General Information about Fluorescent Lamps and Compact Fluorescent Lamps

Discussion in 'General Discussion' started by ashleigh, Sep 10, 2009.

  1. ashleigh

    ashleigh Moderator

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    There have been a few posts here over the last year or two about Fluorescent Lamps and Compact Fluorescent Lamps.

    Specifically, for more exciting reading, see the threads about Megaman CFL's, which brings up some information about T5 linear (strip) fluoros as well.

    http://www.cbusforums.com/forums/showthread.php?t=4097

    http://www.cbusforums.com/forums/showthread.php?t=5434

    General Information about Fluorescent Lamps

    A strip fluoro running off a dedicated dimming ballast can be dimmed, and in theory this reduces the light output as well as power consumption.

    Switch on current & power consumption

    Don't get into the argument about the turn-on current being really high so it's better to leave a fluoro lamp running. The peak current during strike/ignition is high, but of short duration. In terms of pure energy use, it's a silly argument. In terms of ENERGY USE, the lamps should be turned off when not in use. (But... see below - switching cycles affect lamp life)


    Filaments, Plasma, Switch-on, Dimming and Lamp Life

    All fluoro lamps have FILAMENTS or HEATERS. This is a small tungsten wire just like in an incandescent lamp, but it's coated with some special elements (usually Calcium, Strontium and Barium), these elements are used to get the filament, which is also an electrode, to throw electrons off faster.

    All this cleverness is needed so that the lamp can run (throwing off electrons) but also so it can be struck or ignited - which is what gets the gasses inside to form a plasma the current flowing between the electrodes at each end. IN ORDER TO GET THERE you need to get those electrodes nice and warm, and throwing off electrons.

    The heaters/filaments AND ALSO the coating is what helps that on its way.

    So if the heater fails, or all the nice chemicals are depleted, the lamp generally wont work any more because it won't start.

    Your typical fluorescent lamp is a Hot Cathode type - it relies on the heaters to get the lamp running. The starter - such as the old fashioned mechanical glow-type, is there to push some current through the heaters, get the lamp struck, and then the heaters turn off - the current through the plasma makes enough heat for it to all be self-sustaining.

    (As an aside, a Cold Cathode lamp has no heaters, they can still be dimmed, and their principle of operation is similar. They generally also need a higher voltage between ends of the lamp.)

    If you DIM THE LAMP you have to change the current flowing through the tube - difficult but possible. A consequence of this though is that often there is not enough heat to keep the lamp struck - so the solution is to run those heaters a bit - this is one of those necessary evils. If you don't run the heaters, there is not enough self heating because of the reduced current, so the lamp goes out (hence leading to a flash on / run a bit / go out cycle).

    Now - the lifetime of the electrodes / heaters is not infinite. Sooner or later they stop emitting electrons, especially if the chemical coatings are depleted - something which tends to happen more quickly when the heaters are used more. And when that happens the lamp won't run any more.

    Burn-In

    Some lamps also require BURN-IN. This comes about because the chemicals used to coat the filaments have some properties that makes lamp assembly and coating difficult, so the chemicals needed are not what's actually used in the coating. A chemical conversion happens, after lamp turn on, and this requires high temperatures for quite some time.

    So some lamps MUST be run at full brightness - some makers require 100 hours of this - before being dimmed. If dimmed too soon, the chemical conversion is not complete and the lamp won't work very well. The other thing is that if the lamp is dimmed before the conversion is complete, the conversion will not ever complete - so the lamp effectively has a significantly reduced lifetime - and it most likely won't dim properly.

    Likewise, depending on the lamp, when dimmed, the heaters must be used. The more you use the heaters, the shorter they will last. Lamps designed to be dimmed can have suitable heaters - but just like an incandescent lamp where the tungsten filament does not last forever, nor can it last forever in a fluoro. So expect dimmed lamps to have a shorter life than non-dimmed. No doubt the makers will claim otherwise, but you struggle to get away from basic physics-of-failure of the tungsten.


    Brightness

    Fluoro lamps also have a noticeable change in brightness - turn them on and they get brighter over the next few minutes. This is especially noticeable with Compact Fluoros but it does apply to them all. You can also see this effect with a modern LCD TV (or laptop PC, or flat screen monitor) which use (normally Cold Cathode) fluorescent backlighting.

    The brightness change comes about, again because of chemistry. The conversion of the current into scattering and clever particles whizzing around in the tube relies on a little bit of mercury, which forms a vapour, and which causes ultra-violet light to be emitted. The UV light only harms us and is not much good - but it in turn hits a coating of phosphors on the tube wall, these absorb the UV light and get all excited, and emit visible light - which is what we see.

    It turns out that the EFFICIENCY of all these conversions varies with temperature. The greatest efficiency (most light out for the same amount of electrical power) happens when the wall temperature of the tube is about 50 - 55 degrees C. Until the lamp and its innards have warmed up to about that temperature, the light output is lower. For this reason, lamps in cold climates take a little bit longer to "warm up" than lamps in warm climates.

    You might say "but when I touch the lamp its never at 55 degrees" - and its generally not. You only need a small point, on the inside, to be there - the thermal gradient through the tube wall and a distribution of heat generally means it won't feel that hot - but having a lamp feel about 40 to 45 degrees, on the outside of the tube, is pretty normal. Your sink of water for dishwashing is about that temperature, and may even be hotter.


    Compact Fluorescent Lamps

    The CFL is just another fluoro lamp, with a smaller tube bent into a funny shape and with an electronic ballast. The same old issues still apply - you need filaments, heaters, chemical coatings, and so on.

    All the fun and foibles of the strip lamp apply, and then some.

    The electronic ballast / power supply of the CFL is used because a big choke and big magnetics are hard to fit into a tiny package. The clever power conversion does have some unpleasant effects, though.

    CFLs generally have nice low power consumption - that's why they have been forced on us by the ban-the-bulbers. However, that's the AVERAGE consumption - which is what you pay for to your electricity company.

    In practice, though, the CFL does not draw a nice uniform current, instead the current tends to be switched in as a great big spike of quite narrow width. In terms of power, the area under the curve that determines power - make the width narrow and the top higher - the area stays the same, what we pay stays the same. But narrower current pulses are nastier.

    So compared to an incandescent lamp, the CFL might draw less on average, but the peaks are actually higher. When you hook a CFL up to a plain ole switch, or a dimmer, its the PEAK CURRENT that matters!

    It is therefore quite normal to find that you have to use LESS CFL's on a circuit than you could previously use with incandescent lamps. The peak current does all the harm.

    Switching CFL's on and off a lot also tends to be bad news. Non-dimmable CFLs don't have big ugly robust heaters - the lifetime of the lamp seems to be determined more by switching cycles than actual operating life, and when they fail there is usually that nasty brown end - most likely the filament failed (notice how they usually fail at switch-on?), or all the special coatings are depleted.

    Dimmable CFLs have to have a more robust construction with better heaters and a significantly better electronic ballast - this helps get a better lifetime, allow the dimming, and so on. That's why they cost a lot more. The current pulses are still nasty.
     
    Last edited by a moderator: Sep 10, 2009
    ashleigh, Sep 10, 2009
    #1
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