High Intensity Discharge lights work wonderfully in hydroponic applications. Not only do HID lamps produce light that mimics sunlight more closely than light produced by other light bulbs, but it does so using less energy and in a manner that extends the life of the bulb. Any type of HID light that you choose to use for hydroponic lights will require a lamp ballast. There are three basic types of ballasts available for lamps used in a hydroponic garden: magnetic, electronic, and digital ballasts.
A magnetic ballast is needed to control the electrical current that flows through the HID lamp. The ballast both ignites the light and keeps it lit. If electrical current was to flow into the light bulb at full voltage, the bulb would burst, so the controlling factor offered by the ballast is necessary for the correct function of any HID light. A magnetic ballast consists of one or more aluminum or copper coils that sit on a core that is made up of steel laminations. The coils and core work in conjunction with a capacitor, and sometimes an igniter. These four parts are most often built into the ballast casing. Magnetic ballasts tend to be less expensive and have a longer life.
A digital ballast offers the same electricity control as does the magnetic ballast, but with the digital model the current conforms to optimal levels of performance for each specific type and wattage of lamp, possible through digital circuitry. Magnetic ballasts weigh more than do digital ones, a consideration in a greenhouse setting, and they also run hotter than digital units. Because of the more consistent application of electrical current, the lamps last longer and function at a higher level using less power with a digital ballast than they might with a magnetic ballast. With a digital ballast, the light produced is more consistent. Magnetic ballasts, especially as they age, become more noisy when they operate, cause the HID lamps to flicker and also contribute to irregularities in the color that the lamp gives off. The digital units can also sense when the lamp is almost used up, and can shut the unit off when it is not functioning optimally.
The specific type of ballast you will need will depend on the HID lamps wattage and type you use. There are many types of grow lights, consisting of high pressure as well as low pressure sodium, and fluorescent grow lights. Each one offers advantages regarding efficiency, lamp longevity, color stability and brightness that you will want to take into consideration depending on how you will use the lamp with your plants. The stage of life of your plants will factor into which lamp will best meet your plants' needs.