GB2167277A - Improvements in or relating to controllable heat sources - Google Patents

Abstract

A switching circuit for tungsten filament lamps used as a heat source in a hob or cooker comprises switching contacts (18... 23) for connecting the lamps (L1 ... L4) into each of several direction electrical configurations each giving a different basic heat output and a semi- conducting switch (T1) providing the ability to vary the basic heat output in some of the configurations. <IMAGE>

Description

SPECIFICATION Improvements in or relating to controllable heat sources This invention relates to controllable heat sources and has particular but not exclusive reference to controllable heat sources that emit a substantial proportion of radiation in the infra-red region, for example tungsten or tungsten halide filament lamps.

Such lamps operate at very high temperatures and emit a substantial proportion of their total radiation in the infra-red region. Another part of the total radiation is also emitted as visible light whose intensity varies as the energy output of the lamp varies. The variation in intensity thus provides a visual indication of the energy output of the lamp.

When used as heating elements, for example for electric hobs, the energy output of the lamps must be controllable from a very low to a maximum value. Because of the characteristic of the tungsten filament lamp, it is not advisable to employ an energy controller that operates on a cyclically intermittent ON/OFF basis only as this produces rapid thermal cycling of the lamps. This is undesira ble because it produces large bursts of inrush current as a result of the high temperature coefficient of resistance of tungsten, and also because the life of the filaments may be re duced. Wide fluctuations in visible brightness may also be undesirable for aesthetic reasons.

According to the present invention, a heat source comprises a plurality of electric heating elements and switching means for electrically connecting the elements in each of several different electrical configurations, each adapted to provide a different basic heat output, and, in some at least of those configurations, means for varying the basic heat output of that configuration.

When the elements are configured to pro vide a low basic heat output, still lower settings may be achieved by alternate ON/OFF switchings of the power supply to the heating elements.

Higher settings may be achieved by switch ing between two configurations whose basic heat outputs are greater than zero and suffici ently close to each other to obviate undesira ble results.

Variation of the basic heat outputs may be effected on a stepped or continuously variable basis.

Preferably, each heating element is a tung sten or tungsten halide filament lamp and may be of tubular form.

By way of example only, embodiments of the invention suitable for use with tungsten -halide lamps will now be described in greater detail with reference to the accompanying drawings of which: Figs. 1, 1A, 2, 2A, 3, 3A, 4, 4A are explanatory circuit diagrams of a first embodiment, Fig. 5 is an explanatory graph, Fig. 6 is a schematic illustration of a multiposition switch used in the first embodiment, Figs. 7A, 7B are explanatory circuit diagrams of a second embodiment, Fig. 8 is a front view of the dial of a rotary control, and Fig. 9 is an explanatory circuit diagram of a third embodiment.

A typical heating source for an electric hob comprises four tungsten halide filament lamps of tubular form and of equal wattage arranged in parallel, side-byside spaced relationship above a reflector adapted to reflect emission from the lamps upwardly through a specified area of a hob cooking plate of a ceramic material or toughened glass permeable to radiation from the lamps in the infra-red region.

The plate may act also as an optical filter to reduce, to an acceptable level, transmission through the specified area of radiation in the optically-visible region. Alternatively, a suitable optical filter may be interposed between the lamps and the plate. It will be understood that several of such sources will normally be used, each source being associated with a different specific area of the hob.

In Figs. 1-4, the lamps are shown as resistive elements L1...L4. The lamps are energised from AC supply input terminals L, N via diodes D1, D2, triac T1 and multi-position switch contacts 1...17. The moving contact members 18...23 of the switch are operated by suitably contoured cams on a common operating spindle of the switch.

The lamps have end terminals L1T1, L1T2; L2T1, L2T2; L3T1, L3T2; L4T1, L4T2 interconnected as shown in Fig. 1.

The trigger electrode of the triac T1 is connected to means PG for producing firing pulses to switch the triac to conduction on a burst firing basis. The means PG are controlled over lead 24. An acceptable alternative to triac T1, would be a magnetic or other relay-switch. In this event, the means PG may be understood to be the device which supplies actuating current to the coil or other actuating means of the relay-switch. The mark space ratio of the firing pulses is controlled to vary the power output of the lamps via a potentiometer with preset tapping points selectable by operation of the multi-position switch referred to above.

With the moving contact members 18...23 in the positions shown in Fig. 1, the lamps L1...L4 are in series connection as shown in the simplified circuit diagram of Fig. 1A. The first two positions of the multi-position switch provide the electrical connections shown in Fig. 1.

In the first position of the switch, the markspace ratio of the firing pulses applied to the triac T1 is such that the lamps are operated at minimum power output. In the second position of the switch, the markspace ratio is increased or the triac may be continuously conducting so that a slightly greater power output is obtained. For example, the minimum power setting may give an output of 110 watts and the second position an output of 220 watts.

The basic heat output in those first two positions is low enough to permit ON/OFF cycling without undesirable effects.

In general, ON/OFF cycling is acceptable when the two energy levels between which cycling occurs results in a sufficiently small difference in filament temperature exists at the two levels.

Fig. 2 shows the connections that exist in the third and fourth positions of the multiposition switch.

As can be seen from the simplified circuit of Fig. 2A, lamps L1 and L2 are in series connection with diode D2 and lamps L3 and L4 are in series connection with diode D1, these two sub-circuits being parallel connected across the mains input terminals L, N. Triac T1 is joined across the junctions L3, D1 and L1, D2.

In the third position of the multi-position switch, triac T1 is non-conducting with the result that lamps L1 and L2 are energised on each positive half cycle of the mains supply and lamps L3 and L4 are energised on each negative half cycle. This gives a power output slightly greater than that of the second position of the multi-position switch. An example of the position three power output is 370 watts.

Equally, in position 3, the triac could be caused to cycle on and off producing for example a power output of 430 watts. In this event, the lamps would be cycling between a basic heat output of 640 watts (as in position four below) and a basic heat output of 370 watts. The difference between these two energy levels is small enough to prevent undesirable results.

In the fourth position of the switch, the triac T1 is fully conductive and all lamps are energised on both positive and negative half-cycles of the mains supply. The power output in this position is continuous at (for example) 640 watts.

In position five of the multi-position switch which is shown in Fig. 3, lamps L1, L3, L4 are in series connection with the parallel-connected diodes (which are therefore ineffective) across the mains supply terminals L, N. Lamp L2 is in series connection with triac T1 and the parallel connected diodes across that supply. This is shown more clearly in Fig. 3A.

In position five, triac T1 may be cycled on and off producing, for example, an output of 850 watts. In this event, the upper and lower basic heat outputs may be 900 watts and 290 watts respectively. Such energy levels are sufficiently close to allow the cycling of triac T1 to be acceptable.

The connections in the remaining positions of the multi-position switch-positions six to nine-are shown in Fig. 4 and in simplified form in Fig. 4A.

In the sixth position of the multi-position switch, triac T1 is non-conducting with the result that parallel-connected lamps L1, L2 are energised during positive half cycles, and lamps L3, L4 are energised during negative half cycles, of the mains supply. This provides a power output of for example 1044 watts slightly greater than position five of the multiposition switch.

In positions seven, eight and nine of the multi position switch, triac T1 is rendered conducting on a burst firing basis giving differing mark/space ratios in each position providing output powers of, for example 1300 watts, 1550 watts and 1800 watts. Thus, during the periods during which the triac Ti is conductive, all four lamps are energised at mains potential on each half cycle of the supply and therefore produce their full power output totalling, for example, 1800 watts. During the periods during which the triac is non-conductive, lamps L1, L2 are energised during positive half-cycles, and lamps L3, L4 are energised during negative half-cycles of the mains supply producing for example a basic heat output of 1044 as in position five. Cycling between these two basic heat outputs is acceptable.

Waveform (a) represents current flow through the lamps L1 and L2. Up to the first vertical line X is shown current flow with triac T1 in a non-conducting condition. Current flow with triac T1 conducting is shown between the vertical lines X and Y. The waveform after vertical line Y is that applicable with triac T non-conducting.

Waveform (b) represents current flow through the lamps L3 and L4 under conditions similar to those explained in the immediately preceding paragraph in respect of lamps L1 and L2.

Waveform (c) is that of the resultant current flow at the supply and is the sum of flows represented by waveforms (a) and (b).

It should be noted that the waveforms are diagrammatic only and do not show the changes in amplitude resulting from the changes in filament temperature that occur as the triac T1 is cycled on and off.

The period during which triac T1 is conducting depends upon the mark/space ratio selected and this determines the mean heat output of the source.

The multi-position switch may be of the form schematically illustrated in Fig. 6. A control shaft 24 rotatable by a user via a control knob (not shown) carries cams C1, C2, C3...C6 with which sets of switching contacts S1...S6 are respectively associated. The sets of switching contacts S1...S4 are shown dia grammatically in Fig. 6 but are adapted to provide the electrical connections shown respectively in Figs. 1-4. Shaft 24 also carries a contact arm 25 movable over switching segments SS1...SS9 corresponding with the nine positions of the switch and via which switching of the triac T1 is effected.

The multi-position switch will also have an "OFF" position not shown in Fig. 5.

An alternative form of control circuit is shown in Fig. 7A. Equal wattage lamps L1...L4 are connected in the configuration with diodes D3 and D4 shown in Fig. 7A, S7 being a mains ON/OFF switch. Two triacs T3, T4 are employed. A control (not shown) is provided to allow the use of two different effective circuit configurations -that shown in Fig. 7A and a second shown in Fig. 78.

In the Fig. 7A configuration, triac T3 is nonconducting and effectively not in circuit and has been omitted for simplification. Triac T2 is cycled on and off to give, in its non-conducting condition zero power output and in its continuously conducting condition about 61% of maximum possible power output. Variation of the mark/space ratio of triac T2 enables the power output to be varied up to the maximum of 61%.

If now triac T2 remains continuously conducting and triac T3 switched into circuit and cycled, the Fig. 78 configuration applies and a power variation of 61% to 100% of maximum possible output is obtained depending upon the mark/space ratio of triac T3. In this configuration, the difference between the two energy levels (i.e. 61% and a higher value depending upon the mark/ space ratio) is acceptable because the difference in filament temperatures is sufficiently small.

Typically, 61% of maximum may be about 1100 watts.

The change in effective configuration is effected by a single rotary control at a midway position of the latter as indicated in Fig. 8 which shows the dial of the control.

Clockwise rotation of the control from its 'OFF' position first closes switch S7 and then, in position 1, sets the triac T3 into conduction on a burst firing basis and at a minimum mark/space ratio giving a power output of say 50-100 watts. As clockwise rotation of the control is continued, the mark/space conduction ratio of the triac T2 gradually increases until at position 2 a power output of about 1100 watts is obtained when triac T2 is conducting continuously. Further rotary movement of the control sets the mark/space ratio of conduction of the triac T3 to give a power output of slightly above 1100 watts.

Subsequent clockwise rotation of the control gradually increases the mark-space ratio thereby increasing the power output to a maximum of say 1800 watts when both the triacs T2, T3 are conducting continuously.

The control just described gives substantially continuous variation of power output from a minimum of about 50-100 watts to a maximum of say 1800 watts using a single simplified control as compared with that of Figs. 1-4.

In a further embodiment of the invention which is basically a modification of the circuit described above with reference to Figs. 7A, 7B and 8, a single triac only is used in conjunction with a switching circuit enabling the triac to be connected in either the position of triac T2 or T3 of Fig. 78 and cycled at variable mark/space ratios.

Such an arrangement is shown in Fig. 9, the triac being referenced T4. The first position of triac T4 is shown in full lines and the second position is shown in dotted lines.

The change in effective configuration and triac cycling may be effected by a single rotary control as with the Figs. 7A, 7B embodiment.

Rotation of the rotary control from its 'OFF' position to a first position closes switch S7 and connects triac T4 in the full line position.

Rotation of the control to a second position switches triac T4 into conduction on a burst firing basis and at a minimum mark/space ratio giving a minimum power output of gas 50100 watts. Continuation of the rotation of the control gradually increases the mark/space conduction ratio of triac T4 until the latter is conducting continuously, the power output then being about 1100 watts. Further rotation of the control then connects triac T4 into the position shown in dotted lines, mains power being applied directly to the D1, D2 junction.

At the same time, the control sets the mark/space ratio of conduction of triac T4 in its new position to give a power output slightly above 1100 watts.

Subsequent rotation of the control gradually increases the power output to a maximum of say 1800 watts when traic T4 is conducting continously.

It is not essential to use triacs as switching elements, any other form of suitable semi-conductor switch could be used. Alternatively, a suitable electromechanical switch may be employed.

The heating elements may, alternativley, be conventional resistive elements of the sheathed or open wire types. In such cases, the matter of switching at high power, is of course not so critical as it is with tungsten lamps.

Claims (16)

1. A heat source comprising a plurality of electric heating elements, switching means for electrically connecting the elements in each of several different electrical configurations, each giving a different basic heat output, and, in some at least of those configurations, further means for varying the basic heat output of the configuration.

2. A heat source as claimed in claim 1 in which the further means is such that variation of the basic heat output is effected, at low heat outputs, by ON-OFF switching of the power supply to the heating elements.

3. A heat source as claimed in claim 1 or 2 in which the arrangement is such that variation of the basic heat output is effected, at high heat outputs, by maintaining energisation of some at least of the heating elements to provide the basic heat output and by providing additional energisation to those elements to increase the basic heat output to a required level.

4. A heat source as claimed in claim 3 in which the additional energisation is provided on an ON-OFF basis.

5. A heat source as claimed in any one of claims 1-4 in which variation of the basic heat output is effected on a stepped basis.

6. A heat source as claimed in any one of claims 1-4 in which variation of the basic heat output is effected on a continuously variable basis.

7. A heat source as claimed in any one of the preceding claims in which the switching means for electrically connecting the elements in the different electrical configurations comprises mechanically operated switches.

8. A heat source as claimed in any one of claims 1-6 in which the switching means for electrically connecting the elements in the different electrical configurations comprises semiconductor switching means.

9. A heat source as claimed in any one of the preceding claims in which the further means comprises semi-conductor switching means.

10. A heat source as claimed in any one of the preceding claims in which each heating element is a source of infra-red radiation.

11. A heat source as claimed in claim 10 in which each source is a tungsten filament lamp.

12. A heat source as claimed in claim 11 in which the lamps are all of the same wattage.

13. A heat source substantially as herein described with reference to and as illustrated by Figs. 1-6 of the accompanying drawings.

14. A heat source substantially as herein described with reference to and as illustrated by Figs. 7A and 78, or Figs. 7A, 7B and 8 of the accompanying drawings.

15. A cooking hob including at least one heat source as claimed in any one of the preceding claims.

16. A cooker including at least one heat source as claimed in any one of claims 1-14.