Patent GB-1260202-A (GB)
Title: Selective phase angle pulse generating circuit
IPC: H03K17/72
Filed: 1969-02-07
Abstract: 1,260,202. Static inverters; induction heating control systems. WESTINGHOUSE ELECTRIC CORP. 7 Feb., 1969 [14 Feb., 1968; 1 March, 1968],No. 6756/68. Headings H2F and H2H. An inverter comprising controlled rectifiers, e.g. SCRs 51 to 54 (Fig. 1) supplies square waveform alternating current to a parallel tuned load Z, e.g. an induction heating unit, the resonant frequency of which varies according to the characteristics of a workpiece being treated. To ensure maximum heating efficiency and a substantially constant voltage across the load the inverter frequency is made to follow the tuned frequency by means of a pulse circuit arranged to fire the rectifiers at a constant phase angle with respect to the alternating voltage developed across the tuned load irrespective of the frequency and amplitude thereof. The voltage developed is substantially sinusoidal and since in a sine wave the ratio of instantaneous voltage to peak voltage v / V  max = Sin # the pulse circuit determines during each half cycle when this ratio and hence the angle # is a preselected value and produces a firing pulse. The developed voltage V a  is transformed and full wave rectified to produce a voltage waveform Vb which is applied across a potentiometer P1 having resistor sections R1, R2. The fraction of this voltage across R2 is adjustable by the slider and is used to select the phase angle at which output pulses are generated in the pulse circuit. Connected across R2 is a peak detecting circuit D3, Cl. The total voltage waveform V b  is applied through a current limiter R3 and load resistor R4 to bases b1, b2 of a unijunction transistor Q1 During the first 90 degrees of a sensed half cycle of waveform V b , C1 will charge to a peak value V c  proportional to the peak value of voltage V b . After # = 90 degrees the instantaneous voltage begins to drop while due to D3 the capacitor G1 will store the peak charge V c  which is applied to the emitter of Q1. The uni-junction Q1 will remain blocked until its stand-off ratio which is a constant, is exceeded. Thus it is arranged that Q 1 will fire when the said ratio of instantaneous to peak voltage is the selected value thereby discharging C1 through Q and R4 to produce a voltage pulse Vd1 at the angle # from the start of the half cycle sensed. Each pulse Vd1 changes the output state of a flip-flop F which causes the drivers of 51 and 54 or 52 and 53 to supply gate pulses to their corresponding rectifiers. In an alternative embodiment, Fig. 3 (not shown) a pair of transistors Q1, Q2 forming an A.C. coupled regenerative NPN-PNP circuit are used as a comparator instead of the uni-junction. The peak detector voltage V c  is applied to the base of NPN transistor Q1 while the instantaneous voltage is applied to its emitter. When the instantaneous value drops below the peak detected value V c  transistor Q1 conducts at the said angle #. Thereby the PNP transistor Q2 is also caused to conduct driving Q1 more heavily into conduction which in turn drives Q2 to become more conductive and so on. A pulse is produced at the collector of Q1 to operate a flip-flop as before. Cl discharges through Q 1 and the transistors revert to their non-conductive state. A separate power supply is provided for the comparator stage.