Patent GB-1048161-A (GB)
Title: Electrical signal responsive apparatus for use with industrial process instrumentation systems
IPC: B22C5/04
Filed: 1964-01-29
Abstract: 1,048,161. Controlled rectifier switching circuits. FOXBORO CO. Jan. 29, 1964 [Feb. 1, 1963], No. 3864/64. Heading H3T. [Also in Division H2] A controlled rectifier is arranged to conduct half-cycles of alternating current when a signal applied to its control electrode exceeds a threshold value and D.C. positive feedback is provided so that conduction of half-cycles continues until the control signal falls a predetermined amount below the threshold value. In Fig. 1 two similar circuits 22, 28 respectively provide outputs when a signal applied at 10, 12 is above one value or below another each circuit receiving also a sawtooth wave so that the duration of conduction depends upon the amount a signal is above or below the values. Circuit 22 comprises a silicon control rectifier 86 to the gate electrode of which is fed the signal 10 in series with a reference voltage derived from a potentiometer 30 and a sawtooth wave produced on line 36. When the sum of the signal and sawtooth wave exceeds the reference voltage the controlled rectifier fires and conducts alternate half-waves of current supplied from winding 78. The resulting output from winding 100 of the output transformer is rectified at 102 and when on sufficient magnitude fires a further controlled rectifier 108 which operates a device such as an alarm or a motor. The output from a further winding 92 of the output transformer is rectified to provide a positive feedback voltage across a resistor 62 in the gate circuit so that the controlled rectifier 66 will stop conducting the half-cycles of the supply voltage only when the input signal falls a predetermined amount below the value at which conduction started. This prevents uncertain operation due to low amplitude noise and ensures conduction for a predetermined minimum time. Circuit 28 which produces an output at 56 when the input at 12 falls below a predetermined level derives its reference voltage from a potentiometer 34, the two reference voltages (at 30 and 34) being derived from the main supply via a rectifier 122 and being stabilized by Zener diodes 130, 136 and 138. Temperature stabilization and protection of the silicon controlled rectifier 66 is effected by connecting a silicon diode 72 in series with the input circuit. This diode is forwardly biased by a D.C. voltage derived from the mains through a rectifier 76 and the potentiometer 70 is set such that variations of voltage with temperature are similar to the variations with temperature of the gate firing voltage of the rectifier so that these variations are compensated. Protection is effected by choosing the value of resistor 84 such that the diode current is equal to the maximum desired gate current, the rectifier becoming non-conductive when this is exceeded. The sawtooth wave generator comprises a capacitive circuit 40, 42 which is slowly charged through resistors 44 and 46 and periodically discharged through a double base diode 52 the charging being linearized by feedback from the emitter circuit of an output amplifier 50.