Abstract: Switched-mode power supplies having a switching transistor and a transformer, with whose primary winding the switching transistor is connected in series, usually require a starting circuit, which is connected to an input voltage. Since a current continually flows through the starting circuit even during normal operation, it is endeavoured to keep the said current as small as possible. The switched-mode power supply comprises a threshold value circuit, for example a Zener diode connected in series with a resistor, which keeps the driver circuit turned off after the switching-on of the switched-mode power supply, until the operating voltage for the driver circuit has exceeded a specific threshold value which is referred to one end of a winding on the primary side. As a result of this, it is possible to charge a charging capacitor via the starting circuit with a low current, without currents flowing away.

Abstract: The invention relates to a switched-mode power supply having a switching transistor which periodically switches through a voltage which is applied to a primary winding of a transformer, having a base current network and having a switching stage for operating the switching transistor, and having a control stage for stabilizing a secondary voltage, the switching transistor having a switched-on phase, an off phase and an oscillation phase during operation. The switching transistor (TP20) of the switched-mode power supply according to the invention is switched through step by step during switching on, via a high-resistance resistor (RP05, RP06, RP07), a primary winding (6, 7) and a capacitor (CP24) . After this, the switched-mode power supply oscillates autonomously via this winding (6, 7) and is controlled by the control stage (TP24) .

Abstract: The secondary winding is connected, via the series circuit of a capacitor and a second diode, to the charging capacitor which delivers the operating voltage. The centre point of the series circuit is connected to earth via a third diode. This enables energy to be recovered, in that the energy stored in the capacitor is delivered back to the charging capacitor during the inhibiting phase of the switching transistor.

Abstract: In a circuit for operating a fluorescent lamp with controllable luminance, with the fluorescent lamp comprising a discharge path and heater coils, the discharge path is connected through a transformer to a controllable alternating current source. The current through the heater coils is preferably controllable in such fashion that the sum of the current through the discharge path and through the heater coils remains essentially constant.