Anyone who‘s familiar with high intensity discharge (HID) grow lights is well aware that they produce extremely intense light. In fact, a 600 watt high pressure sodium (HPS) bulb produces about 90,000 initial lumens.
However, generating this light requires that a tremendous amount of voltage be present in the HID bulb’s arc tube. If the bulb were allowed to draw current unimpeded, it would explode. So, all HID lighting requires a ballast to provide the bulb with high voltage at startup and then stabilize the arc by limiting the amount of electrical current received by the bulb during operation. A ballast contains all of the necessary electronics to control the bulb so that it operates correctly.
HID ballasts are available in either 120- or 240-volt designs, but they both use the same amount of electricity for a given wattage bulb. While there are some light fixtures that have the ballast attached, most use a remote ballast. The ballast sits on a shelf on the wall of the grow room, and an extension cord connects the ballast to the light fixture.
Older-technology magnetic ballasts are excessively heavy, noisy to operate, and produce a great amount of heat. Magnetic ballasts should always have a fan blowing on them to keep them as cool as possible. Engineers have developed new electronic ballasts (or e-ballasts) to replace magnetic ballasts. These e-ballasts are significantly smaller, lighter and quieter to operate. In fact, some e-ballasts can produce as much as 30% more lumens for the same amount of electricity while producing 30% less heat.
Magnetic and electronic ballast pros and cons
Magnetic ballasts: These consist of a large transformer, a capacitor and a starter contained in a single case. They are cheaper to produce and more durable than electronic ballasts. However, magnetic ballasts are voltage-, wattage- and bulb type-specific; they are designed to operate at either 120 volts or 240 volts, but not both. And using a given wattage ballast with a different wattage bulb will result in fewer lumens produced and a shorter bulb life span. In addition, magnetic ballasts operate with either a metal halide (MH) or a HPS bulb, although there are model that switch to either type of bulb. But these types are best avoided, since they can overpower an MH bulb and shorten its lifespan.
E-ballasts: These use microprocessors to control current and voltage, and the circuitry is extremely sensitive. Even though manufacturers have gotten better at precisely tuning the sensitivity of their ballasts, some unexpected quirks can appear. For instance, e-ballasts can be particular about which brands of bulbs they will accept and which ones they will not. Also, a perfectly good bulb might stop working for no apparent reason; this occurs most often with inexpensive bulbs. If you purchase only premium bulbs, the ballast will likely operate for the bulb’s full lifespan.
E-ballasts are also known for producing radio frequency interference (RFI), which may degrade nearby radio and TV signals. Manufacturers have gotten better at shielding their ballasts, but it’s wise to note the ballast’s RFI output before making a purchase.
Also, e-ballasts can accept both MH and HPS bulbs with the simple flip of a switch. The circuit can include a rheostat that lets the gardener adjust the wattage output of the ballast and thus dim or brighten the bulb to compensate for the different stages of plant growth.
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