Patent GB-652108-A (GB)
Title: Improvements in and relating to protective arrangements for electric blankets and like articles
IPC: H05B3/34
Filed: 1948-02-20
Abstract: 652,108. Protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 20, 1948, No. 5101. Convention date, Feb. 21, 1947. [Class 38(v)]  . An electric blanket 1 has heating elements 3, 4 of a high temperature-resistance coefficient and is normally controlled by a' thermal switch 11 responsive to ambienttemperature variations and having a bimetal element 10 which is also heated by a series-winding 12, the desired temperature of the blanket being chosen by a knob 12a. The blanket circuit also includes the normally-closed contacts of a relay 8, which are adapted to be opened on overheating of the blanket. On overheating, the resistance of elements 3, 4, increases and the current drops so that the voltage-drop across a resistance 13, of zero temperature-resistance coefficient, decreases also. This voltage-drop is opposed by a reference voltage in the secondary 14b of a transformer 14, the voltage-drop normally being the greater so that the cathode 32 and-grid 28 of the section 19b of a vacuum tube 19 are normally at different potentials, the section thereby being in a non-conducting state. With a decrease of the voltage-drop, however, the voltage induced in the gridcathode circuit by transformer 26, falls, so that the section 19b " fires." A voltage-drop is therefore produced across a resistance 33 in the anode-cathode circuit and when this voltage is great enough, a neon-tube 35 strikes so that a voltage-drop produced in a resistance 24 in the grid-cathode circuit of valve section 19a drives the grid 21 negative, thereby making this section non-conducting so as to de-energize the relay winding 8a and open the contacts of the relay 8. An auxiliary heater 12b for switch 11 is thereby connected in circuit and the switch eventually opens and interrupts the anodecathode circuit of section 19b, which therefore stops firing. As a result, the section 19a again fires and the winding 8a is reenergized to close its contacts and reconnect the blanket 1. If the cause of the overheating has been removed, the supply is maintained but if not, opening and closing of its circuit continues. The heater 12b may be omitted, in which case the switch 11 remains closed when winding 8a is de-energized, and the blanket is reconnected by opening the main switch 7 to cut out valve section 19b, and then reclosing it. In a modification, (Fig. 3, not shown), the resistance 13 and transformer 26 are omitted, and a transformer, having one primary winding fed by a reference voltage and another and opposing primary in series with the elements 3, 4, has a secondary in the gridcathode circuit of section 19b, so that a decrease in the current in the elements 3, 4, causes a decrease of voltage in the secondary, so as to make section 19b fire, as before. The magnitude of the reference voltage is limited by a fixed and adjustable resistance, and a further adjustable resistance ganged with the control knob 12a so that the reference voltage is automatically varied as the desired temperature of the blanket is varied.