GB1320644A - Electronic flash device for photography - Google Patents

Abstract

1320644 Electronic flash control WEST ELECTRIC CO Ltd 21 April 1971 [27 April 1970 (2) and 8 June 1970 (3)] 10404/71 Heading G2A An electronic flash device has a series circuit of a luminescent discharge tube 2 and a gate turn off element 3 connected across a main discharge capacitor 1, and a trigger circuit to simultaneously provide trigger pulse voltages to both the discharge tube and the gate turn off element, a control capacitor 6 being discharged to impress a reverse cut-off voltage across the gate and cathode of the gate turn off element upon triggering of a semi-conductor element 8 by a luminescence detection circuit 9 receiving a predetermined quantity of light reflected by a foreground subject. The trigger circuit consists of a series circuit of a trigger capacitor 12, the primary coil of a transformer 13, a switch 11, a resistor 4 and a diode 5, the trigger pulse voltage to the gate turn off element being produced across the resistor and the trigger pulse voltage to the discharge tube being produced in the secondary coil of the transformer. Diode 5 is connected to block the discharging current from control capacitor 6 being by-passed through resistor 4 either during normal operation or as a result of the incidence of external light other than due to the luminescence of the discharge tube 2, the latter occurrence triggering thyristor 8 which would discharge capacitor 6 in the absence of diode 5. In modifications, a variable resistor is connected across control capacitor 6 to provide a suitable gate voltage to the gate turn off element, Fig.2 (not shown); the voltage on trigger capacitor 12 is controlled by a voltage divider connected via a diode to the junction between resistor 14 and the primary coil of the transformer 13, Fig.3 (not shown); a passive element such as a resistance or an inductance is connected in series with the discharge tube, adjustment of which varies the charging voltage of control capacitor 6, Fig.4 (not shown); a series circuit of a resistor and a capacitor is connected across the anode and cathode of the gate turn off element to allow a small glow discharge current to flow through the discharge tube and thus to ensure reliable discharge of the main discharge capacitor.