Note: Descriptions are shown in the official language in which they were submitted.
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A CONTROL CIRCUIT FOR A HEATING UNIT
Thi~ invention relate~ a control circuit for a heatin8 unit
and particular, though not exclusively, to a unit arranged to be
mounted in a oooking hob to fonm a hotplate area of a glass
oe ramic cook top of the hob.
Heating units of thi~ type are disolo~ed in our
corresponding U.K. Patent Application No. 2132060A, wherein, in
a preferred embodi~ent, ea2h unit includes four tungstsn-halogen
lamps supported above a shallow metallic tray containing a layer
of non-metallic, thermally-ir~ulative material. A temperature
control arrangement is capable of switching the lamp filament3
into a number of series and/or parallsl c binations providing a
corresponding number of d1screte power outputs of the lamps to
achieve an optimi3ed characteristic heat output curve. Other
temperature control3 may also be used~ 3uch a3 phase control of
the lamp filaments below a power level of 200~ and/or bur~t-fire
or mark-3pace control above this power level, because it has
been found that the use of bur3t-fire control at lower power
levels causes visual flickering effects of the lamps, which can
be disturbing to a user of the cooking hob.
However, the above-mentioned temperature control of the
lamp filaments may be disadvantageous if les~ than four lamps
are required, because the pos~ible number of ~eries and/or
parallel combinations of the lamp filaments i~ reduced, thereby
reducing undesirably the number of pos3ible temperature settines
on the cooking hob.
Furthermore, it is also desirable to maintain a balanced
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output from the lamp arrarlgement oYer thc whole hotplat~ area,
which becomes increasingly dlffioult as the nu~ber of la~p~ ls
reduced .
It is therefore an ob~ect of the pr sent invention to
provide a heating unit including a more fle~ible control
arrargement, which enable~ an increased number of heat output~
to be achieved from a reduced number of lamps.
It is a further ob~ect of the invention to provide a
heating unit including a control arrangement, which all2viates
the problem of di~turbing flickering of the lamps when
burst-fire control i9 used at relatively low power levels.
According to one aqpect Or the present invention there iY
provided a control circuit for a heating unit including a
plurality of lamps emissive of infra-red radiation, said circuit
inoluding switchine mean~ for selectively ~witching said lamps
into a plurality of series andJor parallel arrangements, each
having a respective power output, u~er-operable means for
~etting a desired heat output from said lamps, and proce~or
mean3 to effeot switohing between at least two of said
arrangements for predetermined proportions of a burst-fire cycle
to achieve ~aid de~ired heat output.
According to a second aspect of the invention there is
provided a oontrol circuit for a heating unit including at least
one lamp emiQsive of infra-red radiation, ~aid circuit including
processor means for effecting burst-fire control of power
supplied to said at least one lamp and for effecting phase
control of 3aid at least one lamp intermediate periods of
continuous energi ation of 3aid at lea3t one lamp during
burst-fire control.
The present invention will now be further described by way
of example only with reference to the accompanying drawing~,
wherein:-
Figure 1 shows a plan view of a heating unit including
three irfra-red lamps,
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Fieure 2 shows a oircuit for controlling the heat output of
the lamps shown in Fl~ure 1,
Figure 3 ~ho~s a table illustrating the various
configurat~on~ of the lampY in Figures 1 and 2,
Figure 4 shows an ~lternative circuit to that 3hown in
Figure 2 for controlllng the heat output from a heating unit
in¢orporating two lamps, and
Figure 5 show3 another oircuit for controlling the heat
output from a heating unit incorporating two lamps.
Referring to Figure 1, a heating unit includes a generally
circular shallow tray 1, preferably made of metal~ having a
layer 2 of t~ermally-insulative materialt such a~ a microporou3
material known a~ Microtherm, di~posed therewithin. Three
infra-red lamps 3 to 5 are supported above the layer 2 by two
~uitably-shaped piece~ 6, 7 of thermally-insulative material
located re~pectively ad~acent the ends of the lamps 3 to 5.
Each infra-red lamp 3 to 5 consists of a halogenated,
quartzS tubular er~velope 8 to 10 respectively, within which a
single coil or coiled coil tungsten filament, 11 to 13
respectively, is ~upported. Each end of each lamp 3 to 5
consist~ of a pinch seal (not shown), having electrical
connections to the respective end of the lamp filament sealed
therein, and each pinch seal is enclosed by a ceramic end cap,
such as at 14, to protect the pinch seals.
Th~ heating unit, preferably together with three other
heating units, is preferably mounted adjacent the underside of a
gla~ ceramic cook-top (not ~hown) of a cooking hob (also not
shown), ~o as to form a number of hotplate area~ of the cook-top.
The heating unit also includes a thermal limiting device
15, which is arranged to monitor the operating temperature of
the glas~ ceramic cook-top, to ensure that it is not damaged by
overheating. The device 15 is arranged to activate a
microswitch 16, which di3connects the power supply to the lamps
3 to 5, if the operating temperature of the glas~ ceramic
exceed~ a predetermined temperature.
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Figure 2 shows a oircuit for providing at least fifteen
temperature ~ettings of the heating unit, a~ shown in the table
in Figure 3, from only the three la~nps 3 to 5, shawn in Figure
1~ To this end, the circuit enable~ th~ hsat output of the
5 lamp3 to be varied by selecti~rely using either phase control,
wherein power is 3upplied to one or more of the lamp~ for
variable proportions of each positive half cycle of the 3upply
waveform or burst-f~re control, wherein power is supplied
intermittently to the lamp or lan~ps for a predetermined number
10 of cycles o~ the ~upply waveform.
The circuit shown in Figure 2 includes the three lamp
~ilaments 11 to 13, the power supplied to which is controlled by
three triacs 17 to 19, re pe¢tively.
Overall control of the circuit is governed by a
15 microprocessor 20, pre~erably of type TMS 1000, which has inputs
at 21 to user-operable temperature setting controls and may also
include temperature feedback controls for ~onitoring the
temparature of cooking utensils on the hotplate area of the
cooking hob.
Outputs from the microprocessor 20 control a number of
gates 22 to 25, and gates 22 to 24 control respectively
activation of the triacs 17 to 19. The microproce3sor 20 i~
also connected to a zero cross-over detecting circuit 26, a
phase control generating circuit 27, and a triac inhibit circuit
25 28. The three gates 22 to 24, and thu~ triacs 17 to 19, are
arranged to effect burst-fire control of the power 3upplied to
each of the lamp filaments 11 to 13, in dependence on the
setting of the user-operable control and corre~ponding output of
the microprocessor 20.
The circuit also includes a relay consisting of switches 29
and 30. With ~witch 29 in position A, a9 ~hown, triac 19 will
effect burst-fire control of power supp].ied to filament 13.
However, if ~witch 29 is changed to position B, triac 19 will be
connected to the phase control generating circuit 27, 90 that
35 power supplied to filament 13 will be pha~e controlled.
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Switch 29 therefore snable3 the output of ~llament 13 to be
selectively controlled by eithar pha~e control or bur.~t-~ire
control. Switch 30 i~ arranged to connect filaments 11 and 13
in ~erie when pha~e control i8 u~ed, a~ de3cribed hereinafter.
A voltage divider circuit 31 provides the appropriate
voltage for operation of the relay, from output 32, and of the
logic component~, from output 33.
Figure 3 ~how~ lamp filament configuration~, provlded by
the circuit in Figure 2, to achieve fifteen heat output~ of the
heating unit, with the maximum power o~ each lamp preferably
being 600W.
Setting no. 15 generate~ the highe~t power output of 1800W
by having all three filaments 11 to 13 connected in parallel at
maximu~ power level, i.eO continuou~ly energi~ed.
Setting no~. 14 to 12 eaoh have filaments 11 and 13 at
maximum power and triac 18 is arranged to control power to
filament 12 by bur~t-firing. Setting no~. 14 and 13 generate
power o~tputs of 1600W and 1400W, re~pectively, by filament 12
being energi~ed for 66~ and 33~ re~pectively, of each
bur~t fire cycle, and ~etting no. 12 generate~ an output of
120oW with filament 12 continuou~ly de-energi~ed.
Setting no. 11 generates an output of 1000W by controlling
all three filament~ 11 to 13 by bur~t-firing, ~ith filament~ 11
and 13 energi~ed for 58.3~ of the cycle and filament 12
energised for 50% of the cycle. It may be preferable to ~tagger
the energisation periods of one or more of the filament~ 11 to
13, to even out the load di~tribution on the main~ ~upply and
enqure that at lea~t one lamp is on at any given time.
Setting nos. 10 to 6 provide pha~e control of power
~upplied to filament~ 11 and 13 and bur3t-fire control of power
~upplied to filament 12. The pha~e control i~ aohieved by
~witching 3witch 29 to po~ition B and al~o ~witch 30 from
po~ition C a3 ~hown, wherein filament~ 11 and 13 are in
parallel, to po~ition D, to connect filament~ 11 and 13 in
~erie~. Triac 17 i~ then inhibited by triac inhibit circuit 28,
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~hioh i~ connected to an input of ~ate 24, 90 that the output~
of bs~th f`ilaments 11 and 13~ in series 7 are controlled by
activation OI triac 19. Triac 18 i9 activated, a~ before, to
achieve bur~t-fire control of the output of filament 12.
Fllaments 11 and 13 are phase controlled for ea¢h ~etting 10 to
6 ac 20W, and settings 10 to 6 generate outputs of 800'd, 600W,
450W, 35a~1 and 250~ re~psctively by burst-firing filament 12 for
100~, 70%, 45~, 30% and 12~ of the cycle, respectively.
Setting no~. 5 to 1 generate outputs of 180W, 14~W, 100~3
10 80W and 60W, re~pectively by appropriate pha~e controlling of
the power supplied to filaments 11 to 13, connected in series,
and having filament 12 continuou~3ly de-energi3ed.
It can be seen that, in each of the configurations,
fllaments 11 and 13 are arranged to generate the same outputA
15 and filament 12 generates a lower output than filaments 11 and
13, thereby en uring that a balanced vi~ual effect of the three
lamps i~ maintained and also enabling more uniform cooking of
certain food~, ~uch as pancakes, which tend to require more
intense heat around the periphery of the hotplate area
Figure 4 ~hows an alternative circuit, which can be u~ed to
control the heat output of` a heating unit accommodating only two
lamps having filaments 34, 35. The circuit includes a
microprocessor 36, which ha~ input~ 37 from user-operable
temperature setting controls and possibly also temperature
25 feedback controls, and also an input from zero cross-over
detector circuit 38. The ~icroproces~or 36 has outputs 39, 40
to control operation of triacs 41, 42, re~pectively, which are
connected re~pectively to t~e lamp filament3 34, 35. It may
also be necessary to include RFI components 43, 44 to reduce
30 unde~irable disturbances in the power ~upply to the filaments
34, 35.
The microprocessor 36 is arranged to select either phase
control or burst-fire control of the power supplied to the
filaments 34, 35, thereby reducing the number of circuit
35 components required and al~o ~implifying the circuit lay-out.
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The oircuit ~hown in Figure 4 could, of ¢ourse, be modlfied
so as to ¢ontrol a heating unit including more than two lamps
3imply by providing additional triac~ and corre~pondlng outputq
from the microprooessor 36.
From the two de~cribed circuits, it can thuq be seen that
the pre~ent invention provides aelectable bur3t fire and pha~e
control~ of the outputs of the lamp filament~, thereby providing
a flexible arrangement that i~ capable of generating a
~ub~tantial number of different heat output~ from the heating
unit.
It is also pos~ible with the preRent invention to alternate
the~e ~electable control~ by pha3e controllin~ power to a
filament~ preferably at 20 W , intermediate period~ of
energiQat10n of the filament during bur t-f~re control. In thi3
way~ however law the power output, the lamp remain3 vi~ibly
energi~ed, thereby alleviating flickering problem~ o~ the lamps
at 1QW power ~etting~, whioh can be di~turbing to a user of the
heating unit. Furthermore, by varylng the proportion of
bur~t-fire control to pha~e ¢ontrol, a large number of different
heat outputs ¢an be obtained.
When a number o~ heating unit~ are mounted in a cooking
hob, the microprocessor of the c1rcuits could be u~ed to control
the output~ of more than one of the unit~ a~ ~hown in Figure 4
by additional outputs 45 from microproces~or 36 to the lamp
filaments of another heating unit.
Figure 5 sho~ a control circuit for controlling the power
output~ of two heating unitq, each including two
tung~ten-ha'ogen la~p~ and each forming a hotplate area of a
cooking hob (not ~hown).
In the circuit, the four lamp filament~ 50 to 53 are each
connected in ~erie~ with a triac 54 to 57, respectively.
Filament~ 50 and 51 are provided to heat hotplate A and
filamentY 52 and 53 are to heat hotplate B. Triac~ 55 and 57
are connected in 3erie~ with RFI components 58, 59,
reYpectively, as ~hown in the circuit in Figure 4.
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A microproces~or 60 oontrols conduction Or ths triac3 51~ to
579 and thus energisation of tha filament~ 50 to 53, ln
accordance with user-oparable switches SA and SB, which ~et
the required heat output~ from hotplate~ A and B, respectively.
5 Switches SA and SB preferably each con~i~t of a Gray Code
slider or rotary switch, which generate~ a binary output
directly readable by the microproce~or 60 and is advantageous
in that only one digit change~ on ar,y transition from one
setting to another.
S~itche3 SA and SB have inputs I1 to Ill into the
microproce3~0r 60 and, if both ~witche3 SA and S}~ are on
~imultaneously, ie. heat output i9 required fro~ both hotplate3
A and B, the microproce~or 60 ~electively sample~, via output~
1 and 2~ the re3pective inputl from ~witches SA and SB.
The microprocessor 60 i3 also connected to a zero-cros~oYer
detector circuit 61, via output 03 and input I1, and to a
triac ~iring circuit, via output3 04 to 07, which
re~pectively operate~ triacs 54 to 57.
Relays RA~ and R.q2 are inoluded in the circuit for
20 hotplate A and relays RB1 and RB2 for hotplate B, 80 as to
3witch the filament~ 50 and 51 in hotplate A and filament~; 52
and 53 in hotplate B into serie3 or parallel arrangements, ~uch
that, when filament3 50 and 51 are in 3erie3, power to both of
them is controlled by triac 55, and, when filament3 52 and 53
25 are in 3erie3 power to both of them i~ controlled by triac 57.
Outputs 8 and 09 from microproce3~0r 60 control a
relay protection and drive circuit 63, which operates the relays
RA1 and RA2 and/or RB1 and RB2 and prevent~ arcing
through the circuit when the relays are changed.
Inputs I5 to Ig to the microproces30r 60 control the
clock frequency of the microproces~or.
Different power outputs from the filaments of each hotplate
can thu~ be achieved by pha3e control, ~eries conneatlon of the
filament3, parallel connection of the filament~, series
35 connection in combination with a diode, and parallel connection
in combination with one or more diode~.
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Po~er output~ bolow 200W can be achieved by phase control
alone. To alleviate the aforementioned ~lickering effect of the
lamps, above 200W9 outputs are preferably achieved by switching
the filaments between a number~ pre~erably two~ o~ the above
conneotions, with or without dlodes, for proportiors of a
predetermined burst-fire cycle.
The diodes are proYided by the triacs 54 to 577 which are
cau~ed to function as diodes by the microprocessor 60.
A specific example of power outputs achieved by the circuit
is ~hown below, wherein fifteen temperature settings are
provided with two la~p filament~, each of 900W.
SETTIN PHASE CONTROL OF FILAMENTAPPROX. PO~ER
NO. ARRANGEMENT FOR % AGE OF CYCLE OUTPUT (W)
1 Phase control 60
15 2 Pha~e control 76
3 Phase control 98
4 Phase control 124
Phase control 159
6 Phase control 200
20 7 hase control-54~. Series with diode-46% 260
8 erie~ with diode-93~. Series 7S 35o
9 eries with diode 69~. Series 31%419
eries with diode-28%. Series - 72%534
11 eries - 84% Parallel with diode~ 16S 681
2512 eries 40% Parallel with diode~ 60% 868
13 arallel with diodes - 90%. Parallel - 10% 1107
14 arallel with diodes - 51S. Parallel - 49~ 1411
Parallel - 100% 1800
If~ for example, setting no. 9 is required for hotplate A,
this setting ls set on switch SA, which passes the correct
signal to the microprocessor 60, via inputs I1 to I4. The
microprocessor then positions relays RA1 and RA2, via
outputs 8 and 09 and relay protection and drive cirouit 63,
so that filaments 50 and 51 are in series and power to the two
filaments is then controlled, via output 05 by triac 55,
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whieh operates a3 a diode for 69% o~ a bur3t-fire cyole pre-set
by the microproces~or 60 and continuously conduets for the
remaining 31S oP the eycle.
The filament arrangements are preferably only ~witoh~d
between two arrangements having power outputs adjacent eaeh
other in order of magnitude of the power outputs of all ths
arrangements, so that any undesirable flickerlng of the lamp~ is
mlnimised.
It can be seen that, by this circuit, it qhould be possible
to achieve an infinite number of temperature setting~q by
varying the proportionq of the eyele oeeupied by each filament
arrangement.
However~ uqually le~qs than ~ifteen 3ettings are, in Pact 7
required, the re~peetive outputs of whieh ean be salected a~
required.
Although the cireuit in Figure 5 i3 intended to control the
filament~ for two hotplates, only a ~imple modification would be
required to enable the circuit to eontrol only one or more than
two hotplates by changing the number of triacs, swit¢he~,
2Q relays, ete.
Furthermore, by ehanging the number of triacs and relays,
the circuit could easily be modified to eontrol one or more
heating unitq, eaeh ineluding more than two lamps, whieh would
then be capable of providing other arrangements eonsi3ting of
eombination~ of serie3 and parallel eonnneetions, as shown, for
example, in Figure 3.
