GB1323551A - Control circuits - Google Patents

Abstract

1323551 Microwave heating supply HIRST ELECTRIC INDUSTRIES Ltd 16 July 1970 [16 April 1969] 19507/69 Heading H2H In a microwave heating apparatus which employs a magnetron, the power supplied to the magnetron is controlled by means of a pair of back to back thyristors, which conduct in bursts, and are switched when the current is zero. The anode supply voltage for the magnetron is rectified by a bridge 16 which is in turn fed from a transformer 14. The power reaching the transformer 14 is controlled by means of thyristors 18 which are switched so that they conduct for a whole number of half cycles of the A.C. input. Zero crossing points are detected by means of unit 20, and the number of cycles for which the thyristors conduct is controlled by means of a potentiometer 24. A phase shift circuit may be provided to ensure that the thyristors switch off at the current zero rather than the voltage zero. A second pair of thyristors (32, Fig. 3, not shown) may be used to control the power supply to the magnetron heater, so that while the anode supply is on the heater supply is off and vice versa. The zero passage of the A.C. is detected by means of a full-wave rectifier MR2, Fig. 4, and a zener-diode D2, the output of these elements being in the form of truncated unsmoothed sign waves. This A.C. signal is used to synchronize a pair of relaxation oscillators using unijunction transistors T3 and T4. The circuit is controlled by means of a unit 40 which converts a voltage input at 22 to a square wave with a variable marked space ratio determined by the input voltage at 42. This is fed to a transistor T1 which forms half of a Schmitt trigger. Depending on the sign of the input voltage to T1, either T1 or T2 conducts. If T1 conducts the relaxation oscillator comprising unijunction transistor T3 is inhibited, and vice versa. Thus pulses to control the switching on of thyristors 18 are produced at 20A and pulses to control switching on of thyristors (32, Fig. 3, not shown) are produced at 20B. A transistor (T5, Fig. 5 (not shown)) may be inserted between the transistor T2, or T1, or both and the unijunction transistors T3 and T4, to ensure that the thyristors are switched on gradually. The transistor is gradually switched to the "on" condition, and capacitor 52 is compelled to discharge through it, so that the frequency of the relaxation oscillator is gradually changed, i.e. the point in the cycle at which the thyristors are fired is gradually altered. The current fed to the magnetron may be controlled by means of a transformer having a capacitor (70, Fig. 6 (not shown)) in its secondary circuit, the capacitor and secondary of the transformer forming a resonant system. The core of the transformer is adjusted so that it saturates when the nominal voltage is fed to the primary. The thyristors may be controlled to fire at the current zero points by means of a pulse circuit, Fig. 7. The input supply is fed via a phase shift network which ensures that the pulses fire the thyristors at the zero current rather than the zero voltage, via a pulse generator 82 which produces pulses corresponding to the zero passage of the current. These pulses are then fed via a gate 84 to a bi-stable 86 having two outputs E and F used to control the anode and heater thyristors. A third output of bi-stable 86 is fed via delay circuits 92 and 94, which comprise monostables to a pair of timing circuits 88 and 90, one of which is variable and the other of which is fixed. These timing circuits are used to control gate 84. Thus gate 84 is opened after a fixed timer period, or a variable timer period, to pass pulses, which arrive at the zero passage of the current to binary circuit 86 which in turn controls the trigger electrodes of the thyristors.