Abstract: In a ballast for parallel operation of discharge lamps in potentially explosive areas, an inverter is provided which has a branch which is assigned to the two discharge lamps, and in this respect is used twice. Two lamp-specific branches can be switched off separately if a fault occurs at the assigned discharge lamp.

Abstract: An electronic, externally controlled ballast has a control oscillator which feeds a fluorescent lamp via an invertor bridge and a resonant circuit. To keep the voltage constant, there are derived from the lamp AC voltage charge packets whose size (and/or phase angle) characterizes the size of the lamp AC voltage. The charge packets are conducted onto a frequency-determining capacitor of the oscillator, which is designed as a sweep oscillator, voltage control thereby being achieved. The ballast provides for reliable operation of gas-discharge lamps. Its design is robust and yet simple, avoids undesired hunting and delivers a good degree of voltage stability.

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.

Abstract: An electronic ballast is provided for the operation of one or more gas discharge lamps. The electronic ballast has at least one inverter half bridge and a driving circuit that prescribes the inverter frequency. In the event of overvoltage across a gas discharge lamp, the driving circuit is brought into a locking state in which it blocks the inverter half bridge. This is performed via a circuit resembling a thyristor, which reduces the supply voltage of the driving circuit below a predetermined value. Provided for the purpose of unlocking and restarting the driving circuit is a detector circuit which detects the insertion of a new gas discharge lamp into its respective holder. This is accomplished by pinpoint detection of a steep pronounced voltage rise at a terminal of a gas discharge lamp which is connected to the intermediate circuit voltage via the filament of the gas discharge lamp.