GB1100366A - An electronically controlled heater - Google Patents

Abstract

1,100,366. Automatic temperature control. MALLORY TIMERS Ltd. July 29, 1966 [Aug. 2, 1965], No. 34194 / 66. Heading G3R. An automatic temperature control system comprises means whereby the initial heating of the apparatus is effected by passing both positive and negative half waves of the A. C. supply through the heating element, the apparatus being subsequently maintained at a constant temperature by passing alternate half waves only through the heating element. The apparatus includes a heating element 10 and a thermistor 11 responsive to the heating effect thereof, a variable resistor 20 being provided to set the temperature at which the apparatus is required to operate. Initially, when the thermistor is cold, its effect is to trigger a unijunction transistor 19 approximately at the beginning of a half cycle thereby producing a gating signal 26 to render a thyristor 13 conductive whereby substantially complete half cycles pass through the heating element 10 whilst the half cycles of the reverse polarity pass through the heating element 10 by way of a diode 34 and a normally closed contact 30 of a relay 29. Thus the apparatus rapidly attains the desired temperature by the application to the heating element 10 of full wave power. When the desired temperature has been attained the condition of the thermistor 11 is such as to inhibit conduction of the thyristor 13 whereupon a capacitor 33 is charged by way of a diode 28 thus to operate the relay 29 having a self maintaining contact 31. Operation of relay 29 opens contact 30 thus preventing the passage of the reverse half waves through the heating element 10 and the temperature of the apparatus is maintained by the application to the heating element 10 of half wave power only controlled by the thyristor 13 in conjunction with the thermistor 11.