A ballast is an electrical device which is used to provide power to a load, such as an electrical discharge lamp, and to regulate that power, both as to its voltage and current. An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 kHz. High Intensity Discharge (HID) lamps, such as mercury vapor, metal halide, high-pressure sodium, and low-pressure sodium, are used for a variety of lighting tasks. HID electronic ballasts have been gaining in popularity due to their efficiency and capability to increase the life of a lamp associated with the HID ballast. An electronic ballast for a discharge lamp comprises a half-bridge inverter, a current transformer, and a load circuit including a discharge lamp. The inverter provides a square wave output voltage, which switching frequency is imposed by the lamp controller. The square wave output voltage is processed by a resonant output circuit that provides low current to warm the filaments, high voltage to ignite the lamps and a controlled current to power the lamps. Phase sequences and management is driven by the lamp controller. Electronic ballasts with full bridge circuits are preferentially employed for the operation of high pressure gas discharge lamps, but find employment also for low pressure discharge lamps or fluorescent tubes as well. The typical fluorescent lamp is composed of a glass tube containing an inert gas and a small amount of mercury. Phosphors coat the inside of the glass tube, and each end of the glass tube includes an electrode. In operation, a ballast provides current to the electrodes. Electronic ballasts, due to its small form factor, light weight, less power consumption, and stable light beams, have become the mainstream of fluorescent lamp ballast. Basically the electronic ballast is a combination of circuits that converts alternating current (AC) into direct current (DC) and then from DC back to AC. In electronic ballasts, protection circuits are implemented in order to protect the lamp from damage due to excessive voltage, current, and heat. When a fault condition occurs, the electronic ballast is shut down or shifted to a different mode of operation.
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