1. Field of the Invention
The present invention relates to the field of power inverters used for electronic lighting ballasts and the use of stand-by battery power in order that the same ballast may be used to provide emergency lighting at a lower level to conserve battery life in the event of power failure.
2. Brief Description of the Prior Art
There are many varied public domain circuits involving the generation of high frequency inputs for driving fluorescent lamps, compact fluorescent lamps, high intensity discharge and other forms of gas discharged lighting. There are ballasts that operate on direct current that are driven from a battery to supply emergency lighting. Where emergency lighting is needed the standard fixture will contain a regular line current driven ballast as well as a direct current ballast and a battery. In some cases a single battery may be used to supply direct current to several fixtures with emergency lighting capability. Unless, with added expense, transfer relays are employed a separate lamp must be installed in the fixture. Emergency lighting provides the minimum light required to evacuate a building. This is necessary to allow the battery to last as long as possible and not be overly large and expensive. A charging method for the battery must also be provided. In spite of the direct current ballast being smaller because of the lower power required there is still considerable additional expense involved is providing emergency lighting.