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AC or DC....that is the question

Discussion in 'Lighting and Electrics' started by propmonkey, Jun 29, 2005.

  1. propmonkey

    propmonkey Well-Known Member

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    Ok this maybe a really dumb and pointless question but its been on my mine on and off(no pun intended) the last few days. but since lamps are running on AC, switching directions 50-60 times a second there for in fact dimming them 50-60 times a second does that cause them to wear out? what if they were running on DC so they are not dimming 50-60 times a second, and therefore extendign the life of the lamp?

    sorry if this was a waste of time its just somethign i wondered and hopefully it makes sense to alteast oen person
     
  2. ship

    ship Senior Team Emeritus Premium Member

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    No question is a waste of time and your question is actually a very good question.

    Should you wish for me to give you an immediate answer, please contact me off line, otherwise let's open this up for discussion. I will not anwser or voice opinion for a week. And this time I promiss I will be more nice.


    Umm... filament notching.... grrr....
     
  3. koncept

    koncept Active Member

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    what you are saying does make sense. i would think it would extend the life since you are not "turning the lamp on and off" 50 - 60 times per second.

    but i also wonder now about some of the new dimmers particularly the sld 96 from strand which has the sine wave technology in it and what that acheives?


    edit:
    it is amazing what you can find when something sparks your interest...
    http://members.misty.com/don/bulb1.html#dc
     
  4. avkid

    avkid Not a New User Fight Leukemia

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    It seems perfectly logical to me that if a lamp is dimmed fewer times it's life would become longer. My opinion is not based on any science or research,just what I think is a logical hypothesis.
     
  5. jumpjet

    jumpjet Active Member

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    I would think that the stress of changing temperature would cause a shorter lifespan so turning the unit on and off frequently would shorten the life, but at 60hz, there is going to be no significant thermal difference between cycles...
     
  6. SteveB

    SteveB Well-Known Member

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    Lamps have thermal inertia, which is why you don't see them flicker. The on/off cycle is so fast that the filament has no chance to respond. Note that even in a zero count blackout (the only kind of blackout), the lamp still has fade.

    So generally, it doesn't harm the lamp.

    Dimming quartz halogen (and incandescent) lamps can extend their life. G.E. use to put out a calculating wheel that shows the increase in life at variable decreases in voltage, along with the corresponding decrease in color temperature.

    At some point though, the lamp temperature gets too low for the halogen cycle to function. This is where the burning tungsten reacts with the halogen gas and gets re-coated on the filament. If the temperature is too low, this cycle is stopped and I cannot remember at what point that occurs. Perhaps a post on rec.stagecraft could get a reply.

    Remember also that lamp life is an average, and I believe that it's something like 50% of a test batch lasted at least that long, with some going longer and some less. They also assume that all lamps operate at exactly the rated voltage at 60 hz, the voltage varying greatly under field conditions.

    SB
     
  7. SteveB

    SteveB Well-Known Member

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    Meant to add that there's a very good discussion over on the lightnetwork.com about SineWave type dimming.

    In short, lamp life isn't one of the advantages.

    I'm sure the ETC website etcconnect.com has some good info. as well

    SB
     
  8. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Remember that most dimmers will have a “pre heat” or “sensitivity” setting which actually means that at zero percent, they are not actually off. This keeps the filament hot enough to cope with the sudden increased intensity when it is operated.

    Now I am by no means an expert on lamps but I am pretty sure that it is a reduction in voltage that extends lamp life. So placing a 118V lamp in a 110V fixture will result in an extended life. Running 110V lights in the same fixtures with one dimmed and one not dimmed, the lamps should last the same amount of time. Some fixtures have a “lamp save” switch on them that reduce the voltage going to the lamp. The older style rheostat dimmers work in this way (I believe)

    SCR and Triac Dimmers actually turn the lamp on and off very quickly. Our eyes cannot differentiate between the on and off, so as a result, we see a lower intensity of light. The voltage remains the same. Take a white plastic ball and poke a screwdriver through the axis so that it can spin. Now, holding the screwdriver in the horizontal plane, paint the top half red and then the bottom half blue. If you spin it quickly enough, you will see a purple ball. Similar principle.

    Like I said, I am no expert when it comes to lamps and especially when it comes to trying to understand how manufacturers determine lamp life. However, one thing is clear and that it the fact that there is no standard when it comes to the parameters used to determine this.

    Anyone care to correct any misconceptions on my part?
     
  9. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Also noticed that there is another post on lamp life and pre heat, which cutlunch explained much better than I did here
     
  10. MSwan

    MSwan Member

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    Even if using DC would allow lamps to last longer it is much more dangerous. Your muscles work on very low levels of DC current (along with lots of chemicals and the such). Defibrilaters use DC current and we all know how a human body reacts to that. If you were to touch a DC source at 120v 20A your body would freeze into place and you would not be able to let go, it can happen with AC as well but generally it doesn't as the changing current will at some point send your muscles the other direction causing you to release the source of voltage (hopefully before 0.6A passes through your heart).
     
  11. ship

    ship Senior Team Emeritus Premium Member

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    “since lamps are running on AC, switching directions 50-60 times a second there for in fact dimming them 50-60 times a second does that cause them to wear out?” - propmonkey

    Steve B is correct on the thermal inertia question. It extends their life or at least lets it attain it’s designed lamp life as opposed to full non Hz voltage.
    The filament has under full current sufficient resistance in the wire that it incandesces - that wire gets so hot it glows. Dependant upon how hot that filament got, and it’s size it will take a while to loose enough heat so that any damage is done to it by instantly heating it up again and again because it’s not getting cold enough will not have sufficient effect given the duration of this power being shut off. The larger the filament, the longer it takes to cool down. Instead as also thought of, it’s kind of like putting the lamp on a dimmer when cycling on and off at 60Hz.

    The lamp is designed for a specific voltage every 60Hz. Should you speed this up or have 120v of non-cycling current it would get much hotter and wear out faster given sufficient volume in voltage. Not designed for this. The lamp would be as if at a higher dimmer ratio, get hotter still and wear out faster. Should you slow down this speed or dim the lamp by way of current chopping or voltage lowering, the filament is operating at a cooler temperature. Since the filament is operating in a sustained one step below melt down when at peak voltage, it does wear out and get thinner in some areas with time - even with the halogen effect. Lowering how close to melt down the resister is operating at thus prolongs the life because it’s not getting so close to melt down and less of the filament is burning off.

    The halogen effect is that of spent particles of the filament in having burned off as if a candle, being re-deposited once it cools and is absorbed by the gas, back onto the hottest part of the filament. A halogen lamp however is not perfect in that while for the most part it re-deposits the filament to continue burning or resisting, it’s not always placing the filament particles back onto the points it burned off of. There are some parts of the filament that are cooler than other parts by way of resistance thus those parts will still wear down in the extended life and higher operating temperature lamp still wearing out. Because the filament can not burn hotter without wearing out as fast, it thus operates or incandesces under the halogen effect at a higher temperature due to the heat and burning up being replentished but only for the most part. This also is reflected in a larger luminous output due to the increased resistance, and because it’s replentishing the filament often an extended life. Just as with incandescent lamps, you have certain checks and balances. You can have higher voltage, life, color temperature or luminous output halogen or not. Change the balance of one, you change that of all. A tungsten - halogen or xenon repressed filament or vacuum tube type will melt down once it recieves voltage sufficient it gets somewhere beyond 3,500K, or with time wears out. Simple as that, in most cases, you balance lamp life with output and/or color temperature, and price of materials used to suppress this burning up. This along with voltage rating.

    This Hz factor and materials used is unlike turning on and off the same lamp a few thousand times but waiting for it to cool down between turning on and off. The lamp that’s had time to cool down will suddenly get shocked into incandessing. This damages the filament because of the shock and is why most dimmers have a “warming current” running thru the lamps when at 0% on the dimmer. A lamp with some current running thru it, not enough to incandess but enough to have some heat will both go to full faster and be less damaging by way of shocking the lamp’s filament that otherwise at one moment is cold and in the next is burning somewhere between 2,800K and 3,500K. This is the main difference in wondering why a lamp that’s turned on and off will wear out fast, but in cycling thru an alternating current, it’s not really wearing out faster. It is in actuality but the AC lamp is designed around this 50 or 60 Hz cycling. A lamp rated for 60 cycle would wear out faster given 90 cycle because it’s not designed to get that hot, instead the lamp is designed to be on a dimmer 60Hz, and it’s specified voltage. Lower the voltage to a say 120v lamp and it might be better able to withstand a faster cycling of current peaks. Also conversely, put a 12v lamp on 120v and it will work, but wear out really really fast. I call it going super nova as that in many ways is what that filament will do in being really really bright but short in life. Have a 110v lamp next to a 130v lamp and given all else is similar, the 110v lamp will wear out faster but in burning be much brighter. It also won’t be able to take voltage spikes or turning on and off as it were as easily. Turning on the lamp when cold is a voltage spike just as if a lightning strike. What effect turning on the lamp will have is in relation to the magnitude of the current running thru the resistor and it’s rated resistance. Your 115v lamps will be very bright, but will be less sufficient in turning on and off in a cold start than that of a 120v lamp. This especially if the actual voltage while at full to the lamp is at about 118v. Over voltage for one in having that extreme output, under voltaging for the other. Yep, your Par Can will stand up to a cold start better than S-4 Leko.

    Take a piece of 12ga cable. It should be able to have 25 amps go thru it, or a sustained 20 amps going thru it for a period of time. This is not to say that it can’t have more, it’s just that due to size and resistance in the wire it will heat up and wear out faster if not also incandess right before it melts down given high enough amperage or a lower amperage sustained amperage. Your 12ga cable given enough current can right before it starts a fire or melts down be a filament lamp. On the other hand, a strobe lamp that is at times very high in current does not really wear out the wire given time between that current draw in the wire and time to cool down.

    Think also steel while in a molten state as similar to a filament incandescing. That’s a lamp filament. Note also the differences in tempering and quenching steel and it’s effects on the differences in strength of the metal. Tool steel or a knife edge verses that of mild steel that can take an earth quake without breaking due to brittleness. Lots of things you can do to steel in making it or damaging it. Also take away the heat under the steel and it’s not going to get hot enough. Materials of the filament, how they are treated or designed for use, and flame/incandescing supprescents also play a factor in lamp design. Lamps lower than 40w are often just vacuum tubes without air. Others use argon or bromine gas under pressure to suppress the burning up of the filament. - this even for halogen lamps. Add a very expensive krypton or better still xenon gas, and it suppresses the filament burning up further in making for a higher output lamp still. Add alloys to the tungsten fiber and it has the same effect in extending the life of the lamp etc.

    (For more information: Go to the Sylvania website, click onto photo optic lamps and do a browse/search for something like the EVC lamp. Click on one of them it pulls up. This will bring up that lamp’s specifications and at the bottom of the page will be a further reading documents section. Click on it and you will have a PDF document called “Technology and Applications, Tungsten Halogen Low Voltage Lamps Photo Optics.” This free book with another one you can find in searching for a arc lamp like the HSD 250/78 in the same photo optics section and looking for further documents on it are books that are rated amongst the best books on the subject and for free. Print them and read them. You will know all about filament lamps in a language that’s both expert and easy enough to read and understand at 43 pages. Very highly recommended reading.)

    “what if they were running on DC so they are not dimming 50-60 times a second, and therefore extending the life of the lamp?” - propmonkey

    An AC lamp for the most part can be universally run off DC voltage. This given there is not many 120v DC systems out there, so it’s primarily speaking as a general thing for low voltage lamps.

    Than on low voltage and DC lamps we get from GE:
    “The best filament design for use in a shock and vibration environment is a low voltage, high amperage filament. This represents a short, thick filament. The higher you go in voltage, the longer the filament, which then necessitates the use of hangers. The hangers will increase the higher voltage filament reliability, but the effects of notching due to the soret effect are increased. Orientation of the filament in relationship to the axis of vibration will also affect the lamp performance in a shock and vibration environment. The most critical consideration in protecting a lamp against the effects of shock and vibration is the packaging by the user. Filament damage, due to shock and vibration, can be reduces or amplified by the way the lamp has been assembled into the user’s product.”
    Most Stage and Studio type lamps are to some extent shock resistant to some degree by way of filament humm and vibration being designed against due to well supported filaments that don’t vibrate with the current. At least when given the alternating current a humm that’s detectable. The more supports or shorter the wire, the less detectable humm and more rugged the filament in support. On the other hand, as above the more supports, especially once we add a DC current to this effect, the more a factor this support or even grid will have on the lamp in how it cools down or on the molecular level is effected by the current passing thru it.

    From GE’s “Sealed Beam and Specialty Lamp Catalog” we get this:
    “DC voltage can reduce life by approximately 50% to 70% due to the “sawtooth” or notching effect. Notching, also known as electromigration, is a change in the molecular structure of tungsten due to the DC voltage. The results are hot spots that accelerate the thermal conditions, the evaporation rate and embrittlement. DC voltage operation is not recommended for filament lamps.”

    Think about it this way and in what’s not stated where the halogen effect and filaments also burning up is involved. All the current comes from one direction. Since it’s coming from one direction, one part of the filament will have more resistance than another part of it. Dependant upon materials or instances of the lamp’s structure and manufacture, such parts of the lamp most effected by the magnetism might not be at the polar end of the lamp either. More heat from damaged parts of the filament means more burning and heat. While by way of resistance, all the filament gets hot enough to incandess, one part of it is going to be hotter than another part. This will cause it to be brittle, electrons to line up due to the magnaitism and in the case of halogen gas re-depositing spent filament molecules back onto the hottest part of the filament, one part of the filament that does not get replenished as much. As shown in the catalog, under a microscope, you can see actual notches in the wire that develop in the filament thus the term “filament notching.” These are most likely breaks in the filament due to microscopic impurities in manufacture or material. It’s a bent wire, urr yep, it can have problems in having been bent if not bent under the right conditions. Due to a specific current flow, filament notching and heating of one part of the filament or certain parts of it over others can be much more of a problem on DC current than on AC current that balances this out. Filament noise in question might also be interesting with DC current.

    Consider this halogen effect, or in general the cooling of the lamp as you get further away from the filament as per a wind that’s generated by the thermal effects of the heat. Touch the glass of a halogen lamp and either that heat is reflected or attracted to that spot touched. This as opposed to normal cooling cycle by lamp verses filament shape engineering than effects it’s heating and cooling cycle as if a wind blocking wall or tunnel effect. One can only imagine the amount of cooling cycle thermal wind inside of a lamp - sufficient to make it explode at times. Often the filament will be either attracted or blown towards or away from where it’s touched - depending upon say skin oil reflects or dirt attracts and retains heat left on the surface. Normal cold lamp +1 AU (Atmospheric Unit) pressures inside a lamp once heated at the filament are higher than those areas further away from the filament no matter the +AU filler gas or -AU vacuum in use, pressure both increases heat, but also suppresses it once it cannot expand further. Used to be you could take a bunch of lamps and throw them at a brick wall and get a big pop from them as great fun. Not as much fun in taking a bunch of HPL lamps and throwing up against a wall. This noise was from the vacuum in the lamp. Most normal lamps don’t pop unless super heated in very high AU pressure.

    (A xenon follow spot lamp given it’s pressure when hot will explode with the concussion of a hand grenade due to AU pressure. Break it while at room temperature and it’s a major mess but not nearly as loud or dangerous.)

    Instead the use of noble gasses (and we all hate chemistry class) does not require the vacuum in suppressing the fire as it were in the filament. No high or low pressure, no pop. On the other hand, you drop a even high pressure mercury vapor lamp and it’s the same pop and explosion in this case as opposed to implosion but similar noise. (Doot’ there goes $250.00 in lamp and an hour to clean up.) Noble gas fillers are safer to be around at room temperature and much more efficient in suppressing the burn up.

    It’s the same with any gas or vacuum within the lamp in that this wind goes from hot to cool, but has to return in a sort of weather pattern back to the heat source/filament due to vacuum effect.


    In getting back to your flash light or car lamp, given such an above effect on the lamp is known, by design, while a AC lamp is just a wire and it’s resistance incandesses, for DC situations you can design a lamp that when polarized as to which direction the current comes from design a lamp stronger in one area than another, much more heavy duty or just down rate the lamp for DC usage over what it could be otherwise. Also given it’s lower current, such effects of notching etc, will be to a much less extent. The DC specific lamp once designed for such effects DC current will have on it would not react well to AC current. At times you will find some DC specific lamps if not a lot of arc lamps that require a specific direction of current to strike and maintain the arc.

    On the other hand, (I trust no offense on his part)
    Steve B was correct but incorrect in the case of this “halogen effect” as it relates to dimming of the lamp. The actual statement was from something brought up in a I think 1998 Philips catalog:

    “Low Voltage halogen lamps should not be dimmed by more than 10% of their rated voltage since this will result in a reduction in life. Standard tungsten filament lamps (with no halogen filling), can be dimmed to zero volts, resulting in virtually endless life. However if low voltage tungsten halogen lamps are dimmed by more than 10%, the lamp will be operating at too low a temperature and the free halogens in the gas fill, will attack the cooler parts of the tungsten filament i.e. where enters the quartz or glass envelope. The wire at that point will then be eroded and eventually will fail. So if dimmed by 10% or more low voltage tungsten halogen lamps will not have an extended life but are unlikely even to reach their rated life.” - Philips Website, Optical p-1


    Higher 120v+ voltage lamps have sufficient volume of current going thru them that such an effect of dimming is not much a factor here. A 120v lamp even at 20% power and no longer incandescing - realistically dimmer curve level of 0% if well maintained at about 20%. That’s sufficient in power to heat the iodine in the gas sufficient that it’s still functioning to catch the molecules of the filament and re-depositing it on the hot spot of the lamp’s filament. It’s also by volume of voltage still much more than the common 12 and 6v lamps stated above as a problem. On the other hand, once at 0% by dimming curve (80% range not 0-100% in reality), once you get to 0% if your dimmers are left on day and night, just as a fully loaded 12ga cable under full load will wear out and burn up with time, this lamp in getting constant heat and burning even if not incandescing will still wear out. As also stated above, the halogen effect might be too cold to continue working. While it’s doubtful that the iodine in the gas will be sufficient to attack the pinch seal if 120v as a base, this lamp under warming current will still for all intensive purposes be an incandescent lamp thus if left on 24/7 have the same effects on lamp life as that of a lamp on a dimmer in extension of life but it still burning up. In this case, at some point the halogen effect will stop for all intensive purposes working and it will only be a incandescent lamp in life. The iodine in getting board might not attack the pinch seal or cooler parts of the filament, but it’s also not interested enough to replenish the lamp’s filament. By assumption or theory, you will probably see more blackened halogen lamps similar to that of incandescent on dimmers left on day and night (unless given loss of signal they do go to a actual 0% in voltage I don’t think they do.) This would be from a now non-halogen effect lamp burning up it’s filament over time. Might only be a warming current, but it’s still heat burning up the filament. Once the filament is expended and not replenished, it won’t be ever again to any significant extent heated up again sufficiently to go back to the filament. Once burned off in other than halogen effect conditions, that part of the lamp life and filament is instead spent permanently.

    Low voltage by code is what... about ?64 volts and it’s also supported in this less than range for consideration of what’s a low voltage lamp by lamp specification. Just as in design, you tend for highest color temperature and output want to install lamps in the fixtures most close to the expected use, on a low voltage halogen lamp, power is not always good. Yea, you can install a FEL in a Altman 360Q and it will be more power. But on the other hand, if normally at 60%, you loose both color temperature and output over that of a 575w lamp or even 400w lamp given improved efficiency and it at operating capacity. That 400w lamp at full next to a 1Kw lamp bit dimmed to 60% might just have more output and a better less amber shift color temperature.

    For this reason, low voltage lamps are possibly best specified to be not dimmed. Instead just change the wattage of the lamp to fit the need. Cam you dim them for a show, sure, should you use them under a dimmer at home, perhaps not as smart. There is also some especially domestic as with DC lamp corrections possible for this stated effect. Often the premium brand especially stage and studio line of low voltage lamp with improved pinch technology will suffer less from these problems than other brands and types. You get what you pay for.

    While iodine - the primary halogen effect chemical is not an acid, it has the same effect on the pinch seal otherwise by way of escaping thru the glass sealed metal envelope or attacking the metal in this coolest area. But this effect is only where the voltage is too low to sufficiently heat the gas but hot enough to incandess the small filament. It should not be the same with line voltage lamps under a dimmer - this unless their voltage while powered up at some point matches that of the 24v or lower common low voltage lamps above stated. For 24v as say a normal for low voltage, 10% of this is 21.6v. That’s right about at the warming current for a 120v lamp thus the effects of leaving your dimmers on 24/7 is of concern but not yet studied by the lamp companies. I don’t think the dimmer companies on the other hand note a serious problem with this so the actual problem voltage might be more like 11v.

    Might not have as much a halogen gas eating away at the pinch seal, but it’s still a lamp operating that is expected not to gain the extended life of the halogen effect. Instead it’s only a incandescent lamp while under a dimmer. Lamp life will still be good, just not improved by the halogen effect in being great. Should your trim setting on the dimmer be off either in the plus or minus way, this can also have an effect on lamp life. Get your dimmers maintinenced professionally yearly. This will save money and optimize your performance. A dimmer that’s actually set for a real 26% when at 0% on the dimmer, will clip off at 96% instead of 100%. The other discussion of a dim glow to one’s lamp thus is say at 30% and his maximum output of the lamps given 80% ratio but 120v is only at 90% of the maximum output of the lamp by way of potential but nothing there to maximize it. This as opposed to a dimmer trimmed to 10% that also will only get to 90 % because that’s all that is available in a actual 80% dimming range. 5% less - 15% warmer current can extend expected lamp life to a degree that probably won’t be noticed in maxing out at 95%, but more than this you potentially damage your lamps. Even with this you might be.

    There is some science that is probably easy but at this point is not studied or known for sure at this point. That Euro people and those using low voltage cyc lights working in series find as exception to these rules. The low voltage lamp when working in series for some reason such as in a strip or cyc light for some reason does not have problems with under 10% dimming having an effect on the lamp life - no matter what brand of lamp is used. It no doubt is much in an electrical way due to how the system works, but it’s still not well defined as to why you can dim while low voltage, a lamp in series wiring - say 12 of them at 12v in a 120v system without the halogen effect problems on life. Or in the Euro system, two 120v PAR 64 lamps without problems especially with the now in theory 10% warming current to the lamp.

    Consider this abstraction however more exception as to it’s function of a bunch of small filaments acting as one 120v filament however as opposed to any difference of the rule in general. Don’t worry much about your 300 hr/115v lamp lasting only now 50 hours. Be concerned however about the 12v lamp at 10% on the dimmer most of it’s life.

    Finally in response to Steve B’s very good post, lamp life is dependant upon the brand and as shown in IES #LM46-98, “IESNA Approved Method for Photometric Testing of Indoor Luminaries Using High Intensity Discharge or Incandescent Filament Lamps,” the in general lattitude and longitudnal and atmospheric conditions of the testing faculty. What’s tested for life in Southern China won’t in a absolute sense be what the same lamp in Northern Germany will find in output and life of the lamp. Atmosphere and elevation play a factor even one country to the next in Europe as this boring document studied.

    In addition to this, there is no even if using the same test equipment - noted above to have different data dependant upon location, there is certain testing data that is company specified. Sometimes 10% of the lamps in sample are thrown out before that 50% in lamp life is stated. They don’t mention the 10% in saying how well their lamps test nor in many ways do they need to. So you might have 110 lamps in the test, or might have 100 but only after 60% have died do you state a lamp life. Other testing companies have other data on “average” lamp life - or what they consider average. Even Philips today has two figures for their moving light lamps. Both an expected lamp life, and replace before lamp life data published. There is also initial lumens and sustained lumens and color temperature as different and normal.

    In testing lamps, it’s also a question of how average these lamps are that are tested. Is it just lamps pulled from one lot of them - initial specification grade of them or to what extent of mixed and un-known in lot the lamps are. Is of 100 lamps, it all of the same lot in manufacturing or some and one of each in being tested? Is on the other hand, it a question of an average of average of testing of these lamps continuously or the average of one test? Lamp specs constantly change in published tested specification. How new and up to date is your source on what to expect with a new lamp verses that of an old lamp? Specifications are important but very much subject to question in changing or accuracy. Within the same lot number of lamps, dependant upon the time of day (how early in the morning, near vacation, near lunch or quitting time) you will get some that will last longer, and others that will if rated for say 750 hours die after 360 and others that will last over 1,580 hours without problems. About 80% of listed life - given you can accurately track this is warrantied. After and including this, it’s subject to manufacturer inspection of the lamp for warranty and approval of it. You pay shipping at times both ways. Buyer beware in getting what you pay for in a quality control type of way.

    Finally,
    “Even if using DC would allow lamps to last longer it is much more dangerous. Your muscles work on very low levels of DC current (along with lots of chemicals and the such). Defibrilaters use DC current and we all know how a human body reacts to that. If you were to touch a DC source at 120v 20A your body would freeze into place and you would not be able to let go, it can happen with AC as well but generally it doesn't as the changing current will at some point send your muscles the other direction causing you to release the source of voltage (hopefully before 0.6A passes through your heart).” - M Swan

    Yep I touched stupidly the 20 amp circuit. Dimmer never tripped, conduit melted into the palm of my hand, show lighting browned out and I could not let go up until a continuos effort to do so. The actual effect is as follows:

    Effects of Electrical Current on the Human Body
    0.005-2mA = Just noticeable
    2-10mA = Slight to strong muscular reaction
    5-25mA = Strong shock, inability to let go
    25-50mA = Violent muscular contractions
    50-100mA = Irregular twitching of the hart muscles no pumping action (ventricular fibrillation)
    100- >100mA = Paralysis of breathing
    (PGS Power Guard Systems, LLC.)

    Note it does not specify voltage type nor actual voltage.

    This is my field of study in the industry, but I’m not an absolute expert - more home study into it. Questions or something not covered so well?

    Sorry, but in being a good question, the explained why part of the answer is very long.
     
  12. SteveB

    SteveB Well-Known Member

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    Jesus Ship,

    When the hell do you find the time to write all this ?.

    I'm on vacation, so begged off, but my fingers aches just reading it.

    (Grin)

    SB
     
  13. ship

    ship Senior Team Emeritus Premium Member

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    I type really really faaaaaast.
     
  14. jumpjet

    jumpjet Active Member

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    I don't think polarity works on your muscles. I would think that no matter which way the electrons (or holes) are flowing, your muscles will contract, causing you to hold on to that source whether you like it or not.

    Unless maybe Mswan has electrolytic muscles....
     
  15. jumpjet

    jumpjet Active Member

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    But maybe not...

    From the department of energy's "ask a scientist" archive...
     
  16. jumpjet

    jumpjet Active Member

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    Better answer, from ibiblio.org

    So it isn't that the muscles are contracting in different directions, just that they have a chance to relax, be it ever so slight wherein a person could regain their faculties.
     
  17. Schniapereli

    Schniapereli Active Member

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    The lamp only get's worn out when it switches on and off over time, (why some people just leave their followspots on, and mechanically douse it), and also if it is changed between burning, and non burning state rapidly.

    To prevent lights from burning up when lights need to be brouhg up fast (0-100 in 0 sec.) light operators usually use a warm-up feature ontheir boards, or keep the light at about 3% on the cue before. This warms up the light before it is brought up suddenly. The same thing works with AC. The light doesn't cool down enough between cycles that heating it up again would harm it.

    (if anybody's still watching this post...)
     
  18. SHARYNF

    SHARYNF Well-Known Member

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    Ship is an expert in this stuff and can generate tons of great detail.

    Here are a few other points to consider.

    If you really measure AC voltage, you will find that the 120v etc is really an average, and so it has peaks and dips that are around 40 volts either side of the 120, so in some cases you can take a 120 volt rated lamp and run it at around 100 volts dc and get the same level output. (This is non scientific and Ship might have much more to say about this). In a marine environment where I spend some time, it is interesting, but non scientific to see that lamps run this way last for several times longer than the typical ac life.


    The biggest problem is on the dimming side of things making an effective dc dimmer, that is not just a resistive network which is not very efficient, so most of the modern dimming systems are based on a waveform ac model.

    Just from a historical perspective Edison was in favor of a DC system for lighting. and power It was the ability to use transformers, transmit ac at high voltages, low amperages over long distances on "thinner" wires that pushed.


    Tesla was the father of AC and it was Westinghouse that pushed the domination and acceptance of AC

    Sharyn
     
  19. mackem_techie

    mackem_techie Member

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    this is because its not the number of volts that harms u but the currrent because the current is the charge flowing per unit time, whereas the voltage is how much "force" is applied to the electrons in the circuit. However, voltage is related to this as, V=IR, Voltage=Current x Resistance and so, assuming the resistance of your body is always approx. the same, then it will always be approx. the same voltage that harms you.

    This is my understanding of it anyway, please corrcet me if im wrong.;)
     
  20. SHARYNF

    SHARYNF Well-Known Member

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    Freddy
    the point in your explanation that you need to be aware of that is incorrect, is that the resistance in your body is constant. It is this that can get someone killed. If you ever saw the movie Greenmile with the electric chair scene this point about adding water (salt water) is really key.

    Our sweat, and what chemicals our body secret can have a dramatic difference in our resistance. This is why it gets dangerous, the idea of well I did this and I was ok so I can do it again can cause you serious harm. We tend to think of things as having constant resistance but the Human body varies considerably in many cases by several orders of magnatude. In some cases it could be 1000 ohms in others 100,000 ohms.

    You are correct re the voltage, but there is another subtlty, high voltage TENDS to stay on the surface, lower voltage tends to go deeper. There is a story that probably is true about a guy in the US Navy that was killed with a 9 volt battery, not saying it is true but worth thinking about

    http://www.darwinawards.com/darwin/darwin1999-50.html

    Sharyn
     

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