GB1168118A - Improvements Relating to Electrical Regulator Circuits - Google Patents

Abstract

1,168,118. Automatic control of current and voltage. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd. May 16, 1968 [June 13, 1967], No.27142/67. Heading G3. A power supply regulating circuit includes first means for sensing the magnitude of the output of the circuit to operate current limiting means in the event of a limit current tending to be exceeded and second means, responsive to the proving of the first means by the sensing of at least a nominal current thereby, to with-hold an otherwise over-riding limitation upon the operation of the circuit. As shown, a load 7 is supplied from a D.C. source 19, 20 by way of a rectifier bridge, including thyristors 15 and 16, and a smoothing circuit 21, 22; firing signals derivedfrom a drive circuit 25 control the thyristors 15, 16 to maintain the load voltage constant, this voltage control being superseded by current control when the load current reaches a predetermined value. The drive circuit is actuated to effect the desired regulation by a control signal derived from a comparator 30 which receives a voltage-dependent signal from a voltage divider 23, 24 connected across the load 7 and a currentdependent signal from a current sensor 6, associated with a rectifier 9, connected in the input circuit of the rectifier bridge 15-18. The parallel combination of a transistor 4 and a resistor 31 is included in the control signal path between the comparator 30 and the drive circuit 25, and the transistor is rendered conductive by way of a circuit including a capacitor 10 and resistors 27, 28, 29 when a current is detected by the sensor 6 to provide a low-impedance path for the control signal; should the current sensor 6 or associated circuitry fail, however, then transistor 4 assumes its non-conductive state and the resistor 31 is inserted into the control path so that only an attenuated signal reaches the drive circuit 25 and the output current is automatically limited to a safe value.