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Incandescent lamp AC vs DC

Discussion in 'Electronic Design' started by D Yuniskis, Mar 25, 2010.

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  1. D Yuniskis

    D Yuniskis Guest


    I need to hack together a power supply for an "illuminator".
    Bulb is rated 6.5V 2.75A.

    Easy solution is a DC "brick".

    However, I think the original power supply was probably just
    a transformer (i.e., driven with AC, no regulation (?)).

    This got me thinking as to whether there *is* a difference
    driving incandescent lamps with AC vs. DC -- i.e., what would
    the mains frequency have to be in order for there to *be* a

    And, how would that difference manifest itself, otherwise.
  2. For an ordinary bulb a transformer with an output of 6.5V/3A will do. As
    6.5V may not be available, 6.3V will also do. Mains frequency does not

    petrus bitbyter
  3. Sylvia Else

    Sylvia Else Guest

    Skin effect at higher frequencies would reduce the current, and
    therefore the brightness.

    Incandescent lights strobe slightly when driven by AC, and that's
    certainly detectable, even if it's not obvious in most situations.

  4. Tungsten incandescent lamps typically last longer on AC.

    The effect is called "filament notching" (aka DC notching). It's more
    of a problem with very low power long-life bulbs, IIRC, as opposed to
    something like a car tail light bulb.

    Best regards,
    Spehro Pefhany
  5. D Yuniskis

    D Yuniskis Guest

    You've missed the question:
    "what would the mains frequency HAVE TO BE in order for
    there to *be* a difference"

    Surely you don't mean to suggest that exciting the filament at
    500Kc would be no different than DC?
  6. D Yuniskis

    D Yuniskis Guest

    Hi Sylvia,

    Yes, though presumably at *much* higher frequencies.
    Ah, crap! I forgot about that (I tend to perceive flicker
    more than "The Average Joe" -- or "Sylvia" for that matter! :> )
    And, if I tried to control the brightness of an AC driven
    lamp using a typical "dimmer", I would be inviting this
    problem. :(
  7. D Yuniskis

    D Yuniskis Guest

    Hi Phil,

    Apparently a significant effect -- if you chase down the
    URL Spehro provided. I had thought AC introduced mechanical
    stress on the filament (heating and cooling rapidly to
    effectively "vibrate").
  8. D Yuniskis

    D Yuniskis Guest

    Hi Tim,

    I think other (hysterically speaking) power supplies have
    been little more than a tapped transformer. But, i don't
    have that so I am looking at options (hence the post).
    I don't know how to gauge that effect -- other than to trash
    some bulbs and determine it empirically. My concern is that
    they are smallish bulbs (roughly the size of a "tail light"
    in an automobile) but use a fair pit of power (e.g., 20W).

    Lifetime is typically only 100 PoH -- so I don't want to
    make a foolish choice in powering them and find myself
    burning through bulbs at an alarming rate (they are $5-$10
    each and a PITA to replace/install)
    Dunno. The low lifetime leaves me suspicious as to what
    may be going on with the bulb's design :-/
  9. D Yuniskis

    D Yuniskis Guest

    Hi Spehro,

    So, how do bulbs in autos achieve their long lives?
    Excellent resource! Thanks!
  10. Jon Kirwan

    Jon Kirwan Guest

    I've used a pyrometer to observe incandescent lamps operating
    on 60Hz power. I would see something on the order of a 3%
    change in flux at 120Hz. It was a casual playing around at
    the time, though, and my detector system was filtered for
    950nm center and a bandwidth of about 120nm, memory serving.
    Anyway, I gathered from this rough experiment that the
    filament in a typical 60W light bulb remains fairly hot
    (doesn't cool that quickly) but does have some observable
    variation when using an instrument to look more closely.

  11. Jon Kirwan

    Jon Kirwan Guest

    I would imagine this to be worse at lower frequencies where
    the wire could actually cool down and heat up enough. At
    some point with higher frequencies, this 'vibration' wouldn't
    be physically possible.

  12. GregS

    GregS Guest

    I was just thinking of the old discs you put in a lamp socket to make them last. A
    diode I guess. I have to make and modify many lamps and microscope
    lamps to convert the AC to DC for light noise problems. If your working
    with photodiods or cameras, don't need 120 Hz.
    There is a fix using 3 phase to DC convereter. It sort of works with incandescents.

  13. Apparently this was a 110 V lamp ?

    The 6.5 V filament is much thicker and would have a very large thermal
    inertia, so one would expect that the intensity variation to be far
  14. Jon Kirwan

    Jon Kirwan Guest

    Yes. Sorry not to have included that comment in the above.
    I didn't try those at the time. Just commenting about what I
    did try.

  15. The way I heard it, more notably incandescents have a slight
    trend of having shorter life expectancy with DC than with AC.

    The explanation is that a very small percentage of the tungsten
    vapor near the filament gets ionized by the small amount of UV
    produced by the filament, causing a drift of tungsten vapor
    towards the positive end of the filament.

    That causes the negative end of the filament to suffer slightly
    more from evaporation than the positive end.

    Of course, there are other reasons why the ends of the filament
    may suffer unequally from evaporation:

    1. In gas-filled lamps, there is often a convection current. If
    the ends of the filament are unequally affected, then one end can
    get hotter than the other, or be surrounded by gas lower in tungsten
    vapor than the other end, or one end may have more wind from the
    convection current than the other.

    2. An incident of mechanical shock may have stretched the ends of the
    filament unequally, causing the ends to have unequal temperature.

    3. One end may have more filament radiation reflected back to it than
    the other.


    On the other hand, if the filament is thin enough and the AC frequency
    is low enough, then the filament's temperature can vary significantly
    over each half-cycle of AC. Increase of evaporation during temperature
    peaks will outweigh decrease of evaporation during temperature dips.
    That will cause life expectancy to be shorter with AC than with DC.

    - Don Klipstein ()
  16. For most filaments, the frequency would have to get to the megahertz
    ballpark, at least hundreds of KHz, maybe a few megahertz, to have skin
    effect significantly dim the filaments. AWG 30 copper (diameter approx.
    ..01 inch or approx. .25 mm), at room temperature, has resistance not a
    whole lot more at 100 KHz than at DC. Tungsten has more resistance than
    copper, and at typical filament operating temperature has roughly 15 times
    as much resistance as at room temperature.

    - Don Klipstein ()
  17. 950 nm is just a little on the short wavelength side of peak
    spectral power distribution per unit bandwidth wavelength for
    most tungsten incandescent lamps.

    Amount of radiation per unit bandwidth in this case should be
    slightly more than proportional to temperature to the 5th power.

    (Total radiation is proportional to temperature to the 4th power,
    and total bandwidth in wavelength terms is inversely proportional to
    temperature, and the peak usually gets closer to 950 nm as temperature

    If the amount of radiation in a 120 nm band centered at 950 nm varied by
    3% over an AC half cycle for that particular tungsten incandescent lamp,
    then I expect the filament temperature for that particular lamp varied by
    slightly less than .6%.

    I also expect visible light output varied more. I seem to think that
    a 120V 60W 1000 hour 845-890 lumen "A19" incandescent lamp has filament
    temperature around proportional to voltage to the .39 power, maybe .4,
    and light output proportional to voltage to the 3.4 power, maybe 3.5.

    (based on, "extrapolated a bit from my knowledge", from what a 100 watt
    120V 750 hour 1670-1750 lumen "A19" does, as best as I know as reported

    That means photometric output being roughly proportional to temperature
    to the 8.7 power at 2870 K or so, very slightly more at the slightly
    lower roughly 2800 K of the 60 W lamp that I described above.

    Based on this, I would expect the 60W lamp that I described to have
    photometric output varying roughly 1.5 times as much throughout an AC
    half-cycle as such lamp's 950 nm output does.

    - Don Klipstein ()
  18. I have had experience with those "discs" of two types:

    1: Diode: Power consumption reduced to ~58-60% of "normal", light
    output reduced to ~26-29% of "normal". Life expectancy has good chance of
    being increased by a factor of 50-90.

    2: Thermistor: My experience is that those "soft-start" tungsten
    incandescent lamps and claim to double their life expectancy.

    I did one notable experiment where I found that when the "disc" was
    fully warmed up, it had enough resistance remaining to dim the lamp to
    an extent worth a goodly 50% life extension, maybe 55%. That means
    10%, maybe 11% less light while combined power consumption of the lamp
    and the "disc" is at most 2% less than that of the lamp alone.
    By any chance is this the thingy that I somewhat remember as an
    unfiltered 6-diode rectifier for "Y"/"wye" 3-phase AC? (4.5% less average
    voltage and similar less RMS voltage than with filter capacitor, 6.8-6.9%
    less power consumption than with filter capacitor, ~13-15% less light
    without filter capacitor than with, while life expectancy is increased
    maybe 70-80%?)
    - Don Klipstein ()
  19. Sylvia Else

    Sylvia Else Guest

    I thought your plan was to run it on DC anyway.

    DC would surely have to be kinder to the filament by removing the cyclic
    thermal stress.

  20. D Yuniskis

    D Yuniskis Guest

    Hi Sylvia,

    No, I was trying to see what the pros and cons (from the
    lamp's perspective) of AC vs DC drive would be.

    E.g., DC is easiest to "get done" -- dig through box of
    "bricks" looking for 20W @ ~6V, cut off existing "DC"
    connector, attach appropriate connector, done!

    And, DC is easiest to *dim* (since that *should* lengthen
    life expectancy of the bulb).

    But, before investing the time to do this, I wanted to be
    sure there were no other issues that would make this an
    unwise approach.

    E.g., Spehro's posted URL suggests DC related failures.
    I, OTOH, had always *assumed* AC presented more mechanical
    stress to the filament.

    The zinger is the "100 hr" life expectancy. Sheesh! Unpopped
    popcorn lasts longer than that! :-/
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