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resistence of 60W glow lamp

Discussion in 'Electronic Basics' started by zooeb, Jan 14, 2005.

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  1. zooeb

    zooeb Guest

    Today I experienced a strange fact. I took a glow lamp (classic home
    lamp 230 V, 60W) and I tested the resistence with my digital tester:
    the value displaied was 64 ohms. But this value is wrong (in theory),
    because 230*230/64=827W, while the correct resistence value must be:
    230*230/60=880 ohms. Now, I would like to know if someone of you
    experienced the same thing by himself (and the motivation of a similar
    value) or if my tester is broken. Thank you.

  2. If you measure it with the filament at the same temperature
    that it operates at, you should get the value you expected.
    What you overlook is that the resistance depends strongly
    on temperature. In fact, this effect is why lamps are used
    as crude current sources.
  3. Your tester is not broken, the resistance of the filament goes up with
    temperature. A cold filament has a very low resistance compared to the
    heated filament. A 100W bulb has less than 10ohms of resistance when
    cold in the US (110V).
  4. JeffM

    JeffM Guest

    took a glow lamp (classic home lamp 230 V, 60W)...
    When I think *glow lamp", I think *neon*.
    An ohmmeter will give a reading of infinite.
    The word you're looking for is *incandescent*.
  5. Metals, in general tend to be ohmic (current proportional to voltage)
    only at a fixed temperature. Those rare alloys that hold a very
    nearly constant resistance (volts per ampere for a given shaped chunk)
    are highly prized ot make resistor elements. Tungsten is not such a
    material. It has a very strong positive resistive coefficient of
    temperature. This means that you can use a filament lamp as a
    resistive temperature sensor element (nice and stable, because the
    metal is sealed in a near vacuum, so no corrosion), or as a simple
    current regulator. Or you can break the glass and use it as a self
    heated (just warm) vacuum sensor since it will get warmer and drop
    more voltage in a better vacuum, with a fixed current heating it.
    Just don't expect it to behave as a fixed resistor.
  6. Guest

    Sounds about right to me. There is about a factor of 15 change in the
    resistance of tungsten between room temperature and operating
    temperature (in the neighborhood of 2800 K).

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