GB1446421A - Electrical switching systems - Google Patents

Abstract

1446421 Street lighting control systems ROYCE THOMPSON ELECTRIC Ltd 6 Aug 1973 [4 May 1972] 20807/72 Heading H2H A switching system controlling the supply of alternating current flowing in a load comprises a solid state switch in one of the supply lines and means to provide a gating signal for the switch, the gating current leading with respect to the supply current such that the gating current is sufficient to cause the switch to conduct fully at the beginning of each half cycle of the supply. The current flowing in a load, such as a street lamp from an A.C. supply L, N is controlled by means of a triac T. The gating signal for the triac T is developed by a photocell P according to the ambient light conditions. During daylight the triac T is non- conductive and the street lamp is switched off. As darkness approaches the voltage across the photocell P increases until at a fixed predetermined light level, say 50 lux, the voltage at a point A assumes a first predetermined value and a transistor TR2 becomes conductive and a transistor TR1 non-conductive whereby the output of the diode bridge D1 becomes open circuited. The bridge therefore ceases to draw current from the lines L, N with the result that a gate current flows to the gate of the triac T which becomes conductive whereby the street lamp becomes illuminated. The gate current leads with respect to the supply current because of a capacitor C2 in the gate circuit. As it becomes darker the voltage across the photocell increases until at a second predetermined higher level, say 5 lux, a thyristor TH becomes conductive to shunt the photocell P so altering the characteristic of the circuit R10, P in box 7. As morning approaches and the light level increases the voltage across the photocell decreases until at a third predetermined light level, say 25 lux, higher than the second but lower than the first, the transistor TR2 turns off, transistor TR1 turns on to complete the output circuit of the diode bridge D1. The bridge then draws current from the supply L, N whereupon the gate current is reduced to render the triac T non-conductive to switch off the street lamp load. Thus the street lamp is switched off at a lower ambient light condition to that at which it was switched on. A further increase in the daylight level causes the thyristor TH to turn off whereupon the system reverts to its normal condition. Section 5 of the circuit provides D.C. bias for the transistors TR1, TR2 and section 6 renders the system insensitive to instantaneous changes in ambient light. In a second embodiment sections 3, 8 of the system are replaced by transistor circuitry. The manner in which the transistors TR1, TR2 are controlled forms the subject matter of the invention claimed in Specification 1,446,422 which has been divided from this Specification.