Patent GB-941308-A (GB)
Title: Improvements relating to pulse generators
IPC: H03K3/45
Filed: 1959-12-09
Abstract: 941,308. Automatic temperature control. GARRETT CORPORATION. Dec. 9, 1959 [Dec. 8, 1958], No. 41917/59. Heading G3R. [Also in Division H3] A control system, in which control is effected by output pulses whose duration varies with the magnitude of an input error signal, the output pulses serving to modify a controlled quantity to reduce the error signal, includes a static lowlevel amplifier and a static power amplifier coupled in cascade, positive feed-back means for producing bistable operation of the power amplifier and negative feed-back means from the output of the power amplifier to the low-level amplifier for alternately reducing and increasing the gain of the latter, so that the power amplifier pulses between its stable states. The output from a D.C. bridge 12, Fig. 2, is fed to control windings 50, 52 of a low-level magnetic amplifier 16, the windings being such that increase in the signal of one polarity will cause one output polarity to increase and the other to decrease. The amplifier has self-saturable reactors 54, 56 provided with bias windings 51, 53. Load windings 58, 60 and 66, 68 are provided in respective arms of rectifier bridges 64 and 72. The bridge outputs are connected to provide a differential signal output voltage for application to the control windings 82, 84 of saturable reactors 86,88 of a power amplifier 18. The power amplifier load windings 90,92 and 94,96 are each in series with a rectifier and supply the output from reactors 86 and 88 to output terminals 177a and 181a. These output terminals supply positive feedback to windings 100, 102 to provide bistability, and also energize field windings 24 of an electric motor M. Reactors 86, 88 have bias windings 101, 103. Interstage negative feed-back is provided from output terminals 177a and 181a to low-level amplifier 16. The negative feed-back circuit includes oppositely directed Zener diodes 112 providing a predetermined minimum feed-back level. Time delay circuits 114, 116 introduce an adjustable time delay in the saturation and desaturation of reactors 54, 56. When the signal from bridge 12 is below a certain value there is no output from power amplifier 18 and when the signal is above a certain value reactor 54 or 56 and corresponding reactor 86 or 88 conduct to drive the motor continuously until the signal from bridge 12 is reduced. Between these two signal values one of the power reactors produces D.C. pulses, the pulse length varying in accordance with the magnitude of the bridge signal. An A.C. output is given from a modified power amplifier, Fig. 3, which includes saturable reactors 170, 172 feeding a motor 174 and replaces the portion of Fig. 2 within rectangle 80. Control windings 183, 184 and D.C. bias windings 186, 187 are provided. Reactor 170 includes load windings 188, 189 each individually in series with a rectifier 194 and both in series with a resistor 196 and field winding 200 of the motor. The output of a rectifier bridge 198 is connected through resistor 202 to positive feedback winding 190. Reactor 172 is similarly arranged. Across opposite ends of the field windings 200, 200a is a rectifier bridge 204 whereof the D.C. output terminals 177,181 supply negative feed-back to the low power amplifier through a circuit including a single Zener diode. In either embodiment half wave reactors 54, 56 may be employed. In a modification, Fig. 8, three motors or other output elements are each connected to the output of the power amplifier through a filter, the filters being designed to pass pulses of different repetition frequencies. The control system is described as applied to temperature control of a room by regulation of air from hot and cold air ducts, the bridge 12 being made up of a sensing thermistor 14, a reference resistor 10 and two thermistors 42,44 arranged in a common supply duct, thermistor 44 being thermally insulated. Specification 860, 227 is referred to.