GB2237940A

GB2237940A - Power controller for electric hotplate

Info

Publication number: GB2237940A
Application number: GB8920877A
Authority: GB
Inventors: Michael Harcourt Buttery
Original assignee: Electrolux Components Ltd
Current assignee: Electrolux Components Ltd
Priority date: 1989-09-14
Filing date: 1989-09-14
Publication date: 1991-05-15
Also published as: GB8920877D0

Abstract

In a LOW range of operation a heater element L1 is energised via an adjustable energy regulator and an element L2 is not energised, whereas in a HIGH range element L1 is energised continuously and element L2 is energised via the regulator. Elements L1, L2 may be of thick film material deposited on the underside of a ceramic hot plate, the resistance of L1 being twice that of L2. The control arrangement reduces inrush current problems associated with the high temperature coefficient of resistivity of such elements. A thermal relay may be provided to prevent energisation of element L2 before the hotplate has been warmed by element L1. The energy regulator may have a shaft carrying cams to operate switches S1 to S4 and a regulator adjustment cam, the cams being shaped so that the regulator progresses to full power energisation of element L1 over the first 180 DEG of rotation, then switches element L1 to full power and connects element L2 to the regulator whereby further rotation to 270 DEG increases energisation of element L2 from zero to full power. A stop on the cam shaft may prevent it being turned directly from OFF to the HIGH range. <IMAGE>

Description

CONTROLLED ELECTRIC HOTPLATE The present invention relates to hotplates, and in particular, but not exclusively, to hotplates employing thick film elements.

One type of hotplate to which the present invention applicable employs elements comprising a thick film of resistive material deposited on an insulating surface, which maybe the underside of a ceramic hob plate.

The materials generally available for manufacture of such thick film elements usually have a relatively high temperature co-efficient of resistivity. One problem that arises as a result is that because the resistance o the element is low when cold there may be a large inrush current on first switching on a cold hotplate. This is a particular disadvantage if an energy regulator is employed, as at low settings power will be switched on and off to the element repeatedly before it has warmed up to anyappreciable extent.

The present invention aims to provide a design of hotplate and control circuit of general applicability, and in which, when applied to thick film hotplates, the above difficulty is minimised. The arrangement according to the invention has the advantage that particularly fine control of simmering at low power settings can be achieved.

The invention is defined in the appended claims and will now be described further with reference to the accompanying drawings, in which Figure 1 is a control circuit for controlling the power output of a controlled hotplate according to the invention, and Figure 2 shows graphically the relation between the control setting and power output in the two ranges of operation which the present invention provides.

Referring first to Figure 1, the hotpLate has two thick film resistive tracks deposited on it, shown, diagrammatically in Figure 1 as the resistive loads L1 and L2. Of these, L1 has approximately twice the resistance of L2, and is used alone at low power settings Control of the power applied to the hotplate is effected by a single knob carrying a control shaft on which are mounted the control cam of an energy regulator and operating cams for four pairs of switch contacts S1, S2, S3 and 54. The energy regulator is of conventional type, but its cam, as will be seen from the description that follows, is not of the usual form, being arranged to progress to full power setting at approximately two thirds of the full operating range, at which point the switchcontacts S1, S2 and S3 change over.The cam then returns to it low setting, and progresses to full on further rotation through the remainder of its range.

As will be seen from Figure 1 one end of each of the elements L1 L2 is connected to the neutral supply line through a pair of switch contacts 54. The other ends of the elements are each connected to one side of a pair of switch contacts S1, 52 respectively. The pair of contacts S1 and 52 form a change over switch, whereby the energy regulator may be connected alternatively in series with either L1 or L2, and a further set of contacts 53 enable L1 to be connected continuously to the supply.

The sequence of switching operations is shown in the following table: Mode Si S2 S3 54 Off Open Open Open Open Low Power Closed Open Open Closed High Power Open Closed Closed Closed When the control switch is in the off position all contacts are open and no power is applied to the elements. As the switch is rotated contacts S1 and S4 first close, putting the element L1 in series with the energy regulator across the supply. Further rotation of the control knob up to the 1800 position increases the energy regulator setting until L1 is fully energised, as is shown by the lower portion of the curve in Figure 2.

Operation up to this point constitutes the low range setting.

Further rotation of the control opens S1 and closes 52 and S3. As will be seen from the circuit diagram the opening of S1 and closing of S2 now place the energy regulator in series with the lower resistance winding L2 and the closure of S3 leaves the higher resistance winding L1 connected across the supply. At this point when the switches change over, the energy regulator cam is shaped so that the energy regulator returns to its zero setting. Further rotation of the control knob again increases the setting of the energy regulator, so that L2 delivers an increasing supply of heat, while L1 is fully energised. This is the high setting of the circuit corresponding to the upper part of the curve in Figure 2.

Because of the higher resistance of L1, power can be supplied to the circuit when this element is cold without causing an excessive inrush current. Further, if the control knob is rotated from the "off" position towards the "high" position the element will have begun to warm up before the power is supplied to L2, again tending to reduce the initial inrush current.

Preferably, a stop is supplied on the control shaft to prevent it being turned directly from the "off" to the "high" position.

It may be desirable in some cases to put a thermally operated relay in series with L2 to prevent power being applied to this element before the hotplate has been warmed by current through L1. An alternative would be to arrange for the energy regulator heater to be switched through separate contacts and to cause the contacts to close on heating, instead of opening on heating, as is conventional.

The invention is also advantageous when applied to conventional wire-wound hotplates.

Claims

1. A controlled electric hotplate comprising in combination, a hotplate having two heating elements, an energy regulator and switching means arranged to supply power to one element through the energy regulator to provide a LOW range of operation, and to energise one element continuously and supply power to the other through the energy regulator to provide a HIGH range of operation.

2. A controlled electric hotplate according to claim 1 in which the two elements are of unequal resistance and the switching circuit applies power to the element of high resistance in the LOW range.

3. A controlled electric hotplate according to claim 2 in which the switching circuit applies full power to the higher resistance element and power through the energy regulator to the lower resistance element in the HIGH range.

4. A controlled electric hotplate according to claim 2 or claim 3 in which the high resistance element has substantially twice the resistance of the low resistance element.

5. A controlled electric hotplate according to any preceding claim in which the switches of the switching circuit are operated by the shaft which carries the energy regulator cam.

6. A controlled electric hotplate according to claim 5 in which means is provided for preventing the operating shaft from being turned directly from OFF to the HIGH range setting.

7. A controlled electric hotplate according to any preceding claim in which the heating elements are thick film elements.

8. A controlled electric hotplate according to claim 7 having elements of unequal resistance and provided with thermal relay means for preventing power being applied to the lowerresistance element when the hotplate is cold.