Patent GB-1132476-A (GB)
Title: Thyristor-controlled power supply system
Inventor: MORRIS JOHN DENIS
IPC: G05F1/455
Filed: 1966-07-12
Abstract: 1,132,476. Automatic control of current and voltage. HIRST ELECTRIC INDUSTRIES Ltd. May 24, 1967 [July 12, 1966], No.31262/66. Heading G3R. In a thyristor-controlled power supply system of the type in which firing pulses for phase-angle regulation of the thyristors are derived from a circuit responsive to output voltage and current variations, and wherein, for protective purposes, voltage control is superseded by current control when the output current exceeds a predetermined level, there is provided a further circuit which protects the system and a connected load such as a transformer-rectifier supplying a magnetron, against the excessive initial inrush of current which can occur when the power is first switched on. As shown, a voltage-monitor signal is applied to an input terminal 11 and at point 13 is compared with a reference signal derived from a circuit 14-18 the resulting error signal being applied to the base of a first transistor 20 which controls further transistors 30, 33, 36 to produce at terminal 39 an appropriate firing pulse for the thyristors of the system (not shown); in addition, a current-monitor signal is applied to terminal 21 and is compared at point 24 with a reference signal derived from circuit 25, 26, the point 24 being the junction between resistors 23 and 27 which are associated as shown with respective capacitors 22', 26'. The signal at point 24 supplied to the base of a second transistor 28 which is thus held off, the transistor 28 becoming conductive only on the occurrence of a current-monitor signal corresponding to an excess output current, the transistors 30, 33 and 36 then being controlled by transistor 28 to produce at terminal 39 a firing pulse which will decrease the output of the controlled thyristors. The current limiting thus effected takes place relatively slowly, e.g. in about one tenth of a second, and would not prevent with sufficient rapidity the occurrence of an excessive initial inrush current; accordingly, there is provided a further protective circuit constituted by a relay 45, preferably a reed relay, having a normally closed contact 45' in series with a low valued resistor 46 shortcircuiting the capacitor 26', the relay 45 being energized only when the controlled A. C. is switched on. Thus, capacitor 26' is fully discharged when the A. C. is switched on and the contact 45' opens, and it thereafter charges with a time constant dependent on its capacity and on the value of resistor 26. This time constant is arranged to be greater than that for the capacitor 22'; the slow charging of capacitor 26' is equivalent to switching on the A.C. supply with the current-reference source 25, 26 set to limit the current to zero and thereafter slowly adjusting potentiometer 25 until it reaches the setting at which it is actually set. Thus, excessive inrush current at switch-on is prevented. A further relay 47, with a normallyopen contact 47' connected across a capacitor 43 in a negative feedback path from terminal 39 to the base of transistor 28, is energized when the current-monitor signal exceeds a second, higher, predetermined level, contact 47' then closing to short-circuit capacitor 43 so that transistor 28 is biased hard on in a very short time, e.g. two milliseconds; this arrangement protects the system from sudden overloads produced, for example, by a short circuit in the load.