Patent GB-958311-A (GB)
Title: Circuit arrangements for starting a self-generating controlled semi-conductor rectifier oscillator
IPC: H03K3/352
Filed: 1962-01-30
Abstract: 958,311. Controlled semi-conductor rectifier oscillating circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. Jan. 30, 1962 [Feb. 2, 1961], No. 3457/62. Heading H3T. [Also in Division H2] A circuit arrangement for starting a self generating controlled semi-conductor rectifieroscillator comprises a capacitor connected to a direct-voltage source through a resistor, a discharge element which in series with a directcurrent impedance is connected across the capacitor, so that at a given value of the voltage across the capacitor the discharge element becomes conductive and a pulse is produced across said impedance, which pulse is fed to the control electrode of a controlled rectifier of the oscillator. The self-generating oscillator comprises two push-pull connected silicon-controlled rectifiers 11, 12 which have their anodes connected together through resonant circuit 13, 14 and their control electrodes connected to windings 19, 20 on a ferromagnetic core 6, the output of the oscillator being taken from winding 16 of transformer 15 and fed to a load 18 which may be a fluorescent gas-discharge tube. The oscillator starts to self-oscillate when a current pulse is fed to the control electrode of one of the rectifiers 11, 12, the current pulse being produced across a resistor 5 when a gasdischarge tube 4 is caused to ignite when the voltage across capacitor 1, connected to the supply 3 via resistor 2, reaches a sufficiently high value. During the oscillation period of the oscillator negative pulses from the anode of rectifier 11 are applied to the junction of capacitor 1 and resistor 2 via rectifier 7 and current-limiting resistor 8, with the result that these pulses charge the upper electrode of capacitor 1 negatively so that it cannot become so positively charged via resistor 2 and hence not have a voltage across it sufficient to ignite the tube 4 after the oscillator has once been started. A rectifier 9 is connected across the capacitor 1 in order to prevent the tube 4 from firing if the negative pulses at the junction of capacitor 1 and resistor 2 are such that the potential of this point would become more negative than that of the negative terminal of voltage source 3. In a second embodiment, Fig. 2 (not shown), the current pulses produced by the discharge of the capacitor 1 via the gas discharge tube 4 are amplified by a PNP transistor before they are applied to resistor 5. In the circuit of Fig. 3 the discharge element formed by the gas-discharge tube 4 is replaced by the emitter-collector electrode path of PNP transistor 4<SP>1</SP> having a strong feedback applied thereto by means of transformer 28. The values of resistors 2, 24, 25 and of capacitors 1, 26 are such that the positive potential at the junction of capacitor 1 and resistor 2 increases at first more slowly than that at the tapping point of voltage divider 24, 25 which latter potential cannot exceed a predetermined value. When the potential at the junction of capacitor 1 and resistor 2 becomes more positive with respect to the tapping point of the voltage divider, the emitter of transistor 4<SP>1</SP> becomes positive with respect to its base and because of the feedback becomes abruptly and strongly conducting and passes a current pulse through resistor 5 which starts the oscillator oscillating. As in the embodiment of Fig. 1 rectifier 7 and resistor 8 ensure that capacitor 1 is not charged positively to a sufficient extent to render transistor 4<SP>1</SP> conducting when the oscillator is generating. In order to prolong a time delay between the connection of the starting circuit, and of the self-generating oscillator to be started, to the voltage source 3 and the production of the first starting pulse, e.g. when heated cathodes of luminescent tubes are to be preheated before applying the striking and running voltages a second resistor, Fig. 4 (not shown), is connected between the supply 3 and resistor 2 and a second capacitor connected to the junction point of these two resistors. Specifications 735,047, 935,235 and 940,669 are referred to.