| 摘要 |
<p>791,696. Electric hot-plates. GENERAL ELECTRIC CO. Aug. 16, 1955 [Aug. 17, 1954], No. 23616/55. Class 39(3) [Also in Group XXXV] A hot plate having heating elements 25a, 25b is controlled by a push-button switching system 21 which provides for (a) automatic temperature control of the plate in dependence upon the temperature of the vessel being heated (b) continuous operation of the plate at any one of a number of different loadings and (c) disconnection of the plate from the mains. The switches are interlocked in such a manner that operation of any push-button, in addition to causing actuation of its own associated switch, restores any previously operated button and its switch to the unactuated condition. In the system shown four of the push-buttons provide, respectively, for four different continuous loadings, their switches controlling the connections between conductors 136, 137, 139 and the lines 101-103 of a three-wire supply such that either one, or both, of the elements 25a, 25b may be connected either between one of the outer wires and the neutral wire of the supply, or across the twoouter wires. When the button for automatic control of heating is operated the two heating elements are connected for parallel supply from the outer wires 101, 102 over contacts of a relay 110 which is now energized over conductor 138, rectiner 130 and voltage stabilizer tube 131. Current for energizing the relay is drawn through a resistor R1, through which also passes the current taken by a shunt circuit comprising a fixed resistor R2, a variable resistor R3 which is adjusted in value 'by a knob 49 for selecting the desired operating temperature, and a thermistor 47 carried by the hot plate in a member which makes good thermal contact with the vessel being heated but which is thermally insulated from the hot plate itself, so that the thernnistor is responsive only to the temperature of the vessel. When the vessel reaches the desired operating temperature the voltage drop in resistor R1, caused by the increasing current taken by the shunt circuit as the resistance of the thermistor falls, is such that relay 110 can no longer hold in. The heating elements are thus disconnected from supply until increasing resistance of the thermistor, consequent upon cooling of the vessel, again allow the relay to operate and initiate another heating period. The temperature of the vessel thus cycles about a value selected by the knob 49. Over the higher temperature settings of the knob 49 a switch 91, 92 actuated thereby connects an additional condenser C2 across the relay coil to reduce the temperature range of the cycle. Adjustable resistors R4, R5 are provided for reducing the differential of the relay and preventing hunting. Specification 144,767 [Class 39(iii)] is referred to.</p> |
