The Luminance Decreasing Phenomenon of the Cold Cathode Discharge Lamp Operating with a Capacitor Ballast

Cold cathode discharge lamps are usually operated with a capacitor ballast, and are supplied by a high-frequency sinusoidal voltage. At some frequencies of the supply voltage, the current waveform of the positive period differs from that of the negative one and the luminance of the lamp is lower than when the current waveform is symmetrical. In an asymmetrical waveform, the current waveform in one half-cycle is pulse-like with a high peak and the waveform in another half cycle has a low amplitude. We calculated the equation of a simple equivalent circuit, and found that the discharge current consists of a sinusoidal steady component and a transient pulse component which increases as the delay time for re-ignition increases. The asymmetrical waveform of the discharge current occurs when the supply voltage, the operating frequency, the lamp diameter, or the capacitance of the ballast is decreased. Since decreasing the value of those parameters increases the delay time for re-ignition in one half-cycle, the re-ignition voltage drops and the delay time for re-ignition becomes shorter in the other half-cycle.