HID Metal Halide and HPS Ballast Information
What basic safety precautions should be taken when troubleshooting HID ballasts?
Basic safety measures should always be taken when troubleshooting HID systems. Most procedures will require that power be
applied when electrical measurements are made. Wearing gloves and eye protection is a good practice when doing electrical
measurements on HID systems
How does a Constant Wattage Autotransformer (CWA) work?
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
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.
Adding an ignitor reduced re-strike time to 4 to 5 minutes from the standard metal halide lamp time of 10 to 20 minutes.
What purpose does a Reactor (R) Ballast serve within HID lamp technology?
A single coil ballast can be used when the input voltage to a fixture meets the starting and operating voltage requirements of an
HID lamp. In this situation, the reactor ballast performs only the current-limiting functions since the voltage necessary to initiate
the ignitor pulses, and start and sustain the lamp comes directly from the input voltage to the fixture. The reactor ballast is
electrically in series with the lamp. There is no capacitor involved with the operation of the lamp. Because of that, the lamp
current crest factor is desirably low, in the 1.4 to 1.5 range.
Without a capacitor, the reactor ballasts are inherently normal power factor devices (50%). Where necessary, to reduce the
current draw during lamp operation, a capacitor may be utilized across the input line to provide high power factor (90%)
operation, but the addition of the capacitor will not affect how the ballast operates the lamp.
Reactor ballasts limit the number of fixtures that can be used on a circuit because they draw substantially more current during
lamp starting (warm-up) and/or open-circuit operation (burned-out or missing lamp), than when the lamp is operating normally.
What purpose does a High Reactance Autotransformer (HX) Ballast serve within HID lamp technology?
When the input voltage does not meet the starting and operating voltage requirements of the HID lamp, a high reactance
autotransformer ballast can be used. In addition to limiting the current to the lamp, the ballast transforms the input voltage to
the lamp's required level. Two coils, called the primary and secondary, are employed within the core. The operating
characteristics are similar to the reactor.
The high reactance autotransformer ballast is also inherently a normal power factor (50%) ballast but can be corrected to a
high power factor (90%) with the addition of a capacitor across the primary coil. As with the reactor ballast, the addition of this
capacitor does not affect the lamp's operation.
One weakness of both reactor and high reactance ballasts is their inability to regulate lamp wattage as the line voltage
delivered to the fixture or ballast varies. For example, a simple 5% change in line voltage results in a 10-12% change in lamp
The diagram on my ballast label shows that a capacitor is optional. What is the affect on the ballast and/or lamp if the capacitor
is not used?
Capacitors are optional for Reactor (R) and High Reactance (HX) ballast circuit designs. The optional capacitor is used for
power factor correction only and does not affect the ballast or lamp operation. The capacitor, when added, will reduce the
ballast input current while the HID lamp is operating. However, the optional capacitor increases the ballast input current
requirement to level greater than the normal operating current during open circuit condition that occur when attempting a hot re-
strike a hot HID lamp after a momentary power loss or shut off.