Patent GB-827476-A (GB)
Title: A d.c. amplifier of the chopper type
IPC: H03F3/40
Filed: 1956-03-20
Abstract: 827,476. Automatic temperature control systems. UNITED AIRCRAFT CORPORATION. March 20, 1956 [March 24, 1955], No. 8644/56. Class 38 (4). [Also in Group XL (c)] A D.C. amplifier of the chopper type has a feedback connection including a resistor and capacitor in series and a rectifier connected between earth and the junction of the resistor and capacitor to limit the feedback. In Fig. 1 a variable delivery engine-driven pump 26 supplies fuel from a tank 28 through a conduit 40, valve 46, adjustable orifice 30, and conduit 56 to fuel injection nozzles 22 in the combustion chamber of a jet turbine 10. The delivery of pump 26 is adjustable by a piston 64 to both sides of which fuel from the pump is supplied through a conduit 58. The supply to one side of piston 64 is through a restricted orifice 60 so that bleed through a conduit 72 is effective to move piston 64 to reduce pump delivery. This bleed is normally controlled by a flapper valve 78 operated by a spring-loaded piston 80 which takes up a position dependent on the pressure drop across orifice 30. Pump delivery is thus automatically adjusted to maintain rate of fuel supply constant at a value determined by the setting of orifice 30 and the spring loading of piston 80. The output of a thermocouple 34 responsive to turbine discharge temperature is applied to an electronic amplifier 36 the output of which, when the temperature exceeds a predetermined value, operates a polarized electromagnet 32 to control a flapper valve 92, in a manner similar to that described in Specification 820,316, so that a further bleed through conduits 72 and 88 over-rides the effect of flapper valve 78 and reduces pump delivery to prevent excessive engine temperature. Additional bleeds through conduit 72 to reduce fuel delivery when other variables, e.g. compressor pressure, become excessive, are referred to. Amplifier 36 is shown in detail in Fig. 2. The output of thermocouple 34, which is provided with a cold junction compensator 100, is opposed to a reference voltage from a potentiometer 105 and the difference applied to a continuouslyoperated switch 116 of a chopper-type thermionic valve amplifier. The output from a synchronously-operated switch 128 of the amplifier is applied through leads 130, 132 to electromagnet 32, Fig. 1. When the temperature is below the maximum permissible value set by potentiometer 105 the input to the amplifier is positive, the output is negative and electromagnet 32 maintains valve 92 closed. When the maximum permissible temperature is exceeded, however, polarities are reversed and valve 92 is opened to reduce pump delivery. Negative feedback is applied to the amplifier through a condenser 148 so that amplifier gain is high under steady state conditions and reduced when temperature is changing. A by-pass condenser 152 in the feedback circuit ensures that high-frequency changes in the amplifier output are not effective to reduce amplifier gain. The feedback circuit also delays the reversal of polarity when the temperature rises from below to above the maximum permissible value so that overheating is permitted temporarily when a manually-controlled throttle calls for rapid acceleration. To prevent too great a delay in this respect a rectifier 150 limits the voltage to which condenser 148 may charge during undertemperature conditions. Normally, under excess temperature conditions the opening of valve 92 is proportional to the temperature error. A network 134 compensates for the inductance of electromagnet 32. Specification 822,443, [Group XXVI], also is referred to.