Parasitic capacitance in pulse-start HID lighting systems

The ignition behavior of pulse-start HID lamps is affected by parasitic cap acitance in lighting systems. Lamp wiring, feed-throughs, sockets, and even the lamps add to the parasitic capacitance that should be minimized for op timum lamp ignition. Parasitic capacitance observed in pulse-start lighting systems manifests itself as a loss of pulse energy available to the lamp a nd as a drain of pulse current available for the transition from breakdown to a glow state during lamp ignition. Additionally, many low-wattage metal halide lamps are equipped with ignition aids such as UV glow bottles which require pulse energy for efficient generation of UV photons for enhanced la mp ignition. This paper examines parasitic capacitance and the sub sequent energy and ig nition current losses to the lamp in a typical low wattage metal halide lig hting system retrofitted with several different pulse-start ballasts. Ignit ion pulse characteristics such as rise-time, pulse width, and ringing can a ffect the amount of energy lost due to parasitic capacitance effects. An an alysis and comparison of ignition pulses which minimize and maximize losses are also presented. Parasitic capacitance values for lamp wires, feed-thro ughs, sockets, and lamps are estimated from leakage current and ignition pu lse voltage waveforms. Finally some comments on minimization of parasitic capacitance and the corr esponding energy loss due to parasitic capacitance are given along with a s imple schematic diagram depicting parasitic capacitance in a basic lighting system.