Patent GB-1373625-A (GB)
Title: Pulse generators and applications thereof
IPC: H02M1/08
Filed: 1970-11-06
Abstract: 1373625 Pulse pattern generators PERKINELMER Ltd 1 Feb 1972 [6 Nov 1970] 52903/70 Heading H3T [Also in Divisions G3 and H2] A pulse generator, suitable for controlling the power supplied to a load (e.g. an electric oven) produces a pulse pattern which is variable from one extreme in which each pulse is followed by a large number of no-pulses, through a range in which the number of no-pulses is reduced until each pulse is followed by a single no-pulse whereafter the number of pulses before each no-pulse is increased. The pulse generator describes (Fig. 1) is basically of the type in which a capacitor C2 charges through a resistor R1 until a unijunction transistor U breaks down and in which the break-down is synchronized by a pulse generator 6 feeding its output to the upper base. The charging rate can be varied by varying R4 so that the number of synchronising pulses which occur before an output pulse appears at 0 is variable. The circuit however is modified by the provision of a circuit R4, C1 having a time constant, at least as long as R1, C2 so that the charging current of C2 is partly drawn from C1 but little discharge of C1 occurs when C2 is discharged through the unijunction transistor. Resistor R1 may be replaced by a field effect transistor which has a high resistance  while C2 is being discharged and a low resistance when C2 is being charged. This makes the output pulse frequency non-linearly proportional to the input control current and has another  advantage referred to below. Fig. 2 shows an oven temperature control system using this oscillator. It comprises a temperature sensor 1 connected in a Wheatstone bridge arrangement 2 the output of which is applied through voltage and current amplifiers 3, 4 to the input of the oscillator (in place of potentiometer P). The zero crossings of the A.C. power feeding the oven are detected in a circuit 6 and applied to the oscillator so as to cause the output pulses from the oscillator to occur at the zero crossing region. The pulses are stretched in 7 and used to control a triac (or S.C.R.) circuit 8 connected in the A.C. path to the heater. The oscillator, operating in a manner described above, so triggers the triac circuit that for low power demand each cycle of power supplied is followed by a long gap. As the demand increases the gap reduces until each cycle is followed by a single cycle gap. A further increase in demand causes the number of cycles between gaps to increase. When an S.C.R. is used, half-cycles of power are fed to the heater. It is stated that by the addition of the time constant circuit R4, C1 the demand for power is known and therefore less hunting takes place. Fig. 3 shows a master-slave circuit for supplying complete cycles of A.C. power to the heating coil H. An input pulse from the stretcher 7 spanning the zero crossing will render TR1 conductive during the positive half cycle of the supply. Accordingly current flows through the heating coil via TR1 and a capacitor C5 is charged via TR1, D1, R7. A sufficient charge in C5 renders thyristor TR2 conducting to provide an additional path for the heating coil. During the subsequent zero crossing of the A.C. supply the thyristors are extinguished but TR2 becomes conducting again because of the voltage on C5 applied to its gate, current flowing through R5 also renders TR1 conducting. At the end of the full cycle both thyristors becomes non-conducting and remains so until a further pulse occurs at the input T. The zero-crossing detector 6 of Fig. 1 illustrated in Fig. 4 (not shown) may comprise a potentiometer (R12, R13) which supplies the upper base of a unijunction transistor, the upper arm of the potentiometer normally being short circuited by a transistor (TR) so that the relatively high voltage is applied to the upper base. The zero-crossing is detected by a diode capacitor circuit and is used to render the transistor momentarily non-conducting so that the upper base voltage is lowered and the unijunction transistor fires if its emitter voltage has reached an adequate level. The pulse stretcher (Fig. 5, not shown) comprising a transistor having a capacitor (C8) connected in its collector circuit and controlling a further transistor (TRB) having its output connected to a transformer, the capacitor C maintains the second transistor conducting following an input pulse while it charges. It is stated that the circuit is also suitable for an open loop system.