GB1284895A - Electronic flash device with automatic light control - Google Patents

Abstract

1284895 Photographic flashlight apparatus WEST ELECTRIC CO Ltd 3 Nov 1969 [14 Nov 1968] 53807/69 Heading H2H [Also in Division H3] A control circuit for a photographic flashlight discharge lamp is so arranged that the quantity of light produced by the tube and the flash duration are controllable in accordance with the light falling on an object being photographed, the energy for the discharge lamp being provided by way of a capacitor. Capacitors 1, 3, 11 are initially charged from a D.C. source 18, 19. A switch 20, which may be the synchronizing contact of a camera, is then closed whereupon capacitor 11 discharges through the primary winding of a transformer 9 thereupon providing a pulse to the starting electrode of a flashlamp 2. At the same time a potential is provided at the junction between resistors 12, 13 which potential is applied to the gate of a thyristor 4 rendering it conductive. Capacitor 1 can then discharge through the lamp 2 to provide a flash. A light sensitive element 6 monitors the light output of the lamp 2 and when a predetermined amount of light has been emitted by the lamp 2-which amount can be preset by a potentiometer 16- a triac 5 is rendered conductive and the capacitor 3 then discharges by way of the triac 5 and the thyristor 4, the reverse polarity of the thyristor 4 rendering it non-conductive to extinguish the lamp 2. Premature discharge of the capacitor 3, should stray illumination impinge on the element 6 before the lamp 2 is illuminated, is prevented by the diode 7. In other embodiments the triac 5 may be replaced by a thyristor or a diac and the element 6 electrically isolated therefrom by a transformer. The light sensitive element 6 may be a photocell.