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What is the purpose of the capacitor in series with HID lamps?

Discussion in 'Lighting' started by ., May 19, 2004.

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  1. .

    . Guest

    What is the purpose of the capacitor in series with HID lamps?
    I am guessing that the reactance of the cap acts like a current
    limiting resistor. Does it also improve power factor?

    Thanks

    Chris R
    Canada

    To send me e-mail remove the sevens
    from my address.

    Chrisd
     
  2. Two things:

    1. If the line voltage changes, the lamp current changes. Because the
    leakage reactance in the ballast is designed to be nonlinear (decreasing
    at higher currents), the combined impedance of the capacitor and the
    leakage reactance (they largely subtract from each other, with the
    capacitor having a higher impedance) increases with higher lamp current.
    This causes a partial negative feedback, partially regulating the lamp
    current as a line voltage change tries to change the lamp current.

    Note that the inductive reactance exceeds the capacitive reactance at
    the third harmonic of the line frequency and at higher frequencies.
    Otherwise, the lamp current waveform can be excessively spiky.

    2. The lamp current is leading the line voltage. This same current
    exists in the primary, although with a different magnitude because of
    the turns ratio of the high leakage reactance autotransformer. Put in
    parallel with this the magnetizing current of the primary (the current
    that is drawn even when the lamp is disconnected), which lags the line
    voltage. In normal operation, these come close to balancing out to
    approximate a resistive load.

    At least these are true with the "CWA" ("constant wattage
    autotransformer") metal halide ballasts, which have a capacitor in series
    with the lamp.

    The same happens in the usual 2-lamp magnetic ballasts for rapid start
    fluorescent lamps. They contain a capacitor inside the ballast case, in
    series with the secondary winding of the transformer.

    - Don Klipstein ()
     
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