GB711027A - Improvements in and relating to the control for electrically heated bed coverings - Google Patents

Abstract

711,027. Automatic temperature control systems. GENERAL ELECTRIC CO. Nov. 24, 1952 [Nov. 29, 1951], No. 29638/52. Class 38(4) The distributed electrical heating element of a bed covering is supplied through a cycling bimetallic switch having two heaters one or other of which is automatically connected in series with the main heating element according to the temperature of the bed covering. As shown an electrically-heated bed covering 1, e.g. a sheet or blanket, has an internal heater 3 which is supplied, through contacts 15 of relay 16 and contacts 26, 27 of a bimetallic switch 18, from and A.C. source (not shown) to which plug 14 is connected. Relay 16 is normally closed and bimetallic switch 18 is operated intermittently by reason of its series connected heater 24. The bimetallic 'switch thus maintains the temperature of the covering at a value dependent on the setting of a stop 28, ambient temperature variations being compensated by their effect off the bimetallic switch. Compensation for rise of temperature caused, for example, by placing another unheated covering over covering 1 is effected by the change in impedance of a material 6 such as nylon, between heater 3 and a winding 5, causing a bridge circuit formed by impedances 31-34 to approach nearer balance so that a relay 22 is operated and an additional heater 25 introduced to increase the heating of the bimetallic switch and decrease the energy supply to heater 3. If for any reason a still greater rise in temperature occurs the bridge circuit becomes more nearly balanced and relay 16 operates to switch off the supply. "On" and " off " push buttons 36 and 37 and an indicator neon 38 are provided.