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What bulb voltage?

Discussion in 'Electronic Basics' started by Richard, Nov 21, 2005.

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  1. Richard

    Richard Guest

    It's funny, but I think it is the case that sometimes the voltage of the
    bulb in the thing you bought does not match the battery voltage. I'm sure
    I've noticed this in the past, often the bulb voltage is lower than battery

    Now, with a 3v batttery source, you would expect that you need a 3v bulb. Is
    this true? Of course, desired brightness (wattage) is then a matter of bulb
    amperage selection. TIA.
  2. The battery label voltage spec is the unloaded voltage. The bulb is
    chosen to match the battery actual voltage under that load. All
    batteries have an effective internal resistance that uses up some of
    the available voltage as current passes through the battery. This is
    why using a bulb specified for a 2 AA cell flashlight on a 2 D cell
    battery will have a short life, but be very bright. The D cells have
    a much lower internal resistance, but the same no load voltage.
  3. Richard

    Richard Guest

    Thanks. Yea, when you look at a bulb out of some battery equipment, you
    quite often fail to see the bulb voltage match the nominal battery voltage.
    I think what this tends to mean is that in most cases the bulb voltage tends
    to be lower than the nominal battery voltage.

    And of course, the main consideration is light output, and I suppose not
    drawing a current higher than the bulb is rated. Voltage of the bulb then
    then does not seem always to be of much concern, certainly it seems to me
    that you seldom find the lower voltage bulbs in battery equipment matching
    the nominal battery voltage.

    I suppose things are slightly different when one is dealing with non battery
    source of power.

    Point noted about different internal resistances of batteries.

    One wonders whether the bulb or battery industry ever produced a table
    having suggested voltage ratings of bulbs for the various batteries.
  4. Richard wrote:
    I think most small, low voltage lamps were designed with a particular
    cell type and number in mind. As cell chemistries have improved, new
    types have been added to work well with these lower internal
    resistance cells. However I have not seen a list of cell and lamp
    combination compatibility. But I'll bet it is available, somewhere.

    Most data is in the this form:

    If you look at page 193 of this catalog:

    and look through the voltage and current ratings for the PR8 through
    CM395X, (all lamps designed for two 1.5 volt cells, I think) you will
    see a general trend that the lower the lamp current, the closer the
    rated voltage is to 3 volts. I think this implies that they expect
    lower internal cell resistance drop for lower lamp current. You could
    probably go to the cell manufacturer's voltage versus current curves
    and deduce what cell chemistry and size would produce these voltages
    at these currents. All these lamp designs were initially requested by
    some customer who needed a lamp for a specific application.
  5. Richard

    Richard Guest

    You know, when one starts out with an interest in electronics first thing is
    messing about with bulbs and battery's. I did not carry the interest on
    professionally, however, I never realised there was so much to say about
    selecting bulbs given a certain set of battery conditions.:c)

    I bought this halloween pumpkin with a lamp in it. It's run off two Duracell
    AA bateries. Actually I bought two, and both bulb have gone already. I think
    they are underated, I mean they must be drawing an overated current. But
    when they were lit the light output was about right.

    Okay, this is what is says on the bulbs: 2.5v 0.2A. Okay so the problem is,
    how do you go about getting a bulb that gives the same light output, but is
    not going to be current overated? Rich.
  6. Richard

    Richard Guest

    When I think about it, I think I would have to measure potential diference
    across the bulb and current to get watts.

    Then seek a bulb that was of the wattage measured.

    And even then I don't suppose the task is an easy one.
  7. I would try to find a similar sized lamp with both a slightly higher
    voltage rating (to increase life) and also a slightly higher current
    rating to produce the same light output power at a lower filament
  8. Richard

    Richard Guest

    Actually isn't that the right answer.

    If when the overated lamp is burning I read 2.4v, 0.3A, then surely I'd need
    a lamp with those specs?
  9. Richard wrote:
    That process would specify a lamp that would run at rated filament
    temperature and that would consume the same power from the battery.
    It would also produce less light than the bulb that was running well
    above its ratings.

    Electrical power to light power conversion efficiency goes up
    dramatically as filament temperature rises. But life expectancy goes
    down dramatically, also.
  10. mike

    mike Guest

    I can't find the reference, but as I recall, there is a twelfth power
    somewhere in that equation...dramatically indeed.

    Design is a tradeoff.
    Most people opt for more light output and shorter battery and bulb life
    in their flahslights. We know that because that's where the market has
    Unless you're an emergency worker, you probably don't use your
    flashlight very much.
    I remember my grandfather used to put a 9V bulb in his 12V hunting
    light. When you've invested $2K in the hunt, short bulb life ain't an
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  11. I used to put a diode in series with the bulb on cordless soldering
    irons to make the bulb last longer. They went from a few hours to a few
    years service when the diode was added.
  12. Richard

    Richard Guest

    Actually, I think you would be reading a voltage higher than the rated bulb
    voltage. If you did not, then you would not be getting the overrated

    Anyway, say the measured voltage was 2.7v, 0.3A. That's overvoltage of 0.2v
    and overcurrent of 0.1A. I ought to be able to approximate the increase in
    light output now given some formula.

    "Light, Life, and Voltage
    For any particular lamp, the light output and life depend upon the
    voltage at which a lamp is operated. For instance, as approximations,
    the light output varies as the 3.6 power of the voltage and the life varies
    inversely as the 12th power of the voltage."

    The normal light output would be 2.5v x 0.2A = 0.5W

    The output when the bulb is being overated would be: 3.6 power of (2.7/2.5)
    = 1.319 times 0.5W = 0.659W

    My math could be wrong here.

    Yes, but how would I proceed from here. I just know that I'm seeking a lamp
    that's going to give out 0.7W

    I've now got to match that with a battery voltage/current characteristic I
    think. But how. This is the tricky bit.

    All this is presuming that I'm seeking a bulb working in it's ratings, which
    I might not if I were to feel efficiency is important.
  13. Jasen Betts

    Jasen Betts Guest

    Also the bulb is often rated at 2.4V, a 3V bulb will last longer.
  14. Richard

    Richard Guest

    Really, all I know is that the battery is supplying 0.5W. I've no idea what
    the light output is, unless I could get to know that from a bulb spec.

    Well, probably wrong. The *light output* will be 1.319 times whatever it
    was in the initial conditions of 2.5v
    All I know is that if the bulb was working at 2.7v rather than the bulbs
    design voltage of 2.5v, the light output was 1.319 times the rated bulb
    light output, i.e, if 2.5v were across it.

    I'm not sure how you would go about selecting a bulb that gave the same
    light output of the over-rated bulb, yet the new bulb within it's maximum
    ratings. Except by a bit of trial and error.

    One ought to be able to calculate these things of course.
  15. Life expectancy varies inversely proportionally with voltage to the 12th

    Light output varies proportionately with voltage to the 3.2-3.5 power.

    Current is usually close to proportional to square root of voltage.

    This "rule" for tungsten incandescent lamps, however, is a
    "1-size-fits-all" and is only approximate, and holds up better for
    "reasonable" applied voltages.

    Now for one more bit on design voltage of incandescent lamps to be used
    with batteries: It is common to have the design voltage only around
    1.2-1.25 volts per cell. Not only is resistance of the cells a factor,
    but also the open circuit voltage of a cell that is halfway used up is
    less than 1.5 volts.

    Keep in mind that flashlight lamps usually have a design life expectancy
    of only 15-30 hours or so at design voltage.

    - Don Klipstein ()
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