Patent GB-1292968-A (GB)
Title: A programmable power control system
Inventor: STOLL MILTON
IPC: G05D23/22
Filed: 1969-11-12
Abstract: 1292968 Automatic control systems M STOLL 12 Nov 1969 55468/69 Heading G3R A programme arrangement for an automatic control system has a voltage comparator receiving one input from a driven potentiometer arrangement providing a reference voltage and another input from a sensor, the relativities of the inputs providing an output which controls disconnection of a load from an electrical source. The voltage from a thermocouple or thermistor 20 is compared with that from a programme arrangement 2 and passed to a proportional power controller 3 which varies the firing angle of thyristors 12, 13 controlling the power drawn by a heater 11 through a manually varied autotransformer 10, 16. Another thermocouple 22 is connectable through a switch to a temperature indicating meter 7 which is reconnectable to indicate the voltage of reference supply 6 the output from which may be reversed by switch 33, 34. The set point voltage is provided by a potentiometer 29, 30 which may be a continuously variable resistor driven at a steady velocity or may comprise a number of discrete resistors connectable in turn by a stepping motor. The Specification envisages the provision of dwell periods so that the set point voltage may be held at constant over a period of time. The potentiometer 29, 30 may be one of a plurality of potentiometers with the selecting devices e.g. multi-deck switches, driven in tandem. The arrangement may include a cold junction compensating circuit comprising resistors (93, 94 and 96) and an N.T.C. resistor (95), Figs.2  and 3 (not shown). Fig.3 (not shown) also discloses a saturable reactor (134) having one winding (140) connected in series with the bridge and thermocouple 20 and another winding (142) connected to a relaxation oscillator including unijunction transistor (143) and capacitor (146), the output windings (147) being connected to a flip-flop including transistors (158, 159) which switch an oscillator comprising transistor (164) and transformer (170) producing signals suitable for triggering the thyristors. This circuit may be balanced at some point other than zero input by passing a D.C. bias current through one half of windings (147). The set point controller may have a five deck stepping switch two decks (187-190, 192-198), Fig.4 (not shown), selecting points on voltage dividers, one of which may give a rising and the other a falling set point, one deck (199-201, 206-207) for switching dwell timers, and the fourth and fifth for modifying any programme (shown unconnected). The switch is operated by a stepping magnet (211) operating from a source on timed pulses (214) through relays e.g. (208- 210). If two programmes are to be provided a switch (184-186) may be reversed causing stepping in the opposite direction. A relay (202-204) forms part of a timer to provide a dwell period. Additional decks (not shown) may be provided to control auxiliary functions for example to operate a solenoid valve (228) or other electromechanical device. A plurality of heaters each having individual temperature sensors may be controlled from a common programme device see Fig.5 (not shown) each heater being controlled separately.