Ballast Selection

The operating life of all germicidal lamps is determined essentially by the life of its cathodes. When the cathodes have lost the emission mix on the filament the lamp will fail to light.

The selection of a proper ballast is critical to lamp longevity and performance. UV Equipment manufacturers must consider the compatibility of the lamp and ballast before deciding upon specific components.

The basic function of the ballast is two-fold. First it provides the necessary open-circuit voltage required to ignite the lamp. The second is to regulate the operating circuit of the lamp.

Lamp Starting

 There are three distinct operating modes of germicidal ballasts available in the market today. Understanding the differences and selecting the right one for your specific application is important to deliver both a maximum uv dose and insure the reliability of the product.

Instant Start Ballasts

Ballast Figure 1 Instant Start.JPG

The simplest starting circuit is the instant start circuit. For this reason the wiring and hardware required is typically less than for other circuits. With this circuit the lamp is ignited by applying high voltage to the lamp starting it “instantly”. There is no heating of the cathodes prior to start-up. The process of starting the lamp in this mode creates “sputtering” of the emission mix from the filament to the lamp wall. For this reason instant start ballasts are generally recommended for applications where the lamps run continuously.

Pre-Heat Ballasts

 Ballast Figure 2 Preheat.JPG

This type of circuit provides a pre-heat current to the filaments prior to ignition. Prior to the development of electronic ballasts an external starter (glow switch) was typically used. During the pre-determined pre-heat interval the voltage across the lamp drops long enough for the cathodes to reach proper temperature before the lamp ignites.


 Rapid-Start Ballasts 

Ballast Figure 3 Rapid Start.JPG

Rapid start ballasts also supply a pre-heat current to the cathodes whenever the power is on. The pre-heat and open circuit voltage is supplied to the lamp simultaneously. The open circuit voltage is not sufficient to bring about ionization when the cathodes are cold, but as they heat up, the increasing electron emission reduces the starting voltage requirement to the point where the lamp will ignite. A proper rapid start ballast and lamp design will minimize the loss of emission mix from the filament over the life of the lamp. This type of ballast circuit is recommended for most applications because it reduces the sputter that results with instant start and pre-heat designs. A general claim can be made that lamps will last longer on a rapid start circuit than any other.

Lamp Operating Current

Proper lamp operating current is critical to insure maximum uv output and lamp life. Inadequate or excessive operating current will drastically decrease lamp life thru sputtering and will effect the internal vapor pressure resulting in low uv intensity (overheating and/or low wattage). It is important that operating current is matched with lamp design to assure the proper cathode selection and gas fill to meet the application.

Starting Voltage

Lamp temperature will affect starting voltage. Design engineers should consider ambient temperature conditions in their application when selecting a ballast. Cold environments will increase the necessary voltage required to ignite the lamp. Another consideration will be the distance the lamp is placed from the power supply. The greater this distance the higher the voltage requirement will be.