HID Ballast

Constant Wattage Autotransformer Ballast (CWA)

The constant wattage autotransformer H.I.D. ballast type should be used where a stabilized light output is required. The constant wattage autotransformer (CWA) is an H.I.D. ballast type that comes in a fairly small economical size, yet still provides a reasonable degree of regulation. The CWA ballast also offers the advantage of a high power factor, low line extinguishing voltage, and line starting currents that are lower than operating currents.

To correct the high current draw associated with reactor and high reactance ballasts, and to provide a greater level of lamp wattage regulation, the 2-coil Constant Wattage Autotransformer (CWA ballast, also referred to as a "Peak Lead Autotransformer") was developed. It is the most commonly used ballast circuit for medium and high wattage (175 – 2000W) applications and typically represents the best compromise between cost and performance. The CWA is a high power factor ballast utilizing a capacitor in series with the lamp rather than across the input. The capacitor works with the core-and-coil to set and regulate the lamp current to the prescribed level.

The CWA ballast provides for greatly improved lamp wattage regulation over reactor and high reactance circuits. A ± 10% line voltage variation will result in a ± 10% change in lamp wattage for metal halide. The metal halide and high pressure sodium ballasts also incorporate wave shaping of the open circuit voltage to provide a higher peak voltage than a normal sine wave. This peak voltage (along with the ignitor when used) is used to start the lamp and control the lamp current crest factor (typically 1.60 -1.65).

With the CWA ballast, input current during lamp starting or open circuit conditions does not exceed the input current when the lamp is stabilized. CWA ballasts are engineered to tolerate 25-30% drops in line voltage before the lamp extinguishes (lamp dropout), thus reducing accidental lamp outages.

Constant Wattage Isolated (CWI)

When electrical isolation for the lamp or premium lamp power regulation is required, CWI ballasts are recommended. They are similar to the CWA in operation and characteristics except the primary and secondary coils are not connected. When the secondary coil does not share a portion of the primary coil, as in the case with CWA ballasts, more efficient shunting of the ballast core flux can be achieved by the shunts of the ballast. This provides better lamp power regulation than with CWA ballasts. The electrical isolation is an important consideration when line to line supply voltages are used. The isolation allows the lamp screw shell to be safely grounded, and not a shock hazard when the lamp is removed.

With the isolated secondary, the low starting current, and inherent secondary short circuit protection, the CWI design eliminates the need for time delay relays, oversize circuit breakers, and heavier wiring required by reactors and lag autotransformer ballasts for the same number of fixtures per circuit.

Lamp wattage and consequent light output from the lamps is maintained in spite of line voltage variations as great as 13% from nominal. Lumen output is regulated to within 2 or 3%. This ensures reliable starting and operation within lamp specifications, and guaranteeing full lamp life and optimum performance. Lamp extinguishing due to line voltage dips is improved over the previous circuit types. The CWI circuit will hold the lamp in operation even if supply voltage dips to 25 % below normal.

CWI is preferable from safety point of view. The use of the CWI ballast is a safety requirement for some country.