Electronic ballast with automatic restarting

Abstract

In an externally controlled electronic ballast, a driving circuit which requires a dedicated supply voltage is provided for the purpose of driving an inventor half bridge. The driving circuit is constructed such that the inventor half bridge blocks when the supply voltage V.sub.cc falls below a threshold value UVLO. Via a self-holding controllable electronic switch and a Z diode DZ2, a voltage monitoring circuit draws the supply voltage V.sub.cc below the threshold value UVLO when an excessively high lamp voltage is detected. The ballast is thereby inactive. A current sensing path which leads via at least one filament of the low-pressure gas discharge lamp serves to detect a change of lamp. The current sensing path controls a further electronic switch, which is likewise connected to the supply voltage V.sub.cc of the driving circuit and can, when it conducts, draw said voltage further to frame than the first switch. The second electronic switch connected to the current sensing path thus permits automatic restarting of the ballast after the change of lamp by bringing the first switch into a non-conductive state again by firstly further lowering the supply voltage V.sub.cc below the value prescribed by said switch.