Patent GB-818372-A (GB)
Title: Improvements relating to means for controlling the power dissipation in electric heaters
Inventor: PANISSET DENIS SYDNEY
IPC: H05B1/02
Filed: 1955-07-01
Abstract: 818,372. Electric heating; current supply. SIMPLEX ELECTRIC CO. Ltd. Oct. 1, 1956 [July 1, 1955], No. 29911/56. Divided out of 818,371. Class 39(3) [Also in Groups XXXV and XXXVII] An electric heater 11 e.g. a hot plate 10 of such mass as to take a considerable time to warm up to a useful temperature has a regulator with contacts 13 for effecting a cyclic variation of current through the heater 11, this regulator being operated or controlled by a thermally sensitive responder e.g. a bimetal 15 which is associated or provided with electric heating means including a primary part carrying a current subjected to a cyclic variation corresponding to that of the heater 11 whereby this responder controls the regulator and including a secondary part arranged in relation to the responder to establish a temperature environment therefore and having preset or adjustable control means to determine the value of this environment. The operation of the preset adjustable control is wholly or partly independent of the temperature of the heater 11. The secondary part of the heating means comprises a resistor 17 in series with the heater 11 and main contacts 13. The temperature environment is formed by a lagged housing 16 in which the controlling members and their heating means are mounted. The thermally sensitive responder may comprise a bimetallic strip 15 heated by passage of the main heating current therethrough or by an adjacent resistor 15a in parallel with the heater 11; or the responder may have a sensitive element of magnetic material undergoing a useful change in permeability with temperature variation. The secondary part of the heating means i.e. the resistor 17 is controlled by a thermal responder comprising a bimetal strip 19 operating contacts 18 shortcircuiting when closed the resistor 17. The bimetal strip 19 however may be heated only by virtue of its deposition in the housing 16 and need not carry any current or be provided with a local heating resistor. The temperature of the housing may be adjusted e.g. between 80 and 300‹C. by adjusting the bimetal 19 by moving the upper contact 18 by a cam having a control knob on the housing or by varying the value of the resistor 17. The temperature environment of the housing renders the control immune to variations in ambient temperature; and when the housing is at a higher temperature, less current passes to the heater 11. In Fig. 2 the heating resistor 15a of the bimetal 15 is in parallel with the main heater 11 and the resistor 17 is also in parallel with the heater 11. Thus the control of resistor 17 is removed from that of the main contacts 13. In Fig. 3 part 12 of the heater 11 is of material having a high temperature coefficient of resistance e.g. nickel and is in series with the resistor 11. The resistor 12 comprises the primary control element of the contacts 13 and is connected in parallel with the whole or part of the secondary resistor 17. As the hotplate warms up, the temperature and hence the resistance of the resistor 12 increases, whereby more current pases through the resistor 17; and hence the primary contacts 13 more frequently close. In Fig. 4 the resistor 17 is in parallel with the main heater 11, and the associated thermostat comprises two bimetals 19, 19a mounted on a block to pivot at 19c. Both bimetals deflect to the left when heated. The shorter one 19c bears on an adjustable mechanical stop 20 and is heated by a resistor 19b in parallel with the nickel element 12. The bimetals act in opposition, but the longer bimetal 19, when a cold vessel is placed on the hotplate when hot, will move about the pivot 19c to the left, thereby decreasing average current through the resistor 17 until the hot-plate regains its working temperature. The control may be arranged to allow an overload current to the hot-plate and for this purpose, the rise in temperature of the housing may be delayed, as set out in Specification 818,371, which is referred to. The hot-plate may have several resistors, all or only one of which may be controlled in this manner. Specification 802,372 also is referred to.