Circuit for switching a.c. power to a load

摘要

1,108,963. Fire alarms. BOEING CO. 22 June, 1965 [26 June, 1964; 14 Sept., 1964], No. 26343/65. Heading G4N. [Also in Divisions G1, H2 and H3] A circuit for controlling the A.C. power supplied to a load comprises a bridge having four rectifiers together with a pair of controlled rectifiers so arranged that one of the controlled rectifiers, when conductive, inhibits conduction of the other controlled rectifier and that the change of state from non-conduction to conduction of the controlled rectifiers occurs at a selected point in the cycle of the A.C. power supply. A load 13 is supplied with alternating power from an A.C. source 10 by way of a bridge having four uncontrolled rectifiers 14-17. Across the D.C. terminals 18, 19 of the bridge are connected two silicon-controlled rectifiers 20, 25, the polarities of which are similarly arranged. The anodes 21, 26 are connected together by way of a resistor 29, the cathodes 22, 27 are directly connected together and the gate electrode 23 of SCR 20 is connected to anode 26 of SCR 25 by way of a diode 33. With the switch 30 in the position shown on contact 32 the gate 28 is connected to the anode 26 thus rendering SCR 25 conductive. Negative potential is thereby applied to gate 23 of SCR 20 to maintain it non-conductive and the load 13 is consequently maintained de-energized. To energize the load the switch 30 is moved to contact 31. Irrespective of the point in a cycle of-the A.C. supply at which this changeover is made the SCR 25 will remain conductive until the end of the positive half-cycle and at this point the SCR 20 will be conditioned for conduction which will take place when its anode voltage begins to increase positively from zero thus allowing current to flow in the load 13. An external source 35 provides a small positive signal which is applied to the gate electrode 28 so as to ensure conduction of SCR 25 when the switch 30 is in the OFF position. To counteract a slight time lag which occurs in conduction of an SCR when the anode voltage thereof rises positively from zero means are provided whereby the gate signal of the SCR leads the anode voltage by 60 degrees. In a second embodiment the system is used as a smoke detection apparatus in which case the switch 30 is replaced by a photosensitive resistor and the load 13 replaced by an audio or visual alarm such that alteration in the amount of light falling on the resistor will cause operation of the alarm.