GB720094A - Improvements in or relating to electric control circuits - Google Patents

Abstract

720,094. Automatic control circuits. P.A.M., Ltd. June 19, 1952 [June 21, 1951], No. 14750/51. Class 38 (4). [Also in Group XL (c)] A thermionic-valve relay circuit for use with temperature-controlling apparatus as described in Specifications 631,595,631,808, [Group XL(c)], and 710,417, [Group XXXVII], comprises a pair of control contacts 1 which when closed cause the contacts of a relay 3 to open. A potential divider 6 applies a portion of the heater voltage desired from mains transformer T to the grid of valve V1 so long as contacts 1 are open and the resultant amplified voltage is applied to the control grid of valve V2 in opposition to a voltage applied thereto from the transformer T. The potential divider 6 is so adjusted that no resultant alternating voltage occurs at the grid of V2 so that current flows through the relay coil 7 and the contacts of the relay 3 close. When, however, contacts 1 are closed, the control grid of valve V1 is earthed by connection to the sheath of the twin-core metal-sheathed cable 8 so that an alternating voltage is applied to the grid of the valve V2 in antiphase with that which is applied to its anode whereby no current flows through the valve V2 and the relay contacts remain open. It can be seen that this arrangement fails " safe," i.e. should the main supply fail or should any of the leads in the cable 8 break or become earthed the contacts 3 will open. Further, the component resistances and condensers may be contained in a single plug-in element as described in Specification 631,808 which if removed from its socket will cause contacts 3 to open. Unidirectional anode voltage for the valve V1 is derived by means of the rectifier 2 and smoothing condenser C1.