GB1455875A - Electric control circuits - Google Patents

Abstract

1455875 Lighting control systems GEORGE H SCHOLES & CO Ltd 14 Feb 1975 [20 Dec 1973] 48879/73 Heading H2H A lighting control unit 12 comprises a panel 13 upon which are mounted the elements for controlling the current in the circuit connected to terminals of the unit, the elements including a triac T1, a manually variable resistor VR 1 and a photosensitive resistor PR1 which responds to ambient light. Terminals 10, 11 are connected to an A.C. supply and an incandescent lamp such that the current in the lamp is controllable by the setting of the resistor VR1 which determines the charging rate of a capacitor C1 which in turn determines the point in the A.C. cycle at which a diac D1 breaks down and the triac T1 fired. The photosensitive resistor PR1 controls the charging rate of a capacitor C2 thereby also controlling the breakdown of the diac D1 and the firing of the triac T1 thus providing further control of the lamp current. A capacitor C3 and an inductor L1 form a filter to reduce interference being transmitted into the mains and, if desired, the resistors R1, VR1 may be shunted by a resistor R4 and a presettable variable resistor VR2 in series. The variable resistor VR1 mounted in the panel 13 is operable by means of a knob 14 and the knob is provided with a skirt having an opening 17 which cooperates with an opening 16 in the unit 12 behind which opening the photo-sensitive resistor PR1 is disposed. Thus when the openings 16, 17 are aligned the lamp is controlled by the photosensitive resistor PR1 in accordance with the ambient light and when the openings 16, 17 are misaligned the lamp is controlled by means of the variable resistor VR1, The photosensitive resistor PR1 is of the type which is insensitive to the spectral light distribution of an incandescent lamp. The lamp may alternatively be of the fluorescent type.