GB1169212A - Improvements in or relating to Control Apparatus - Google Patents

Abstract

1,169,212. Control systems for heating resistors. SATCHWELL CONTROLS Ltd. 6 March, 1967 [16 March, 1966; 30 June, 1966]. Nos. 11573/66 and 29435/66. Heading H2H. [Also in Divisions G3 and H3] Apparatus for controlling a physical parameter comprises an electronic switch controlled by a two state circuit having an astable condition, the electronic switch being arranged to be conductive in only one of the two states thereof. The parameter may be temperature and the control may be automatic or manual. In an automatic temperature control system a thermistor 25 is situated to sense the temperature being controlled the value of which is settable by a potentiometer 29, which together with the thermistor 25 and other resistors forms a bridge circuit having a transistor 33 connected across its output diagonal. The transistor 33 controls the state of a two state trigger circuit which includes transistors 37, 39. When transistor 33 is turned off transistor 39 is continuously conductive and transistor 37 is turned off. The voltage developed across resistor 44. when transistor 39 is conductive is used to render a thyristor 56 conductive so as to provide a signal causing a bi-directional controlled rectifier 49 to become conductive thereby allowing current to pass through the heating coil 50. When the condition of the transistor 33 is fully conductive the transistor 39 is turned off and the transistor 37 continuously turned on, no current then passing through the heating resistor 50. When however, the temperature is such that the condition of the thermistor 25 is such as to cause partial conduction only of the transistor 33 the two state circuit is controlled to function in an astable state so that the transistors 37, 39 conduct alternately with a mark-space ratio determined by the condition of the transistor 33. The alternate conduction of transistors 37, 39 is caused by the charging and discharging of a capacitor 41 and a resistor 42. In a second embodiment the element controlling the flow of current through the heating resistor 50 may be a thyristor diagonally connected across a diode bridge. In further embodiments (Figs. 5, 6, not shown) the bridge circuit including transistor 33 is replaced by a variable resistor thus providing a manually controlled apparatus. The controlled apparatus may be a cooking utensil or an oven and the temperature sensing thermistor may be placed adjacent the cooking utensil inside the utensil and in the case of an oven inside the oven. The apparatus may also be arranged to control a refrigeration plant.