. Anybody care to guess what happened to it to cause it to start breaking down at 250V? Was it my fault, or was it just a coincidence that it started blowing fuses just as I left the grill on? If the cap is all that's bad, could certainly be a conincidence. Sounds like the capacitor is there for power factor correction, I'm assuming you're in the UK or somewhere else with 240v mains, over here most ballasts are the rapid start type and the power factor correction capacitor is internal. |
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Fluorescent light capacitor function and failure mode
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#5
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Fluorescent light capacitor function and failure mode
I've never seen a fixture with a large electrolytic across the power line. Are you from the UK since you use the word "mains" for the power line? Any capacitor is probably just to filter out high-frequency noise from the lamp arc, 50 or 60 Hz and many, many harmonics. The capacitor is probably breaking down when exposed to the full line voltage, but not under the low voltage from the ohmmeter section of the multimeter. Almost any capacitor rated at twice the line voltage will reduce the "noise going back onto the power line. It is most evident when listening to AM radio. If you can use the radios in your house with the light on, without the capacitor, then it probably is ok to continue to use it without interfering with your neighbors radio reception. But if you are in an apartment house, you may cause interference to your neighbors radio due to the ac power lines radiating the noise. The capacitor has nothing to do with filtering noise, it's to raise the power factor. Your diagnosis that it's breaking down under the high voltage is likely correct though. |
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#6
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Fluorescent light capacitor function and failure mode
"James Sweet" writes:
I've never seen a fixture with a large electrolytic across the power line. Are you from the UK since you use the word "mains" for the Yes, I'm in the UK. [...] The capacitor has nothing to do with filtering noise, it's to raise the power factor. I see. I had assumed it was to prevent flicker, but it doesn't flicker with the capacitor out, so I guess not. Your diagnosis that it's breaking down under the high voltage is likely correct though. Yeah, of course -- I was wondering why, though. Perhaps condensation formed between the contacts and heated up the casing, damaging the internals, but if so I dunno what the physical mechanism of the breakdown is. Thanks for your help, everyone. John |
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#7
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Fluorescent light capacitor function and failure mode
: I've never seen a fixture with a large electrolytic across the power
: line. Are you from the UK since you use the word "mains" for the : power line? Any capacitor is probably just to filter out : high-frequency noise from the lamp arc, 50 or 60 Hz and many, many : harmonics. The capacitor is probably breaking down when exposed to : the full line voltage, but not under the low voltage from the ohmmeter : section of the multimeter. Almost any capacitor rated at twice the : line voltage will reduce the "noise going back onto the power line. : It is most evident when listening to AM radio. If you can use the : radios in your house with the light on, without the capacitor, then it : probably is ok to continue to use it without interfering with your : neighbors radio reception. But if you are in an apartment house, you : may cause interference to your neighbors radio due to the ac power : lines radiating the noise. : H. R. (Bob) Hofmann The cap might be there for power factor correction. But on a single fixture? I doubt it unless local code specifies so. -- debaser at ecn.ab.ca -@+.% |
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Fluorescent light capacitor function and failure mode
wrote in message news:8H8tb.12766$f7.662436@localhost... : I've never seen a fixture with a large electrolytic across the power : line. Are you from the UK since you use the word "mains" for the : power line? Any capacitor is probably just to filter out : high-frequency noise from the lamp arc, 50 or 60 Hz and many, many : harmonics. The capacitor is probably breaking down when exposed to : the full line voltage, but not under the low voltage from the ohmmeter : section of the multimeter. Almost any capacitor rated at twice the : line voltage will reduce the "noise going back onto the power line. : It is most evident when listening to AM radio. If you can use the : radios in your house with the light on, without the capacitor, then it : probably is ok to continue to use it without interfering with your : neighbors radio reception. But if you are in an apartment house, you : may cause interference to your neighbors radio due to the ac power : lines radiating the noise. : H. R. (Bob) Hofmann The cap might be there for power factor correction. But on a single fixture? I doubt it unless local code specifies so. -- debaser at ecn.ab.ca -@+.% See this link http://www.ambercaps.com/lighting/po...n_concepts.htm Virtually all rapid start ballasts in north america have a power factor correction cap, but it's traditionally sealed within the ballast casing. These can be identified by the lable saying High Power Factor, next time you're in a hardware store have a close look at a typical F40T12 rapid start magnetic ballast. In the UK the mains voltage is high enough that most fluorescent fixtures use a simple choke ballast, it's more efficient than the autotransformer ballasts we have here, but you need more than 120v to reliably start tubes longer than about 2 feet. Choke ballasts here generally don't have a capacitor (never seen one that did) likely because small tubes are rarely used more than a few at a time. |
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#9
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Fluorescent light capacitor function and failure mode
"James Sweet" wrote in message news:hx9tb.200610$Fm2.189138@attbi_s04... wrote in message news:8H8tb.12766$f7.662436@localhost... : I've never seen a fixture with a large electrolytic across the power : line. Are you from the UK since you use the word "mains" for the : power line? Any capacitor is probably just to filter out : high-frequency noise from the lamp arc, 50 or 60 Hz and many, many : harmonics. The capacitor is probably breaking down when exposed to : the full line voltage, but not under the low voltage from the ohmmeter : section of the multimeter. Almost any capacitor rated at twice the : line voltage will reduce the "noise going back onto the power line. : It is most evident when listening to AM radio. If you can use the : radios in your house with the light on, without the capacitor, then it : probably is ok to continue to use it without interfering with your : neighbors radio reception. But if you are in an apartment house, you : may cause interference to your neighbors radio due to the ac power : lines radiating the noise. : H. R. (Bob) Hofmann The cap might be there for power factor correction. But on a single fixture? I doubt it unless local code specifies so. -- debaser at ecn.ab.ca -@+.% See this link http://www.ambercaps.com/lighting/po...n_concepts.htm Virtually all rapid start ballasts in north america have a power factor correction cap, but it's traditionally sealed within the ballast casing. These can be identified by the lable saying High Power Factor, next time you're in a hardware store have a close look at a typical F40T12 rapid start magnetic ballast. In the UK the mains voltage is high enough that most fluorescent fixtures use a simple choke ballast, it's more efficient than the autotransformer ballasts we have here, but you need more than 120v to reliably start tubes longer than about 2 feet. Choke ballasts here generally don't have a capacitor (never seen one that did) likely because small tubes are rarely used more than a few at a time. I should add that older preheat ballasts had this as well, hence the reason that they contained PCB's, which were used in the capacitor. |
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#10
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Disposing of caps. containing PCBs (poly-chlorinated biphenyls)?? [was: Fluorescent...]
"James Sweet" writes:
"James Sweet" wrote in message news:hx9tb.200610$Fm2.189138@attbi_s04... [...] I should add that older preheat ballasts had this as well, hence the reason that they contained PCB's, which were used in the capacitor. Ack! I guess this mean this capacitor sitting beside me might well contain PCBs. Luckily, it seems to be intact (but I'll certainly keep it in an outdoor cupboard now, just in case). Will my local dump know what to do with it, do you think (I'm in the UK)? The light is certainly an old one, not the instant start variety, and for all I know, it might be 35 years old. It's about 13 cm long, excluding contacts, and this is what's printed on it: AME C.22451 I.I.C. CAPACITORS LTD 5.0 MFD +- 10% 250V RMS WKG 50 C/S -25 TO +50 C INTERNAL RESISTOR FUSE FITTED I presume that's *milli* farads, since the thing is so big? Actually, the "fuse fitted" bit has me confused: why was it causing shorts if it has a fuse?! John |
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