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Cost of electricity for light dimmer

Discussion in 'Electronic Basics' started by Zarbol Tsar, Oct 26, 2004.

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  1. Zarbol Tsar

    Zarbol Tsar Guest

    I have an ordinary room lamp. In the mains lead of the lamp there is
    a dimmer device. It is continuously variable from very dim to full

    If I set the dimmer to give me a dim light then do I pay less for the
    electricty than if I used the light bulb at full strength?

    Thank for any info.
  2. someone

    someone Guest

    Yes, but you pay more than you would by using a lamp of lower wattage.

    For example with a 100 watt bulb, and the dimmer set to deliver 50 watts,
    the lamp may give out approximately the lumens equivalent to a 25 watt
  3. Zarbol Tsar

    Zarbol Tsar Guest

    Do you mean that if the dimmer is set at its midpoint to 50 Watts
    (insted of the full power of 100 Watts) then I pay only for 50 Watts
  4. SQLit

    SQLit Guest

    answer depends on the dimmer. Some dimmers just burn off the excess voltage
    as heat. Ever notice some of commercial dimmers and the huge heat sinks on

    I will bet that if you try an 100 watt lamp and measure it at 100% and then
    again at 50% your not going to see much of a difference in the usage. (ahead
    of the dimmer)

    I once had a museum customer that had huge amounts of track lights on
    dimmers. They would put in 100-150 watt bulbs and then dim to around 50%.
    Dimmers toasted every 6 months. The load was only about 80% of the rating of
    the dimmer. I convinced them to change to 75 watt bulbs and dim 25%. I have
    not replaced an dimmer in 5 years. I have not idea if there was just an bad
    batch of dimmers or what. Under constant use as far as I am concerned
    dimmers are not my friend. I use levels of light which means more fixtures
    in my home.
    Just my view from the cheap seats
  5. Tim Wescott

    Tim Wescott Guest

    Yes, your consumption goes down so your bill goes down (or should).
    Unfortunately your light output goes down faster than your bill, because
    light bulbs can't be efficiently dimmed.
  6. someone

    someone Guest

    I don't know where the point would be to get the 50 watts, but the point is
    that you would be paying for power that would be consumed by a 50 watt bulb
    but getting only the light you would get from a 25 watt bulb. From the
    money saving point of view you are ahead to put in a 25 watt bulb (and pay
    for only 25 watts of power).

    A dimmer is for aesthetic purposes. One little advantage is that the bulbs
    usually last longer when run at low power. Savings there still is tiny
    compared to the loss of lumens per watt.
  7. You're wrong both-sorry but that's it.A dimmer is *not* a potentiometer, so
    there's no energy consumed on it.Usually it has a triac (which is a simple
    power electronic device, like two thyristors in anti-parallel connection).
  8. The heat sink is for the triac.
  9. Phil Munro

    Phil Munro Guest

    Here is my take on dimmers.

    1) Dimmers do NOT burn off power to accomplish dimming. At least not
    the dimmers that we use in residential switching applications, and not
    the dimmers that have been available for 30 years or so.

    2) There is a very small power loss in the dimmer, but that is NOT
    what causes the dimming. Rather the current to the lights is switched
    on and off, causing the average (effective) current to be lowered and
    therefore the incandescent bulbs are "dimmed." The small size of the
    dimmer in the wall box is what causes the "hotness" of the dimmers.
    And the higher wattage dimmers have the larger visible heat sinks
    because they can carry higher current and therefore dissipate more
    (but still small) power.

    3) Bulbs should be chosen to allow the level of lighting that would be
    the MAXIMUM desired at a location (or the maximum fixture rating), and
    then dimmers can be used to allow LOWER levels of lighting when that is

    3a) A more efficient way to accomplish dimming is with switches. For
    example, a three bulb fluorescent fixture works nicely with two wall
    switches to allow one bulb on, two bulbs on, or three bulbs on. The
    bulbs run at maximum efficiency whenever they are on. BUT this requires
    an extra wiring and an extra switch, plus it does not allow continuous
    dimming. (For fluorescent lighting, dimming is more difficult, so this
    method is especially nice.)

    4) If the bulb wattage is ALWAYS higher than the desired level of
    lighting, then a lower bulb wattage should be used since it gives more
    light per watt than a dimmed higher wattage bulb. For example, using a
    60 watt bulb to get a certain level of LIGHT is more efficient than
    using a dimmed 100 watt bull to get that same level of LIGHT. Look at
    the lumens ratings on the two bulb sizes to understand this. Two 60
    watt bulbs equals the same LIGHT output (lumens) as one 100 watt bulb.

    5) Dimmed bulbs will last longer since their operating temperature is
    lower, also:

    6) Using a ROTARY (or slide) on/off dimmer (which brings the light level
    up from zero) is better for the life of the bulbs, since it minimizes
    start-up surge current. I know there has been difference of opinion on
    this, but I am convinced that my opinion is fact. Do not buy the PUSH
    on/off dimmers; they will shorten bulb life, and may shorten dimmer
    life, also (see #8).

    7) Dimmers go bad when the loads they are controlling is higher than
    their ratings. Also, putting more than one dimmer in a box causes
    a derating of its power capability. For example, the standard 600 watt
    dimmer (with no extra heat sink) gets a smaller rating when more than
    one of them are in a ganged box.

    8) Dimmers also go bad when a bulb filament sags and causes a high
    current surge when it burns out. This happens more often with the
    push on/off dimmers since the push on/off dimmers cause a surge of
    current which mechanically breaks an old filament. Ever heard the
    "singing" of a bulb on a dimmer circuit? This is a similar mechanical
    vibration of the bulb filament.

    9) A single standard 600 watt dimmer should last a long time on 400
    watts or so. I know lots of examples of this.

    OK, my time is up. What have I forgotten? --Phil
  10. R.Lewis

    R.Lewis Guest

    Not for the past 40 years they have not - they would be too expensive to

    Ever notice some of commercial dimmers and the huge heat sinks on
    I am sure that there will be plenty of respondents explaining the purpose of
    these (relatively small) heatsinks.
    Have you no idea of the size of heatsink required to 'burn off', say, 100
    watts in air and keep the surface temperature of the heatsink below the
    hundreds of degrees!!
  11. someone

    someone Guest

    Were you replying to a different post than those listed below?

    The illumination output of a standard light bulb varies approximately by the
    square of the applied power. Cut the power in half and you get about one
    fourth the lumens. That is only approximate and varies somewhat with the
    type of filament.
  12. Sylvia Else

    Sylvia Else Guest

    This in particular is why you shouldn't use dimmers with lamps that are
    mounted so that the filament is above the socket. When such lamps burn
    out it is not uncommon for a broken piece of filament to fall and
    briefly short across the two thick wires that feed the ends of the filament.

  13. You pay less with a given lightbulb being dimmed than with the same
    lightbulb not being dimmed.

    However, if you use a lower wattage lightbulb or fewer lightbulbs you
    save even more. Incandescent lightbulbs are very significantly less
    efficient at producing light when dimmed.

    - Don Klipstein (,
  14. When dimmed to half the power consumption, you pay for 50 watts while
    getting less light than a 40 watt lightbulb delivers. This gives
    you about 21% of the light of non-dimmed operation, but due to
    some economies of scale this is brighter than a 25 watt lightbulb
    but still dimmer than a 40 watt one. (It takes about seven 25 watt
    lightbulbs to make as much light as a 100 watt one.)
    As for the midpoint of the dimmer - it is probably other than this.

    Maybe better off with compact fluorescents - get full light with around
    25-33% of full power, and equivalent incandescent dimmed to that low a
    power consumption is around or just somewhat more than a nightlight.

    Splurge a little and you can get a system with dimmable compact
    fluorescents. This requires bulbs, fixtures, special dimming
    ballasts, and dimmer controls to be compatible with each other - get the
    system as a whole.
    Disadvantages: 1) The lower limit is probably "brightish nightlight"
    2) The color does not change to a warmer color the way
    incandescent does when dimmed. Color changes are
    much less and more irregular.

    - Don Klipstein ()
  15. People fear the resistive losses in potentiometer (or rheostat) type
    dimmers that have been obsoleted by triac-based ones for decades.

    However, a remaining issue is that incandescent lamps operate much less
    efficiently when dimmed. As a rough general rule, efficiency of a given
    lightbulb at producing visible light varies roughly with the square of
    power fed into it. (Roughly, only roughly that is.)

    - Don Klipstein ()
  16. Dave Platt

    Dave Platt Guest

    And, as an additional issue, it's generally a bad idea to use a dimmer
    with quartz-halogen lamps. When dimmed, the bulbs run a good deal
    cooler, the halogen-sequestering-and-redeposition of the tungsten
    doesn't work as well, the tungsten tends to plate out on the inside of
    the tube and dim the bulb, and the bulb lifetime is greatly decreased.

    All in all, as others have said, it makes more economic sense to use
    smaller bulbs at full power rather than dimming a high-wattage bulb.
  17. True, but this argues against dimming.

    Better would be to point out that a 100 watt bulb dimmed to the
    brightness of a 60 watt one consumes about 73-74 watts.
    And that a 100 watt bulb dimmed to consume 60 watts produces about 21%
    of its full output, which is less than that of a 40 watt lightbulb.

    Now for the bonus extra trickery: Using fewer lightbulbs is better than
    using lower wattage ones, as long as you get adequate light distribution.
    Higher wattage lightbulbs are slightly more efficient. There is more than
    one reason, but one is that thicker filaments can be operated at a higher
    temperature (better for radiating visible light as opposed to infrared)
    for a given life expectancy.

    True, but for among different ways of achieving a given light output
    lower wattage (or better still fewer) undimmed bulbs will cost less than
    dimmed ones. In most of the USA, the cost of the electricity is so much
    more than the cost of buying replacement bulbs that it pays to consider
    energy efficiency.
    A few bulbs benefit from "soft starting", many and probably most do not.
    Most (but not all) bulbs have zero or negligible fatigue damage to the
    filament from a "cold start" despite a cold start jolting the filament to
    cause a "ping" sound that is audible at close range.
    It is true that most incandescents fail at a cold start. However, for
    most models, the actual damage is caused mainly by operating hours. One
    thing that is true (for most lightbulbs) is that an aging filament becomes
    unable to survive a cold start just a little before it becomes unable to
    survive continuous operation.

    There is a usual prelude-to-failure uneven evaporation of the filament.
    That process causes a "thin spot" that is subject to a temperature
    overshoot during a cold start. In most lightbulbs, such a "deadly thin
    spot" is a deterioration mode of the filament that accelerates at a rate
    worse than exponentially (during operation) once it becomes significant.
    This means that for most lightbulbs, when they become unable to survive a
    cold start their operating hours are numbered. And for most (but not all)
    lightbulbs, cold starts do zero to usually-negligible damage until the
    filament has aged enough for a cold start to be fatal.
    True, but they usually don't blow out immediately unless the overload is
    very severe. Mild to moderate overload merely shortens their life.
    True - the power rating of a dimmer usually assumes that there are no
    adjacent dimmers adding heat (of just a couple to a few watts - that is
    Current surge from burnout is often worse than the current surge of a
    cold start. Often when the filament breaks, an arc forms across the gap.
    The arc can be encouraged by the voltage gradient across the filament to
    expand and go across the ends of the filament, in which case the filament
    is no longer limiting current through the arc. This is what causes the
    "bright blue flash" that sometimes occurs during a burnout, especially a
    burnout during a cold start (when the filament resistance is less and
    allows more current to flow through the arc which makes the arc hotter and
    more conductive).
    Most lightbulbs have fusible links in one of their internal lead-in
    wires so that a "burnout arc" does not pop a breaker or blow a fuse.
    However, this may be inadequate for protection of dimmers.
    I believe probably true. It gets more uncertain when you have a 600
    watt or 540 watt load on a 600 watt dimmer, and it gets worse when you put
    more than one dimmer in the same box since each one adds heat to the
    others (despite the loss in each dimmer being only a few watts).

    - Don Klipstein ()
  18. Hardly ever true - more like 1-2% of the line voltage times load current
    becomes heat.
    Usually the heatsinks are for dissipation of heat amounting to only 1-2%
    of full load power!

    Please consider that a 40 watt soldering iron, maybe as little as a 15
    watt one, can get the heatsinks much hotter than any normal operation would!
    A 100 watt bulb dimmed to half output consumes about 74 watts. With
    losses in the usual dimmers, this amounts to about 75 watts.
    Most likely:

    1. The dimmers were not addequately conservatively designed
    2. Dimmers were placed close to each other or in the same box as each
    other so that they added heat to each other.

    However, lower wattage bulbs dimmed less do indeed give the same light
    for less power consumption and less dimmer heating than you get with
    higher wattage bulbs dimmed more.

    - Don Klipstein ()
  19. I was reading some promiotional stuff on the Lumileds website. They
    pointed out that the white LEDs have the advantage that they can be
    dimmed, and the color temperature doesn't change, in other words they
    don't have the disadvantage of incandescents that as they are dimmed,
    the amount of light per watt diminishes.
  20. Say for instance you had four 25 watt bulbs and you needed only 25 watts
    of light. Well, you switch on only one. and you get 25 watts of light
    and pay for 25 watts of power. But if you dim your 100W lamp to get 25
    watts of light, then you might be paying for 50 watts of power. You get
    less light per watt when it is dimmed.
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