CA1044319A - Touch-sensitive appliance power control - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A power control apparatus uses a digital electronic counter to gate a triac for controlling the current supplied to a load. The preferred arrangement is that each of ten possible counts achievable by the counter corresponds to a particular rate of current supply to the load. The counter may be caused to count up or count down by means of proximity switches mounted on a control panel and optionally includes a reset input. The control panel comprises an insulating glass plate having three pairs of electrodes thereon with the electrodes of each pair being mounted on opposite faces of the place, each pair of electrodes being associated with a respective one of the count-up, count-down and reset inputs of the counter, the pair associated with the reset input being shaped to surround in the respective plane of the plate the other two electrodes on that surface of the plate.

Description

1~43~9 : ~
The present invention relates to a power control apparatus.
Many devices are known which allow an operator to vary the power dissipated in a load. Such devices may include user-operable components such as simple switches, rheostats, bi-met-allic strip thermostats and the like which may either directly vary the power dissipated in the load or vary the power by means of an intermediate electronic or electro-mechanical circuit. Such devices all have the common feature that they include at least one part which is movable by the operator and this feature gives rise to several problems, for instance, in the case of a control panel for an electric cooker, great care has to be taken when wiping round the control knobs when cleaning the cooker.
It has also been proposed to utilise proximity switches to vary the power dissipated in the load. For instance, a group of proximity switches may be provided, each of which is connected to a respective circuit which causes a predetermined amount of power to be dissipated in the load. Thus by operating the ap-propriate proximity switch it is possible to select which one of a predetermined number of power levels is dissipated in the load. This arrangement has been used to control the heating elements in a cooker, the proximity switch arrangement being used to replace the conventional bi-metallic regulator controls.
However, in order to provide sufficiently fine variation in the power supplied to the heating element it has been necessary to provide 5 or 10 --1-- ~
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1~)44319 switches for each cooker hot plate, thereby requiring something in the order of 40 proximity switches for the various hot-plates and grill of the cooker, with the attendant duplication of the proximity switch circuits. ; -According to a first aspect of the present -invention there is provided in an electric cooker. a power control apparatus including a memory for storing a variable numerical value, the memory having an output, a first control input for causing the value stored in the memory to be increased progressively while such first control input is actuated, and a second control input for causing the value stored in the memory to be decreased progressively while such second control input is actuated, first and second manually operable means for actuating said first and second control inputs respectively, an electric cooker heating element, and means connected to the output L~
of the memory for varying the current supplied to the heating element in dependence on the value stored in the memory, so that each value corresponds to a particular rate of current supply to the load.
~.-The memory may be analogue or digital. If digital, the values stored may be represented for instance, as the voltage existing across the plates of a capacitor, in which case the voltage may be changed, and hence the value stored, -by means of a diode pump arrangement. This allows the value stored by the memory to be continuously variable over a range of values.
In a digital arrangement the memory can hold any one of a number of discrete valuesD Suitably such a digital memory stores the value as a group of binary encoded digits. In this case the memory may include any known means for storing such

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3~9 binary digits, such as a shift register, a series of binary latches or a binary counter.
In a preferred embodiment of a first aspect of the invention, which may be used to control the heating element of an electric cooker, the memory includes a digital counter which can be made to count up between a plurality of discrete counts, there suitably being 10 possible counts in all.
The counter may be driven by pulses which are spaced ~ -at approximately 0.2 or 0.5 second intervals. These pulses may be derived from the fifty hertz mains supply. The outputs from the counter may be connected to a numerical display which indicates the count stored.
Preferably the counter has a clock input, a "count-up"
input, a 'count-down input and a reset' input. Suitablyl the counter counts pulses applied to the clock input when either the count-up or count-down inputs are operated. When the count-up input is operated, the pulses applied to the clock input are counted upwards by the counter so that the count achieved by the -counter is thereby increased. Similarly when the count-down input is operated the counter counts downwards, the pulses applied to the clock input serve to decrease the count stored by the counter. The reset input is suitably operable to reset the counter to its lowest counting value which preferably corresponds to a zero energy supply rate to the load.
Preferably the counter input, count-down input and reset input are each connected to a respective proximity ~ 19 .' .:' detector via respective switching circuits. Suitably the arrangement is such that the presence of an operator's finger in contact with or closely spaced from one of the proximity detectors causes the respective input to the counter to be activated. These proximity detectors may be mounted on a control panel.
The counter used may be of the recycling type, in which case, gating circuits may be provided to prevent the counter -counting up beyond its maximum count or down below its minumum count.
In the preferred embodiment of the invention the load is an electrically heated element of an electric cooker hob, which is supplied with electric current from the alternating -mains supply via a triac whose gate is connected by a suitable circuit to the output of the counter, this circuit serving to gate the triac so that the power dissipated in the heating element is varied in proportion to the count stored by the counter.
The triac is preferably operated in the known "burst fire" mode ~`
in which the triac delivers a proportion of complete half-cycles of the mains supply to the heating element, this proportion varying dependent on the count stored by the counter. In the "burst fire" - -~
mode the triac only switches on and off as the alternating mains supply voltage instantaneously crosses zero, so that the problems of introducing interference into the mains supply which occurs in the known "phase-control" mode is avoided.
The output from the counter ~ay be used to gate . . , -, - ~ . - . ..

4~3~9 the triac in a number of ways. For instance 9 the count - may be converted into a corresponding voltage, resis-tance or current by means of known ~esistive ladder networks. Such a corresponding voltage, resistance or current may then be used as a parameter in a further circuit which varies the mark-to-space ratio of the complete half-cycles of the current supplied to the load by the triac.
Alternatively, the count stored by the counter may 'O be used to burst fire the triac by means of a purely digital network. In this case there may be provided a read-only me ry which suitably stores a numb~r of binary ~wordsn. Thus when the counter stores a particular count a gating circuit causes a corresponding word to - lr` be read from the read-only memory, this word being used to determine the mark-to-space ratio of the complete half-cycles of the current delivered by the txiac.
Suitably the particular word would be read serially~
that is, one bit at a time, and each bit would represent whether the triac were to be gated on or off during the period in which that bit was being read. The operation of the triac in the burst fire mode could be effected simply by making sure that each bit of the word being read is read for a period which is a multiple of the time ^ period of half a cycle o~ the main supply. A purely digital arrangement such as this would eliminate the thermal drift and other errors which could occur in either a completely analogue system or a part-analogue -part-digital system.

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443~L9 In order to operate the heating element of a cooker provided with such a control circuit, an operator notes the -indicated count and decides whether the corresponding rate of heating of the heater element needs to be increased or decreased.
If the operator decides that the rate of heating needs to be increased he places a finger on the "count-up" proximity detector and the counter then proceeds to count upwards, at the same time increasing the flow of current to the heating element via the triac, the count at all stages being indicated by the display. When the operator decides that a sufficient rate of heating has been achieved, he then removes his finger from the "count-up" proximity detector and the counter remains at this count. The triac then continues to regulate the supply of electricity to the heating element at a substantially constant rate.
Similarly, if the operator decides that the rate of heating is to be reduced he places his finger on the "count-down" ~--proximity detector which causes the counter to count downwards and when the dlsplay indicates that a desired count corresponding to a desired rate of heating has been achieved, the operator removes his finger from the "count-down" proximity detector whereupon the counters holds its count at this point.
Operation of the "reset" proximity detector causes the counter to return to a count of zero, thereby shutting off the supply to the heating element.
The relationship between the count achieved by the ` ' ' :' .
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10443~9 counter and the current ~upplied to the heating element via the - triac is not necessarily linear and indeed may be made logarithmic to provide facilities for simmering and boiling, According to a second a3pect of the invention there is provided a power control apparatus including a memory for storin~
a variable numerical value; the memory having an output, a first control input for causing the memory to increase the value stored, and a second control input for the memory for causing the memory to decrease the value stored; first and second manually operable 1~ means for actuating said first and second control inputs respectively, an electric cooker heating element, and means connected to the output of the memory for varying the current supplied to the heating element in dependence on the value stored in the memory, so that each value corresponds to a respective rate of energy supply, a reset control input of the memory, for causing the memory to store a value corresponding to a zero , rate of energy supply to the load and including third manually operable means connected to said reset control input, each of the manually operable means including a respective switch circ~it having an output connected to a re~pective control input of the memory and an input and a respective proximity detector electrode .
for each control input, each said electrode being connected to the input of a respective switch circuit, witheach of first ;.
electrodes being mounted on the same face of an insulated plate 25 member, the electrode operatively associated with the reset ,~
control input being shaped to surround, in the plane of!sald ; ~ ;
face at least the greater part of each of the other two electrode'~,, . The provision of the second electrodes represents an -`
improvement over known proximity detectors in that when a finger ~ j 30 or other body is applied to a second electrode the capacitlve;.
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coupling between the first and ~econd electrodes is independent of the ~hape or size of the finger or the manner in which.it i~
applied to the second electrode, This can be particularly useful in the application of the apparatus to the control of electric cooker heating elements, in view of the environment in which and manner in which the apparatus will be used.
According to a further a~pect of the present invention there is provided a power control apparatus including a memory . ~
for storing a variable numerical value: the memory having an- . :-output, a first control input for causing the memory to increase the value ~tored, and a second control input for the memory for causing the memory to decrease the value stored, first and ~.
second manually operable-means for actuating said first and :
second control input~ respectively: an electric cooker heating element, and means connected to the output of the memory for varying the current supplied to the heating element in dependence ...
on the value stored in the memory, so that each value corresponds to a respective rate of energy supply; at lea~t two proximity detectors each including an insulated plate member, a first ~ -20 'sense electrode on one face of the in~ulating plate and a second .
~en~e electrode on the oppo~ite face of the insulating plate, -the first ~en~e electrode being capacitively coupled with the ~econd qense electrode and the first sense electrode being :.
responqive to close proximity between the second sense electrode and conducting body effective to produce an output signal, only the first sense electrode having an output connection, each electrode output connection bein~ connected to the input of a reRpective switch circuit whose output is in turn connected to .
a respective control input of the memory.

Thus., if used in for example an electric cooker with a heating element as the load, when the front, other face of the - . . ~ . , .
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1~)4~319 plate ~ember is'cleaned, for instance by wiping with a cleaning cloth, the fact that the electrode associated with the reset electrode will almost invariably be the la~t one to be actuated ensures that the counter is reset to zero i.e. corresponding to no heat being ~upplied from the heating element so that the ele-ment will not inadvertently be / f ~

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-11)443~9 1 via a suitable driving circuit (not shown). Although display 6 is shown in Figure 1 as being connected to a separate output, the display 6 could of course be connected to lines 11. Any suitable device may be u~ed as display 6 although a seven segment display of the L.E.D. or liquid crystal type is preferred.
The inputs 12, 13 and 14 of counter 1 are driven by the outputs of proximity switches 2, 3 and 4 respectively and each proximity switch is arranged so that it produces an output only when its operating surface is touched by an operator. Counter 1 is arranged so that it will retain a fixed count until one of the control inputs 12, 13 and 14 is operated.
Each of the proximity switches, 2, 3 and 4 includes a respective proximity detector and associated switch circuit.
The switch circuits may be of any of any of the known types which are adapted to produce an output signal in response to the presence of an object such as an operator's finger in close proximity with or in contact with the associated proximity detector. For instance, each proximity detector output electrode may be connected to the gate MOS field effect transistor and the source or drain circuit of the transistor may include a load resistor so that an output signal may be developed across the drain and source of the transistor.
A strobe 5 is provided which delivers to the clock input 16 of counter 1 pul~es which preferably occur at approx-imately 0.2 or 0.5 second intervals. The strobe 5 may consist of a digital dividing network . ' ' -10~

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driven by pulses derived from the 50 hertz electrical mains supply.
The pulses delivered to the clock input 16 of counter 1 are the pulses which are effectively counted when either of its inputs 12 or 13 is operated. -Each output line 11 is connected to a respective input of a burst fire controller 7, whose output is connected to the gate of a triac 9, which in turn controls the flow of electric current to a heating element or elements 10 which may be included in a cooker hot-plate.
Triac 9 is operated in the "burst fire" mode. That is ;
to say that it is operated to supply pulses consisting of a whole number of supply cycles of mains alternating current to the heating element. By varying the mark to space ratio of the `
pulses, the width of the pulses and thereby the rate of energy ;~
supply to the heating element 10 can be varied as desired. For ~ ;-,.: :
examplè, a long pulse or mark separated by a short space will have `~
the effect of supplying a relatively large amount of energy to the heating element per unit time whereas a short pulse or mark and a long space will cause a relatively small amount of energy per unit time to be supplied to the heating elements 10.
Figure 3 shows a preferred ~orm of burst fire controller ~ `-7 in accordance with the invention. A digital to analogue converter ;
22 produces a DC output voltage which is proportional to the count input carried by lines 11. Preferably the higher the count ~`
registered by counter 1, the higher the DC voltage `
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~ , . ' '' ' . ' -11- ' ` ~ . ` v ' output of converter 22. This DC output voltage is applied to the input of a delay switch 8 whose output is in turn connected to one input of a comparator 27. A free running ramp generator :
23 is provided, the ramp output of which may have a time constant of approximately twenty seconds sothat the effect of switching of the heater current on the mains supply may be minimised.
Comparator 27 is arranged so that triac 9 is only gated-on when the ramp voltage from generator 23 applied to input 29 is less than the voltage applied to the input 30 from delay switch 8. -Assuming that a steady DC voltage is applied to input 30 of comparator 27, output 28 of comparator 27 will gate-on triac 9 until the ramp voltage from ramp generator 23 exceeds ~
the input voltage to input 30. When the ramp voltage has ex- - :
ceeded the input voltage to input 30, output 28 stops producing an electrical output signal, but because of the nature of operation of triac 9 it will continue to conduct until the alternating mains supply waveform passes through zero at which time triac 9 will cease to conduct. Thus since the time taken for the ramp voltage to exceed the input voltage to input 30 will be proportional to ~ .
the magnitude of the input voltage, the proportion of time during ~ -which triac 9 supplies current to heating element 10 will be dependent on the voltage applied to input 30. The ramp voltage from ramp generator 23 will continue to increase thus inhibiting -gating-on of triac 9 until a new ramp generation cycle commences whereupon its output is .:

~04~;~i9 reduced to zero.
Hence triac 9 is only on or off when the alternating mains supply is instantaneously zero, thereby eliminating the possiblity of causlng high-frequency transients which may cause interference with other applicances.
Switch 8 is arranged so that when counter 1 is holding ~ ~
a steady count the output from delay switch 8 is proportional ~ ~ ;
to the input voltage from converter 22. However, if the "count-up" proximity switch 2 is operated it delivers a pulse to input 24 of switch 8 which causes delay switch 8 to produce an output voltage which corresponds to the maximum heating rate from , heating element 10. A thermal sensor 15, e.g. an infra-red sensor may be provided located adjacent the heating element 10. Thermal sensor 15 produces an output signal which is proportional to the temperature measured and which may be fed to an input 26 of delay switch 8. When a predetermined temperature is registered ;
by sensor 15, delay switch 8 returns to producing an output which is proportional to the input from converter 22. Thus control of t~
current supplied to the heating element 10 is returned to converter 22. ~ -Delay switch 8 also includes a further input 25 which is arranged to be activated when the count-down proximity swltch 3 is operated. Delay switch 8 is then caused to produce zero output voltage until a second predetermined temperature is registered by temperature sensor 15. ~

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~r 1~4~319 Therefore when it is desired to increase the rate of heating from heating element lO, the ap~ropriate proximity switch is operated and in the case where the heating rate is desired to be increased the heating element 10 is supplied with maximum current ~or a period, the sui~ply current then reducing to that corresponding -to the desired heating rate. This arrangement permits fast response of the heating element 10 to de~ired change in heating ~te. Simllarly, operation of the count-down proximity detector inhibits the sup~ly of current to heating element lO for a time therefore increasing the speed of response of the element 10 to a reduced rate of heating.
~lternatively, instead of a thermal sensor 15, switch 8 may be provided with a time delay circuit, whose time delay cycle is initiated when either of the inputs 24 or 25 are activated on operation of the respective proximity switch and d~ring the delay time, delay ~-~
switch 8 operates as described above. ~Ihen the delay cycle of the time delay circuit is complete~ control of input 30 of comparator 27 is returned to converter 22, thus providing a similar function to the provision of thermal sensor 15. E}owever, since the time required to reach a desired temperature, and the maximum temperature attained for a particular rate of current supply to heating element 10 will depend on the thermal capacity of whatever body is being heated, the desired time delay, or temperature to be sensed by sensor 15 must be chosen to be optimum for the average conditions of use of ..,~ .

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16~443~9 heating eler,lent 10.
The converter 22 included in burst fire controller 7 need not necessarily have a linear transfer charact-eristic between the count delivered to the input of the converter and the output delivered to the burst fire ;
pulse generator. Indeed it is preferred that the transfer relationship conforms to a substantially logar-ithmic relationship so that a change of o~unt for -~
instance from 1 to 2 produces a much smaller change `
in the rate of energy supply to heating el~ment 10 than does a change in count from, for instance, 7 to 8. With such a transfer characteristic it is possible to provide relatively fine ad'ustment of heating rate at low heats to facilitate simmering.
The outputs from cou~er 1 to the burst fire co~roller 7 and indicator 6 may either each consist of 10 separate lines, each line carrying a signal when a corresponding count is achieved or, for instance, four lines may be used and the count encoded as a four bit ~ `
binary code in which case decoding circuits may be provided in burst fire controller 7 and display 6 respectively to produce outputs corresponding to the 10 ;
possible counts.
The control apparatus described above is adapted ;~
for regulating the supply of alternating current to a load via a triac 9. However, a similar circuit may be used for controlling a direct current power supply. This ~
may be achieved by connecting the output of converter 22 to the regulating input of a direct current regulator, P~ ~ ',', :~ ' ` ' .: .

~63443~9 for instance, to the base of a power transistor which is connected in series with a load.
The preferred embodiment of control apparatus -described above operates so as to count the pulses applied to the clock input 16 of the counter 1. An alternative arrangement exists whereby the count input 16 of counter 1 may be connected to both of the count-up proximity switch 2 and the count-down proximity switch 3, With this arrangement the counter counts up or counts down by one count each time the a~propriate proximity switch is touched. Thus it is possible in this arrangement, to make the counter count up, for example, three counts by touching the up proximity switch three times. -~
Figure 2 shows a control panel incorporating ~roximity detectors in accordance with the invention.
~he panel consists of metallic elements 17, 19 and 20 as shown mounted on a glass plate 31, the metallic elements 17, 19 and 20 being separated from one another ~0 by a zone 18. Corresponding metallic layers 17a, l9a and 20a respectively are located in alignment on the opposite face of glass plate 31. Each of the metallic elements 17a, l9a and 20a is connected to a respective electrical switch circuit which can detect the flow of current to and/or from the element. The elements 17, 19 and 20 are insulated by a thin coat of non-conduding lacquering over their whole surface and from elements 17a, l9a and 20a by the glass plate 31. Preferably the lacquer i3 a gla~s-like or ceramic material.

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, iO~43i9 When an operator places a finger on any one of the elements 17, 19 or 20 9 the corresponding element 17a, l9a or 20a is capacitively coupled to earth by virtue of the .
dielectric properties of glass plate 31, since the operator's body will generally be at ~pproximately earth potential and his electrical resistance will be much less than the glass plate 31.
The proximity detector circuits associated with -each element 17a, l9a and 20a are adapted to produce an output signal whenever the corresponding element 17, 19 or 20 is touched by an operator. Preferably the proximity detector circuit associated with elementsl7a, l9a and 20a are connected to the "reset" 1~, ~Icount-up~ 12 and "count-down" input 13, respectively of counter 1.
Preferably all the circuit elements shown in Figures 1 and 3 except triac 9 and heating element 10 are powered from a low voltage direct current source and -their zero volt rails are connected to the ~ve side of .
the mains supply.
In this arrangement, when an o~erator places his finger on one of the elements 17~ 19 or 20, the electrode touched is effectively coupled to earth and causes a .:
capacitive charging current to flow to or from the - :
corresponding element 17a, l9a or 20a since these ele-ments are connected to switches whose zero volt rails are connected to the live side of the mains and thus all of the elements 17a, l9a and 20a a.re maintained at an alternating voltage of the order of the mains supply voltage above earth. A respective switch senses the :-. . .

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~ ' r~ _ 1~4~3~9 capacity charging current which is thus caused to flow and produces an output signalO Since elements 17, 19 and 20 are electrically conduct:;ve, the capacitive .. coupling between each element and its corxesponding

5 element 17a, l9a or 20a is rendered independent of the size or shape of the operator's finger and the manner in which it i9 applied to the electrode.
A further advantage of the arxangement shown in Figure 2 is that if the fr.ont panel constituted by ele- .
ments 17, 19 and 20 and display 6 is cleaned by, for instance, wiping a cloth, the last element which will be touched by the cloth wi.ll be element 17 and this will cause its associated switch to delivex an output signal : to the reset input 14 of counler 1 thus turning off the electrical supply to the heating element '0. Thus there is no danger o~ the cooker being inadvextently `.
switched on by opera'ion of element 1.9 si.nce such an operation will always be ~ollowed d~.ring cleaning by the operation of the switch associ.a~ed wi~h element 17.
In orde.r to prevent str y fie.lds hehind elements l,r~, l9a and 20a rrom operatin.~ t.heir associated swit- :
ches a further sheet o' dielestric materi.al 32 may be located behind elements 17a, l9a and 20a and behind this a further sheet of conductive matex.ial 33 which may for instance be connected to the zero volt rai.l supplying the control elements.
Sheet 33 will ef~ectivaly form a screen to prevent operation of any of the proY.imity detectors by stray fields from behind.

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,, .. . : . , )443~9 Preferably display 6 is located behind the glass sheet 31 to facilitate connection to its driving circuits and also to protect it from damage.
Although it is not shown in the Figures a pulse trans-former may be used to couple burst fire controller 7 to the gate of triac 9. Whilst not essential this will ensure that there is no danger to either the operator or the other control elements should break-down of triac 9 occur. The operator is further protected in that the only portions of the apparatus accessible -under normal circumstances are elements 17, 19 and 20 andthese are electrically insulated from all the other c1rcuit elements.
The various circuit elements described may be constructed using one or more integrated circuits. Preferably, all the cir-cuit elements are included in a single metal oxide semi-con-ductor integrated circuit. ~-Although the embodiments of control apparatus described ;
are adapted for use with an electrical heating element, the ~ ~ -apparatus may be modified to operate with a gas burner, for in- ~
stance, by the use of an electro-mechanical control valve. `
Various other circuits exist for implementing the power ;

control apparatus of the invention. For instance, the counter 1 ~ ..
may be replaced by a shift register consisting of a number of bi-stables arranged in ~nown fashion and this register may suitably carry a pattern of binary digits. In this case the count-up and count-down input= may be used t~ shift the binary pattern in ~ ' .
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either direction through the register. Outputs of the register may be decoded to produce an output equivalent to the count of counter 1. Such a decoder may include switching circuits which switch into circuit a respective resistor of differing value for each decoded output in known fashion, Figure ~ shows an alternative form of burst fire controller which includes triac 9 whose gate is driven through a diac 41. In this case the triac 9 is control-led by the diac discharge from capacitor 43. Capacitor 43 is fed via a protection resistor 45 and a capacitor 44, capacitor 44 serving to provide maximum current drive at the instantaneous zero crossings of the supply voltage.
This circuit may provide a proportion of complete half cycles of the mains supply, which proportion may vary between 0 and 100%. The precise proportion i9 determined by a variable resistance represented in Figure 4 by a variable resistance ~2. This variable resistance 42 may be a variable resistance generated from either the output of convertor 22 or from the decoder of the shift register arrangement described above. However, since the aurrent flowing through the resistor 42 will alternate in direction, it would be necessary to couple either the convertor 22 or the decoder output into the circuit of Figure 4 via a four-diode bridge, the output from the decoder or convertor being connected to two opposite arms of the bridge and the other two arm~ of the bridge being connected to respective leads of capacitor 43. Suitable ~alues for the components shown in Figure 2 O
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are resistance 42, variable between 0 and 50K,resistor -45,100K, capacitor 43, 0.02uF~ capacitor 44 0.02u~, diac -type D32 a~d triac type Q ~0061~.
A~ mentioned earlier the memory could alternatively be an analoguxe memory. For instance, the value of count to be stored c~uld be represented by a voltage exi~ing across the plates of a capacitor. This capacitor could be charged up or down by use of a Xnown diode-pump arrangement adapted to increase or decrease the voltage across the capacitor. This diode pump arrangement c~uld be connected via switch circuits to the output electrodes of proximity detectors such as th~se de~cribed, there being one proximity detector to increase the voltage across the capacitor and another proximity detector to decrease the voltage across the capaclt4~. The voltage exi~ting across the capacitor could be monitored by very high impe~ance circuit such as that provided by the gate to drain circuit of a field effect transistor which would produce an output voltage or current in it B
source to drain circuit proportional to the v~ltage acros~ the capacitor. ~lternatively, the field effect transistor c~uld be used to provide a variable resist-ance between its source and drain. This variable resistance could be used as the variable resistance ~2 ;~
in Fic3ure 4 to achieve burst fire control of triac 9 or alternatively could be u~ed as a variba~e resistor in any of the known phase control circuits. Alternatively the output of the analogue memory could be u~ed to -effect the variation in the mark to space ratio by an ,~,1 . .
.2~ - , ~Q~ 319 oscillat~r which could be u9ed to gate~ fo~ instance~
a triac. If an AC supply is u~ed then neither the mark nor space of the oscillations ~hould be shorter than half a cycle of the supply.
Although the triac 9 hag been described as opera-tlng in the burst fire mode, there is no reason why it should not ke operated in the known phase control mode when the problem of injection of interference into the mains supply is not important. If operatBd in the phase control mode~ the triac could be associated with a suitable high frequency interference sUppxesston network of any known type.
The embodiment of oontrol apparatus of Figure 1 has as it~ load an electrical heating element of an electric cooker. This is in no way limitative a~d the control apparatus could also be used for in~tance for control-ling other heating elements, lighting arrangement or motors of various kind~. Also the di~play u3ed could comprise nixie tube~ neon bulbs, tungsten lamps, or any other ~uitable form of display.
The outputs of the d~gital memori~ described could also be used to drive one or re electromagnetic relays to va~y the power supplied to a load.

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an electric cooker, a power control apparatus including a memory for storing a variable numerical value the memory having an output, a first control input for causing the value stored in the memory to be increased progressively while such first control input is actuated, and a second control input for causing the value stored in the memory to be decreased progressively while such second control input is actuated, first and second manually operable means for actuating said first and second control inputs respectively, and electric cooker heating element, and means connected to the output of the memory for varying the current supplied to the heating element in dependence on the value stored in the memory, so that each value corresponds to a particular rate of current supply to the load.

2. A power control apparatus as claimed in claim 1, wherein the memory is a digital memory for storing any one of a set of discrete numerical values.

3. A power control apparatus as claimed in claim 2, wherein the memory consists of an electronic counter, operably to count between discrete count values.

4. A power control apparatus as claimed in claim 3, and further comprising an input for the counter for pulses to be counted and a pulse generator, wherein the first control input is operable to cause the counter to count said pulses so that the count stored is increased, and the second control input is operable to cause the counter to count said pulses so that the count stored is thereby decreased and wherein the counter only counts said pulses when one of the control inputs is operated.

5. A power control apparatus as claimed in claim 4, wherein the pulse generator comprises a digital dividing network for deriving the pulses to be applied to the counter from an alternating current mains supply.

6. A control apparatus as claimed in claim 1 and further comprising an alternating mains supply, a triac connected in series with said heating element across said alternating current mains supply and a circuit having an input connected to the output of the memory, and an output for sending pulses to the triac, so that the triac is gated on for a proportion of complete half-cycles of the mains supply, the said proportion varying according to the value stored by the memory.

7. A control apparatus as claimed in claim 1 wherein the relationship between the value stored by the memory and the rate of energy supplied to the heating element is substantially logarithmic.

8. A control apparatus as claimed in claim 1 and further comprising an overriding device for the energy supply rate varying means, operable so that for a period after the value stored by the memory is increased, the rate of energy supply is increased to maximum and so that for a period after the value stored by the memory is decreased, the energy supply rate is decreased to zero.

9. A control apparatus as claimed in claim 8, wherein said overriding device includes a time delay circuit for determining said periods.

10. A power control apparatus as claimed in claim 8, further comprising temperature sensing means operable to end either of said periods when a respective predeter-mined temperature is reached.

11. A power control apparatus as claimed in claim 1 and further comprising a numerical display for giving a visual indication of the value stored by the memory.

12. A power control apparatus as claimed in claim 1 and further comprising a reset control input of the memory, for causing the memory to store a value corresponding to a zero rate of energy supply to the heating element and including third manually operable means connected to said reset control input.

13. A power control apparatus according to claim 1, wherein each of said manually operable means includes a respective switch circuit having an output connected to a respective control input of the memory and an input and a respective proximity detector electrode for each control input, each said electrode being connected to the input of a respective switch circuit.

14. A power control apparatus including a memory for storing a variable numerical value; the memory having an output, a first control input for causing the memory to increase the value stored, and a second control input for the memory for causing the memory to decrease the value stored; first and second manually operable means for actuating said first and second control inputs respectively; an electric cooker heating element; and means connected to the output of the memory for varying the current supplied to the heating element in dependence on the value stored in the memory, so that each value corresponds to a respective rate of energy supply, a reset control input of the memory, for causing the memory to store a value corresponding to a zero rate of energy supply to the load and including third manually operable means connected to said reset control input, each of the manually operable means including a respective switch circuit having an output connected to a respective control input of the memory and an input and a respective proximity detector electrode for each control input, each said electrode being connected to the input of a respective switch circuit, with each of first electrodes being mounted on the same face of an insulated plate member, the electrode operatively associated with the reset control input being shaped to surround, in the plane of said face at least the greater part of each of the other two electrodes.

15. A power control apparatus including a memory for storing a variable numerical value; the memory having an output, a first control input for causing the memory to increase the value stored, and a second control input for the memory for causing the memory to decrease the value stored, first and second manually operable means for actuating said first and second control inputs respectively; an electric cooker heating element; and means connected to the output of the memory for varying the current supplied to the heating element in dependence on the value stored in the memory, so that each value corresponds to a respective rate of energy supply; at least two proximity detectors each including an insulated plate member, a first sense electrode on one face of the insulating plate and a second sense electrode on the opposite face of the insulating plate, the first sense electrode being capacitively coupled with the second sense electrode and the first sense electrode being responsive to close proximity between the second sense electrode and conducting body effective to produce an output signal, only the first sense electrode having an output connection, each electrode output connection being connected to the input of a respective switch circuit whose output is in turn connected to a respective control input of the memory.