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Incandescent bulb flicker?

  1. Oct 9, 2015 #1
    In an argument re an incandescent bulb, yeah those things, will flicker at 100Hz or 50Hz given a 50Hz mains power.


    The reasoning is the peak and trough of the sine wave both deliver a peak in candescence ie two peaks per cycle therefore 100Hz flicker??

    Will be settled by experiment soon as we find a shop that sells incandescent bulbs, the pimple face kid in the hardware franchise does not know what a incandescent bulb is...lol.


    So my reasoning is correct??
     
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  3. Oct 9, 2015 #2

    Vanadium 50

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    How much does a filament temperature change in one cycle?
     
  4. Oct 9, 2015 #3
    That would be a difficult thing to measure directly.

    I am going to simply use a light sensor at a sufficient sample rate to catch a few cycles of light intensity which will vary sinusoidally......at 100Hz or i lose the bet.
     
  5. Oct 9, 2015 #4

    Nidum

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  6. Oct 10, 2015 #5

    Vanadium 50

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    The point is that the light is produced by the filament's heat. To get less light, the filament has to cool down. The less it cools down in the milliseconds of that phase, the less variation in light you will get.
     
  7. Oct 10, 2015 #6

    sophiecentaur

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    The actual amount of flicker visible depends on a number of factors. The thermal capacity of the filament is very relevant and low voltage bulbs of the same Wattage as high voltage bulbs will flicker a lot less; their filaments are lower resistance and physically bigger so the thermal delay is greater and the troughs in brightness are 'filled in'. In workshops, with rotating machinery, it used to be (still is?) practice to have low voltage AC bulbs, illuminating drills and lathes so that it was less likely that they could appear stationary. Coiled coil filaments are the most common these days because they produce more light for a given power (iirc) and are a bit more robust. The flicker could be different from a straight wire filament.
    Americans often notice that the flicker is worse in the UK and Europe.
     
  8. Oct 10, 2015 #7
    Lower frequencies ie 10Hz difference is evidently enough to rule out thermal mass effects if the last sentence above is true.

    I guess it is a matter of what is more sensitive, thermal mass and lag in brightness as explained or brightness versus increments in voltage fluctuations.

    Is thermal mass effects calculated by heat transfer equation?
     
  9. Oct 10, 2015 #8

    sophiecentaur

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    The thermal mass effect is sufficient, even at 10Hz, to introduce a phase shift between V and I due to changing Resistance with temperature. If you display V and I on X and Y plates of an oscilloscope, you can produce some very interesting hysteresis plots, even at very low freguencies when you are looking at a small, low power lamp. No reference but I can report having demonstrated this to an A level class. They did not all appreciate what they saw - but what's new?
     
  10. Oct 10, 2015 #9
    How do you display I on a oscilloscope, I would try that.

    Officially I can't find a supplier of filament bulbs, any leads?
     
  11. Oct 10, 2015 #10

    sophiecentaur

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    12V 'pea bulbs' are available in many places. Replacement bulbs for battery torches are to be found in camping shops or, most likely, in electronics component suppliers (maplin / radio shack?). A bulb holder could help and save having to solder to the bulb.

    To see this effect on a scope, use a low value of resistor in series with the bulb (at the earthy end of the generator and scope signal earth). This is your 'current sensing' resistor and you put a scope probe (X) across it to show variations in current through the bulb. You may need to try a range of resistor values to get the best results.
    The other (Y) probe is placed on the supply, which needs to be variable frequency and adjustable output volts (appropriate to the bulb). The scope needs the facility for displaying the A and B channels as X-Y and there will be a position on the time base settings (obviously the timebase is not needed here).
    You will notice that the measured V, when you connect things this way will be total V across the bulb and the current sense resistor. If the signal generator can give you a 'floating' output then you can connect the scope earth to the mid point of the resistor and bulb and connect the Y to the other end of the bulb and the X to the other end of the R. You then need to Invert one of the A or B inputs so that the two signals have the same sense.
    The display will be roughly a diagonal S shaped line which shows the ratio of V and I instantaneously and the curve is due to the R value changing with the brightness. At a very slow (fraction of 1Hz) input frequency, the spot traces out a simple curve. As you increase the frequency, the curve splits into two paths - one as the filament heats up and one as it cools down.
    If you do have the right sort of scope available and a suitable signal generator, the result is quite impressive and will cause interest among your fellow students or workers. You may even be asked to speak at dinners and to give lectures (not guaranteed).
     
  12. Oct 10, 2015 #11
    Was with you until the floating voltage connection, have nevercseen an oscilloscope with that function. I doubt I could interpret what I wouldcsee, sounds nice experiment tho.

    Brightness is directly related to temperature so a more intuitively pleasing experiment would be to capture brightness and total voltage both as a function of time plotted on the samevtime axes IMO.

    There would be no interpretation.

    Hey I got invited to speak at a dinner for a race winning human powered vehicle I designed for school kids.
     
  13. Oct 10, 2015 #12

    Nugatory

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  14. Oct 11, 2015 #13

    sophiecentaur

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    X-Y is not uncommon in (analogue, of course) scopes. In fact I bought a cheap and cheerful one only a couple of years ago (eBay) which has that setting. Unless you are incredibly youthful, you are bound to have had contact with such a scope. It's just that the setting often hides on the panel as another position on the timebase switch.
    Using the mid point as Earth means that the scope is looking at the Volts across each component separately; better than the first and simpler option.
    A light sensor would be good, if you have one.
    THE X-Y display is far more sensitive to phase and timing lag because the separation of the paths of the 'loop' will show itself before you will spot the difference between two normal time traces. But any experiment is worth doing and is always fun.
    Dinner speech = Applause :smile:
     
  15. Oct 11, 2015 #14
    I get the x-y bit just have to think about the floating voltage, always had trouble with differential voltage measurements on an oscilloscope.

    Dinner is best when its free, my vehicle showed rear aerodynamics alone is more important than front aerodynamics alone or no aero and full aero is horrible for human endurance races in humid conditions lol.
     
  16. Oct 11, 2015 #15

    sophiecentaur

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    This doesn't involve "differential" measurement. The scope will just be showing the voltage of each probe to earth (although it is capable of showing the difference between the two probe voltages (A-B) and can also invert one or more of its inputs). If the generator output is floating then the mid point can be connected to earth and the two terminals of the generator will vary around earth quite happily; they will be in antiphase with each other. The volts (to Earth) on the resistor will tell you the current (V = IR) and the other (to Earth) will show the volts across the bulb.
     
  17. Oct 11, 2015 #16

    CWatters

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    What country are you in? I don't know of any that has a total ban on sales of incandescent bulbs yet.
     
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