HPS is a high pressure sodium streetlight which is a type of HID (high intensity discharge) lamps.  These lamps contain an arc tube inside a glass bulb instead of a filament like conventional incandescent lamps.  The bulb contains a vacuum in which the arc tube is contained.  The arc tube is filled with an excess sodium and mercury arc mixture (amalgam). The ends of the arc tube contain an electrode.  It is between these electrodes that the an electric arc is produced creating the light.  The light is the result of collisions between electrons and ions releasing energy which appears as light.

Electric Arc

To create and establish an electric arc and starter circuit must be used to send  voltage spikes between 2500 to 4000.  This high voltage ionizes the xenon gas to create a xenon arc.  After which the starter circuit turns off and the voltage drops to the lower operating level.  The sodium and mercury arc metals then vaporize and join the arc stream.  The arc current increases until the current rating of the lamp is achieved.  This can take several minutes.


The ballast limits the lamp power during normal operation by acting as an inductive load. At power up, the ballast is  used by the ignitor circuit to create the 4000V starting voltage.

High Pressure Sodium Lamp