|
07 Feb 08 - 05:37 AM (#2255749) Subject: BS: light bulbs- lifetimes- switched on/off? From: Mr Red Anyone know of websites that give decent facts on the degredation of lifetime with each switching on of the average incadescent light bulb. Before we get into a discussion of the actual inrush current it is about 15 times the steady-state current and decays exponentially in about 0.1 sec. and is 90% done in 0.13 secs. That is quite a hit and causes thermal and mechanical shocks to the bulb and the tungsten age hardens. Average life of bulb is about 750-1000 hours on an unspecified switching regime - but probably once/twice a day. So URLs anyone? Facts? |
|
07 Feb 08 - 08:58 AM (#2255855) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Geoff the Duck I'm assured they last virtually forever if you don't turn them on. Not much light though! Quack! GtD. |
|
07 Feb 08 - 10:08 AM (#2255909) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: GUEST,Darowyn I have seen a study somewhere, years ago, which showed that headlight bulbs on Motorcycles last longer (in real time terms, not just in hours of use) if they are left on all the time. Apparently the hot filament survives vibration better. There's no switch on mine, so I could not turn it off if I wanted to. Cheers Dave |
|
07 Feb 08 - 10:42 AM (#2255962) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: JohnInKansas The main way in which a simple incandescant bulb ages is by evaporation of tungsten from the filament while the bulb is on and at normal operating temperature. The specific way in which the filament is "consumed" is that the rate of evaporation is higher where the filament is hotter. Since the same current flows throught all of the filament, the filament is hottest where it's diameter is smallest, since a reduced diameter has a higher resistance per unit length. This means that the hottest spot, already the smallest, loses tungsten fastest, and becomes even hotter. ... etc. Tungsten is fairly brittle at normal room temperatures, and is actually "tougher" at lamp operating temperatures. The current, as noted already, is much lower during operation than during the turn-on surge; so a weak spot that holds together for a while during steady operation may not be strong enough to stand the surge current at turnon. A filament that's "very near end of life" is likely to fail under the surge when turned on, hence the "folk knowledge" that turning the lamp on and off shortens its life. The actual "using up the life" of the lamp occurs almost entirely during normal on-operation, and the life of a lamp is almost entirely the cumulative time spent turned on. Turning a new bulb on and off has virtually no effect on the remaining life of the bulb. If the filament doesn't break it's not affected by the surge. It's only when the filament has been used long enough to develop local areas of reduced diameter that the turn-on surge can "pop the filament." If the filament has degraded to the point where it could remain lit for another x hours if left on, but has a hot-spot that can't stand one more turn-on, then turning it off will lose that last x hours, since you can't switch it on without breaking the filament. (Unless you use a dimmer to bring it up without the surge perhaps?) The time between when one more turn-on surge will cause the filament to fail and the time when it will fail if just left turned on can be a fairly significant number of minutes, or even a few hours; but isn't often a very large percentage of the total life expected. For industrial lamps, and for some "long-life" ones, you may sometimes find a statement that "life" is based on a certain operating cycle, and a usual assumption is 8 hours on per 24 hours. This usually means that a "750 hour" bulb is expected to be on for a cumulative total of 250 hours. The "gotcha" here, is that they almost never will tell you whether the "life" quoted is a mean, median, one-sigma, two-sigma, or another one of the several "statistical measures" that might be used, so they haven't really told you anything very meaningful. They might mean that no more than 10% of bulbs will fail before the specified "life," or they might mean that "we had one once that lasted this long." Casual experience, not rigorously measured, would indicate that about 60% of bulbs may last 3x the "life" on the package in normal service, and usually not more than 2 - 10% fail before you've used up the flavor promised. Usual "business practice" is to turn off incandescant lamps whenever not needed, since power consumed leaving them on is significant and bulb life is almost completely unaffected. For fluorescent lamps, the power required to "re-light" them is quite significant when compared to the power consumed by leaving them on, so recommendations are to turn off if not needed for some specific period. The usual spec is something around a half-hour to an hour of idle time, although I've known companies who said to turn off the fluorescents if you'll be out of the room for 10 minutes or more. (Note that not all "maintenance engineers" are spectacularly proficient statistical analysts, but a very short idle time spec may be based on the assumption that when you think you'll be gone for ten minutes it usually ends up being two hours.) John |
|
07 Feb 08 - 02:23 PM (#2256128) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Schantieman Unless someone better qualified in physics than I am can explain it to me (and that's probably most of you since I didn't get a very good A level), my understanding is that the switch-on surge can't be very much larger than the normal current or the fuse/circuit breaker will blow/trip. n'est ce pas? S |
|
07 Feb 08 - 02:33 PM (#2256138) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: PoppaGator How 'bout them newfangled compact flourescent bulbs ~ does it make sense to leave 'em on rather than turn on-and-off too frequently? |
|
07 Feb 08 - 02:46 PM (#2256156) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: bobad Schantieman, breakers have differing trip points, there are fast, medium and slow blow breakers and the slow blow will take up to six times the current for which they are rated for a period of one second. |
|
07 Feb 08 - 03:50 PM (#2256212) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: artbrooks There is a light bulb in California that has been burning pretty much continuously since 1901. Makes you think that the on/off cycle may have more impact than the filament degradation while lit. Consumers Reports, BTW, makes a comment in a recent report that frequent cycling does shorten the life of compact fluorescents. |
|
07 Feb 08 - 03:57 PM (#2256218) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: skipy The final heat treatment of a filament is carried out by the new owner, as a fully heat treated filament will probably break during transit. So when fitting a new "bulb" do not switch it on to see if it works and then switch it off. Replace old bulb, switch it on for 20-30 mins and then switch it off. I have read "somewhere" that this action will double it's life. Skipy |
|
07 Feb 08 - 04:13 PM (#2256239) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Bill D I use a number of the tiny 4 watt night-light bulbs for various things, and I'm sure they last better if I just leave them burning. I have even done side-by-side tests in similar lamps. Now the new compact flourescents SAY "life based on X hours per day and X number of on-offs"..but they don't give you the graphs. |
|
07 Feb 08 - 05:19 PM (#2256324) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Schantieman Presumably these 'flourescent' bulbs work on ground up wheat? Or maybe they're fluorescent? (ducks) S |
|
07 Feb 08 - 07:04 PM (#2256406) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Joe Offer For decorative lighting, I sometimes wire incandescent bulbs in a series, so they won't be so bright. When I do that, the bulbs last forever because of the reduced voltage. -Joe- |
|
07 Feb 08 - 07:08 PM (#2256410) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: John J Soft-starting a filament lamp will extend it's life hugely, eg switching on with a dimmer. The peak voltage of a 240Vac supply is around 350v volts, if you switching with a normal mechanical switch you have no control of the voltage supplied to the load at the instant you switch 'on' - it could be 350v, equally it could be 0v. Shoving 350v across a bit of wire is going to give it one hell of a kick (albeit for a v short time), at least if you switch on at the 0v instant (zero crossing switching) the lamp filament has chance to warm up for .01 sec. I had a customer who had problems with filament lamps failing due to the high frequency at which the were switched on and off. I improved matters enormously by not switching the lamps off fully. We applied a low voltage to the bulb, not enough to illuminate it but enough to keep the filament hot. When the lamp was switched on fully it wasn't so much of a shock = longer life. John |
|
07 Feb 08 - 07:26 PM (#2256424) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: MaineDog I have already had to replace several cfls, way ahead of their promised/scheduled demise, I'm not sure why. The last one that died began to burn up at the base, and could have started a fire if it had been near anything flammable. Re: incandescents: the resistance of a cold filament is much less than that of a hot one. This is why there is a big surge when turning them on. Unfortunately, most dimmers use "chopping" technology rather than resistance, so that the filament is subjected to many brief hurtful surges, rather that a true slow ramp-up of current. Presumably the thermal inertia of the filament keeps it from overheating enough to outgas significantly, and its temperature should increase slowly enough to prevent some amount damage if the dimmer is turned up slowly. I have lots of bulbs high up in my cathedral ceiling so I have an interest in not having to replace them frequently. I also like cfls because they save lots of energy (read money), but only if they last as they should. I think we need better dimmers, as well as better bulbs. MD |
|
07 Feb 08 - 10:15 PM (#2256493) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Bill D 'Most' of the 7-8 incandescent bulbs I have left are there only because they are in places where I have or prefer dimmers...and a couple of those will soon be changed. I need standard fluorescents in my shop, and a couple of places are just not worth the trouble to remove dimmers, but I'll soon be maybe 75% CFs. I'm sure they'll save time, energy and $$$ over time, but I wish the industry would just publish the data sheets on on/off time and expected life under various conditions. |
|
08 Feb 08 - 03:28 AM (#2256583) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: GUEST,PMB The filament thins because of evaporation of the tungsten. But the death of the bulb is almost always caused by breakage of the filament at hot- spots caused by uneven evaporation, in response to the turn-on surge. I had 4 bulbs blow simultaneously a few days ago- the voltage was probably high when I turned on, and I probably caught the mains at a peak. None of these bulbs had been in use for anything like 1000 hours. I've just measured a standard European 60W bulb. Cold resistance is 72 ohms. The hot resistance is, by calculation, 240*240/60 = 960 ohms, so the startup current will be over 10 times the running current. CF bulbs are fine where lights are on for continuous periods, but I suspect that much of the expected saving will be lost. Because of the pisspoor start-up performance people will leave them on continuously in situations where an incandescent bulb would have been turned off. |
|
08 Feb 08 - 04:51 AM (#2256606) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Mr Red Schantieman Fuses are still bits of wire that take a while to get hot before melting (=fusing). A short surge is expected so they are designed to fuse in an inverse current/time characteristic. Indeed a standard UK 13A fuse will blow at 13 Amps given enough time and the thermal confines of the UK plug and the way some people use it. Probably 100+ hours. Most fuses are rated at 13A - few people fuse at lower values becuse they don't know better. so yer 60watt bulb at 240V is 1/4 amp and surge is 3.75A. even a 1A fuse would most likely not fuse. John in Kansas. The whole point about end of life is that a loo light that is switched on at every visit of a 6 man company drinking coffee all day is that the weakness will be found sooner. UK traffic lights were given soft-start electronics 40 years ago, and that definitely was found cost effective. As an Electronic Engineer I was always told at Uni that frequent switching affected "average" (read mean) life, and on the basis of finding the weakness sooner it is un-arguable. But I was given the explanation why it shortened life at a molecular level and involved crystaline structure and voids (and maybe age hardening) but it has been a long time. The numerical significance of life-shortening is what I don't have. The 750 -100 hours figure is from Wiki and stated as average. The evapouration of tungsten is a function of the vacuum which has improved over the years. The reason i asked was a discussion at work about "THE LIGHT BEING ON" (that kinda discussion) and the damn thing went pop - which turned-out to be the fitting! And had we got a light bulb? and are we allowed to mend it ourselves? you guess. |
|
08 Feb 08 - 08:05 AM (#2256698) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Mr Red artbrooks Is that the light bulb that has had three new filaments, two blulbs, four new bases and 10 new vacuums? But has lasted that long - or is it that it takes no number of Californians to change a light bulb? Because like Arnie they are tough and not afraid of the dark, but he will be back to do the job. Oh and my point about voids and crystals is that once a localised spot goes a little higher resistance - it gets hotter. Evapouration is surface (plenty of that with coiled coil) and continuous, crystalisation/voids goes right the way through and is worse at switch-on. But it all conspires. Vehicle bulbs are usually single coil (thicker wire) because of the vibration. Still hoping for hard facts. |
|
08 Feb 08 - 07:02 PM (#2257306) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Geoff the Duck I still reckon the hard fact is that they last forever as long as you don't turn them on... Quack! GtD. |
|
08 Feb 08 - 07:31 PM (#2257318) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Bee We have had 12 of our light fixtures fitted with compact fluorescent bulbs. The oldest is three years, the newest about six months. Most are about two years old from when the local hardware store had a sale. I've had to replace two, neither of them the oldest. One died within weeks of installing, the other after about a year, and it was one that is on and off a lot. I like them. I prefer the ones with a slight yellow cast, rather than the very white ones, but use the white where I have a need to be precise about colour (studio). My kitchen, which provides some of the lighting for the living room, is where I most appreciate their long life. I was replacing incandescents there every month or two, in fixtures that were a pain to get at. Problem solved. |
|
08 Feb 08 - 07:56 PM (#2257336) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Bert They used to make a small thermistor that you inserted into the base before putting in the bulb. I haven't been able to find one recently. Years ago I scavenged a thermistor from an old television set and wired it into a slide projector. Slide projectors are notorious for the short life of their bulbs but with mine the thermistor burned out before the bulb after several years of use. |
|
08 Feb 08 - 10:43 PM (#2257417) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: The Fooles Troupe The thermistor would have a high start up resistance, which would drop as the bulb warmed up, limiting the startup rush current. Thermistors are still readily available from 'electronic kit part' places fairly cheaply - Tandy Electronics for example, if they still exist. You may wish to read this article The Case for CFLs http://www.slate.com/id/2183606/pagenum/all/#page_start |
|
09 Feb 08 - 09:21 AM (#2257614) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: Mr Red The amazing thing is that one project we have at work involves a whole PCB controlling the lights for a camera. The bulbs get a variable voltage to improve life and help the camera see without flaring, the rig is on an umbilical down a conduit 40 metres away. Not surprisingly to protect the fusing (mostly) but also help the bulb we have a circuit measuring the voltage and holding off the relays until the voltage is low enough not to cause concern. So if they leave the lights at full voltage they have to wind the variac down to near zero. Fuses are easy to replace but you really don't want to change them every time you forget - assuming you have one in your pocket at the time. |
|
09 Feb 08 - 05:57 PM (#2258006) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: John J What about re-setable fuses? I don't know much about them but I gather they are rather slow in reacting to high-currents. Bournes make them and Raytheon certainly used to make them. I like the idea of using a thermistor as a current inrush limiter for a filament lamp as mentioned by Bert. John |
|
10 Feb 08 - 04:15 PM (#2258784) Subject: RE: BS: light bulbs- lifetimes- switched on/off? From: GUEST Well its a goof many years since I was a lighting engineer. We used to make two main types of lamp 1000 hour and 2000 hour. The only difference was in the thickness of the filament. The 2000 hour lamps consumed slightly more power, but lasted twice as long. Most failures were due to mechanical vibration or to magnetostriction. We also made traffic light lamps, which had more supports, and were less prone to vibration. Where lamps are used in pendant fittings, and there are frequent failures, simply shortening the flex a little can improve the lamp life by changing the resonant frequency of the installation. Having tap dancing elephants in the loft can also shorten lamp life. Magnetostriction is due to the filament having a coiled coil, which contracts when a current flows through it. We used to have a lamp shaped like a radio valve, which had lasted thirty years before I stupidly dropped it whilst moving house. I never checked its current consumption, but the light output was a fraction of the 1100 lumens you would get from a normal 100 watt lamp. John |