Protected low-pressure discharge lamp operating circuit

Abstract

A fluorescent lamp control circuit which provides power at high frequency, or example in the order of 35 kHz, includes a push-pull oscillator circuit having two transistors (T1, T2), an inductance element (L1, L1'), and a capacitor (C1, C1') to supply the lamps (3, 3'). To prevent dangerous high voltages upon removal of the lamp load, connected to the oscillatory circuit and to the series resonance circuit, upon removal of failure of a lamp, a protective circuit is provided formed by a thyristor (TH) connected to short-circuit at least one of the transistors (T1) of the oscillatory circuit. Control energy is derived directly from the high-voltage supply by a rectifier (D1, D1') connected to the inductance (L1, L1') of the series resonance circuit, so that sufficient and reliable switching energy for the protective thyristor (TH) is always available, the firing criterion therefor being determined by the breakdown voltage of a breakdown element such as a diac (22) connected to a voltage divider (20, 21) and receiving its control voltage also from the high-voltage connection of the series resonance circuit (4) including the inductance, which also feeds the power supply capacitor (C3) for the thyristor (TH).