Semiconductor-controlled operating circuit for one or more low-pressure

Abstract

To operate one or more serially connected low-pressure discharge lamps, tcally fluorescent lamps, a preheating circuit is provided to preheat the electrodes of the lamps (E1, E2, E3, E4), which changes from low-impedance to high-impedance state after the lamps have been preheated by controlling a semiconductor switch (Q3) in the heating circuit. In accordance with the invention, to eliminate reliance on the resistances of the lamp filaments themselves, which are subject to variation from lamp-to-lamp due to manufacturing tolerances, and later on, to changes due to aging of the lamp, and to provide for reliable switching of the semiconductor switch, a sensing impedance element (Z), which may be an ohmic resistor or a capacitor (Z', Z'), is serially connected to the switching path of the semiconductor switch (Q3) which, typically, is a field effect transistor (FET). The voltage drop across the series circuit formed by the impedance element (Z, Z', Z') and the semiconductor switch (Q3) is set by suitable dimensioning of the impedance element, to be sufficient to retain the main switching path of the semiconductor switch in low-impedance state when it carries full heater current, that is, is already in low-impedance state. To change over to high-impedance state, control signals to the semiconductor switch are removed, for example by shunting a resistor (R2) in a voltage divider, thus turning the semiconductor switch (Q3) OFF.