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Running at half the voltage

Discussion in 'Electronic Basics' started by john stone, Dec 11, 2012.

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  1. john stone

    john stone Guest

    Have a 10Watt, 12Volt Hologen lamp for a christmas decoration. But the
    mains tranformer I have is only 6 Volts.

    Apart from being a bit dimmer, would there be any other consequences running
    at the lower voltage? Thanks.
  2. Leif Neland

    Leif Neland Guest

    john stone forklarede:
    A bit dimmer?

    If the resistance of the lamp was independent of the temperature, which
    it is not, the lamp would draw 1/2 the current at 1/2 the voltage,
    giving 1/4 the wattage, 2.5w.

    However, the resistance _is_ dependent of the temperature. Se more at

    But the bottom line: Nothing will fail catastropically, but you may get
    much less light than expected. But it may look nice and christmassy :)

    (with a lot of but's ;-))
  3. A halogen lamp on the correct voltage runs the filament hotter than a
    standard incandescent tungsten lamp. Without the halogen gas filling,
    this would soon cause the tungsten to migrate from the filament to the
    inside of the bulb, where it would blacken the glass. The filament
    would rapidly become thin and then would break.

    At a certain temperature, the halogen will combine with the tungsten on
    the inside of the glass. It then floats around until it meets the hot
    filament, where it deposits the tungsten back where it came from
    (approximately). This depends on having the glass hot enough to allow
    the halogen and tungsten to combine, which occurs at near dull-red heat
    (which is why quartz glass is necessary).

    If you under-run the lamp, the filament will not blacken the glass as
    quickly, but the glass will not reach recycling temperature, so any
    blackening will remain. The relative temperatures of the filament and
    glass will determine whether the life if the lamp is extended or
    shortened. The light output will be very muchreduced and lacking the
    blue end of the spectrum.
  4. To put it a bit more simply...

    A /slight/ reduction in voltage will cause a halogen lamp to burn out
    prematurely, because the bulb doesn't get hot enough to initiate the
    "recycling" process -- but the filament is still extremely hot.

    Half voltage is a huge drop. You should have no problems.
  5. Guest

    At 6V the halogen cycle won't operate so you'll probably get
    discoloration of the bulb after some time and perhaps the bulb won't
    last as long (to the degree that it's a trade-off of lower power vs.
    the elimination of the halogen cycle). Other than that, it'll
    probably be a very yellow light at pretty low intensity.
  6. Guest

    The envelope is probably black by now. Incandescent life goes with
    something like the 12th power of, er, power, so it's not surprising.
    You'll probably find that it won't last all that long, now, at full
    power. It's filament is now on the inside of the envelope.
  7. We run a halogen (over our dinner table) from a dimmer.
    Actually, it's the 12th power of voltage.

    Even a slight drop in voltage causes a big increase in life. For example, a
    5% reduction almost doubles the lamp's life (1.85 times). At a 10%
    reduction, the lamp lasts 3.5 times as long.
  8. Guest

    Of course.
  9. Sjouke Burry

    Sjouke Burry Guest

    Halogen has a build in cycle to keep the filament healthy.
    That does not work at a lower temperature.
    So running it at the wrong voltage will cause it to fail
    Other then that, for a decoration a dim bulb is no problem,
  10. Phil Allison

    Phil Allison Guest

    "John Larkin"

    ** The formula that predicts lamp life is only valid for a small range
    around the nominal voltage, about 10%. The simplifying assumptions it is
    based on are true only in this range.

    Reducing the voltage by large percentages extends lamp life by factor of 10
    or possibly up to 100 - the halogen cycle has SFA to do with it.

    .... Phil
  11. Even a slight drop in voltage causes a big increase in life. For example,
    I'm guessing yes. A friend had halogen lamps on a dimmer, which he dimmed
    only slightly -- and they burned out pretty quickly.

    The halogen cycle requires a very high temperature. Ergo...
  12. Guest

    Because you DON'T run it at full brightness the envelope isn't cleared
    by the halogen cycle. This is a common failure of halogen bulbs. They
    get black, reducing their light output and fail early.
    But you just said you rarely run it at full power. It can't if you
    s/power/voltage, as pointed out earlier.
    12/16, who's counting.
    The halogen cycle stops.
    There are carbon filament bulbs still operating from a hundred years
    ago. <shrug>
  13. Guest

    A faulty assumption doesn't bode well for any conclusions drawn.
    It must be *really* dimmed, unless it's also run for some hours at
    full power. Every halogen I've dimmed has turned black after some
    It makes a difference.
  14. "John Larkin" wrote in message
    Does anyone ACTUALLY READ WHAT IS POSTED? And THINK about it? Why is it
    necessary to explain something in excruciating detail over and over and over
  15. Phil Allison

    Phil Allison Guest

    "William Sommerwerck"

    ** John has done that and written an excellent reply.

    How about YOU read it and THINK about it -

    instead of childishly stamping your stupid fat feet all over the place.

    ..... Phil
  16. "Phil Allison" wrote in message "William Sommerwerck"
    "William Sommerwerck"

    ** John has done that and written an excellent reply.
    How about YOU read it and THINK about it --
    instead of childishly stamping your stupid fat feet all over the place.

    If you'd been paying attention, you'd have seen that his most-recent remark
    (above), though correct /out of context/, has nothing to do with the issue
    at hand.
  17. Phil Allison

    Phil Allison Guest

    "William Sommerwerck""
    ** What "issue" is that ??
  18. "Phil Allison" wrote in message "William Sommerwerck""
    ** What "issue" is that ??

    There are actually two issues.

    One... A halogen lamp has to run at or above the temperature at which the
    tungsten is redeposited on the filament more rapidly than it evaporates.
    This temperature is presumably well-above the temperature of a conventional
    incandescent lamp. It's reasonable to assume that reducing the filament
    voltage some unstated amount will lower the temperature below the critical
    recycling temperature, but still high enough to cause the filament to
    rapidly burn out. No one here seems to have any information about this.

    Two... "Obviously", if the filament voltage is "low enough", the rate of
    tungsten evaporation will be so low, that it doesn't matter whether the lamp
    is conventional or halogen.

    These have nothing to do with each other, as the temperature for One is
    almost certainly well above the temperature for Two.

    This is what Wikipedia has to say. (The following is 100% accurate and
    unimpeachable, of course.)

    "Halogen lamps are manufactured with enough halogen to match the rate of
    tungsten evaporation at their design voltage. Increasing the applied voltage
    increases the rate of evaporation, so at some point there may be
    insufficient halogen and the lamp goes black. Over-voltage operation is not
    generally recommended. With a reduced voltage the evaporation is lower and
    there may be too much halogen, which can lead to abnormal failure. At much
    lower voltages, the bulb temperature may be too low to support the halogen
    cycle, but by this time the evaporation rate is too low for the bulb to
    blacken significantly. There are many situations where halogen lamps are
    dimmed successfully. However, lamp life may not be extended as much as
    predicted. The life span on dimming depends on lamp construction, the
    halogen additive used and whether dimming is normally expected for this

    The article says that the first halogen lamps (for a carbon filament) were
    patented in 1882.
  19. Phil Allison

    Phil Allison Guest

    "William Sommerwerck"

    ** Irrelevant.

    The TRUTH is that the re-deposited metal does NOT repair the damage done to
    the filament - there are many pics that show this.

    The halogen cycle merely keeps the quartz glass clean !!

    ** The simple fact is that halogen lamps do NOT have especially long lives -
    any more than non halogen lamps with the same filaments.
    ** See - it is all about the darn glass.

    ** Note weasel words.
    ** Still all about the darn glass.

    ** Not many - ALL !!
    ** The predicted life extension ( power of 12 or 14 ) is not usable beyond
    about 10% voltage reduction as the numbers become huge.

    ** Yep.

    And the one thing that matters most is the GAUGE of the wire in the

    Most halogen lamps are LOW voltage, hence THICK filaments - leading to
    longer life than for high voltage ( ie 120V /240V) lamps of the same power.

    It is sooooooo simple - if the surface temp is the same but there is way
    more material then it takes longer for the filament to wear out.

    ..... Phil
  20. Phil Allison

    Phil Allison Guest

    "Phil Hobbs"
    ** Really ?

    What is that supposed to prove ? ?

    Is that not a 120V quartz-halogen lamp ???

    A 24V, 150W, 3300K QI ( 35mm slide) projector lamp has a rated life of 50

    Such lamps have been in use common since the late 1960s.

    ** ROTFL.

    It is NOT in the SAME places and huge gaps open up.

    See the pics on Wiki.

    ..... Phil
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