Note: Descriptions are shown in the official language in which they were submitted.
ELECTRONIC INFINITE EI~AT CONTROL
This invention relates to controls for ad~usting the rate of
application of electrlc power to a load, especially a reslstive load, and flnds
partlcular use ln controls for electrlc heatlng elements Eor stoves, space
heaters and the like.
Typical controls at present, employed for varying the power output of
heaters and stove elements, embody arrangements in which the control contacts
are cycled between an ON and an OFF condltion over regular, relatively short
time perlods, so tha~ the ON duty cycle of the contacts determines the average
power applied to or dissipated in the load. Typically the arrangement is such
that a thermally sensitive element in the control is heated during the ON
period of the duty cycle, and which cools durlng the OFF perlod. Such controls
only approximate to providlng constant power at any one setting due to changing
environmental temperature, are difficult to regulate exactly and are
particularly inaccurate and inconsistent from one unlt to the next at low
settings (that ls low duty cycle). Nevertheless, they are in widespread use
because they provide a continuous range of control.
~ith the development of solid state electronics, there are now triacs
(switchable bl-lateral conductors) on the market capable of handling the heavy
currents ln (and well above) the range encountered in electrlc heating
appliances and stoves. One problem with such devices ls that they evolve heat
and therefore any control mechanism involving them must be capable of
dlsslpatlng thls heat and in an environment whlch itself ls often quite hot.
It is an ob~ect of the present invention to provide an accurate
control device for medium to heavy current AC loads, particularly for resistive
heating loads for stoves and space heating, which gives a more accurate control
but retains the low cost advantage of prior art controls.
More particularly, in accordance with one aspect of the invention
there is provided, an electrical load control which comprises:
a pair of input terminals for receivin~ an alternating current
power supply;
a pair of respective cw;tch cont~cts series connected with each
terminal;
a first load terminal connected to one pair of said switch
contacts, and thus connectible to a first of said pair of input terminals,
the other pair of said switch contacts series connected through a switchable
bi-l~teral conductor to a second load terminal, said first and second load
terminals being cnnnectible to an electric load;
said bi-lateral conductor includin~ a switching gate electrode;
variable phase change means electrically connected on closure of
said switcb contacts between said first input terminal and ssid second load
terminal and having an output whose phase is variable with respect to phase
of the alternating current power 5upply at the input termin~ls, and means
connecting said gate electrode to the output of said phase change means;
a control shaft for said control movnble between an ON and an OFF
condition of said control to closa and open said switch contacts and to vary
-0 said phase change means for providin~ 8 variAble phase signal to said gate
electrode whereby varying output in 8 said load when connected to said load
terminQls;
and means preventin~ said shaPt from actuating said variable phase
change means to provide output to said gate electrode until said control is
in said ON condition.
In accordance with a second aspect of the invention there is
provided an electrical which comprises:
a pair of input terminals for receiving an alternating current
power supply;
a pair of respective switch contacts series connected with each
terminal;
a first load terminal connected to one palr of said switch
contacts, and thus connectible to a first of said pair of input terminals,
the other p~ir of switch said ContQcts series connected through a switchQble
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bi-lateral conductor to a second load terminal, said first and second load
terminals being connectible to an electric load;
said bi-lateral conductor includin~ a switchin~ ~Rte electrode;
a variable resistance and a capacitor electcicQlly series
connected through a junction, on closure of said switch contacts, between
said first input terminal and said second load terminal and means connecting
said gate electrode to the junction of said resistance and capacitor;
a control shaft for said control movable between an ON and an OFF
condition of said control to close and open said switch contacts and to vary
said resistance in said ON condition for providin~ a variable phase si~nal
to said gate electrode whereby varying output in a said load when connected
to said load terminals;
means preventin~ rotation of said sh~ft in said OFF condition of
said control, means permittin~ lon~itudinal movement of said shaft in said
OFF condition;
and means permitting rotation of said shaft following initi~l
longitudinal movement of said shaft in said OFF condition.
In accordance with a third aspect of tbe invention there is
provided an electrical load control which comprises:
a pair of input terminals for receivin~ an alternatin~ current
power supply;
a pair of respective switch contacts series connected w;th each
terminal;
a first loQd terminal connected to one pair of said switch
contacts, and thus connectible to a first of said pair of input terminals,
the other pair of switch said contacts series connected through a switchable
bi-lateral conductor to a second load terminal, said flrst and second load
terminals bein~ connectible to an electric load;
said bi-lateral conductor includin~ a switching gate electrode;
a vaci~ble resistance and a CQpacitor electrically series
connected through a junction, on closure of said switch contacts, between
sQid first input terminal and said second load terminal, and means
connecting said gate electrode to the junction of said resistance and
capacitor;
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housin~ means for said control;
msans mounting ssid bi-lateral conductor for heat sinking on said
housin~;
a control shaft protruding from said housing and ~ovable between
an ON and ~n OFF condition of said control to close and open said switch
contacts and to vary said resistance in said ON condition for providing a
variable phase signal to said gate electrode whereby varying output in a
said load when connected to said load terminals;
interlock means between said housing and said shaft preventing
rotation of said shaft in said OFF condition of said control, and means
perm;tting longitudinal movement of said shaft in said OFF condition;
said mov,ement of said shaft moving said cont~cts between open and
closed condition, and said interlock means permitting rotation of said sha~t
when moved to closed condition of said contacts.
Illumination means may be pro~ided to give a visible indication
when the switch contacts are closed and the illumination means may be
mounted adjacent the control shaft so that light is visible from the end of
the shaft. The resistive track may be profiled to provide a chosen
relationship of resistance to rctation of the shaft, and the resistive track
may comprise a resistive ink deposited on a substrate. A cover means may
form part of the housing throu~h which khe sh~ft exkends.
Specfic embodiments of the invention will now be described having
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reference to the accompanying drawings ln which:
Figure 1 shows a typical circuit diagram of a heating load circuit
and control embodylng the invention;
Figure 2 is a perspective view of one embodiment of a control devlce
forming part of the circuit of Figure l;
Figure 3 is a side view in section of a modified device of Figure 2
with different terminal arrangement;
Figure 4 is a side view in section similar to Figure 3 but with the
control device in ON condition;
Figure 5 is a plan vlew of a typical circuit board of an embodiment
of the invention with components connected below the board illustrated
schematically;
Figure 6 is a detailed side view in section of part of an embodiment
employing a heat sink and Figure 6A shows a similar arrangement but with the
cover forming the heat sink as in Figure 2 and shows a typical placement of
components connected to and accommodated beneath the circuit board;
Figures 7 and 7A show details of embodiments of the indicator llght
assembly and the base of the operating shaft;
Flgure 8 is a sectional view of a modified embodiment showing details
of the slider mount and of a lens, knob and detent arrangement for the
operating shaft;
Figure ~ is a plan view of the base of the control for receiving the
board of Figure 5; and
Figure 10 is a graph of output power in the load, against shaft
position.
With reference first to Figure 1, an alternating current supply is
provided to terminals Ll and 1,2 suitably made of brass. Ll feeds a load
terminal Hl connected to one side of a resistive load 6 through swLtch
contacts SW1. Termlnal L2 feeds, through switch contacts SW2, to a mounting
lug L3 which may also be brought out as a further terminal. L3 i8 connected to
terminal MT1 of a triac 10 (a switchable bi-lateral conductor). The other
electrode MT1 is connected through an rf choke 3 of about 50 ~H, suitably
comprising 49 turns of #17 gauge wire on a ferrite rod (as used in the
prototype)3 to a second load terminal H2 ~oined to other side of load 6.
Lug/terminal L3 and terminals H1 and ~12 may also suitably be of brass. The
sides of SW1 and SW2 remote from their respec~ive termlnals L1 and L2 are
bridged by a series connected neon lamp 1 and resistance 2. An rf suppresæion
capacitor 4 of about .1 ~F also bridges lug L3 and terminal H2. The gate G of
the triac is connected through a diac 16 (a symmetrical bi-lateral solid state
switch) to one side 17 of a phasing capacitor 19 of about .1 ~ whose other
side connects to MTl. A switch arrangement SW3 (single pole double throw, open
circuited in one position) connects this one side of capacitor 19 either to a
variable resistance 18 of about 930 kJ~maximum or to open circuit. The
resistance 18 and the switch SW3 are ganged as will be explained la~er.
It will be appreciated by those skilled in the art that the closing
of switch contacts SW1 and SW2 will light the lamp 1 and result in the
application of the voltage from terminals L1 and L2 across the series connected
triac 10 and load 6. Current does not flow however unless switching current
also flows into gate G. The variable resistance 18 and capacitor 19 (when
connected by SW3) form a timing circuit and only when the voltage at ~unction
17 between the two, and with respect to terminal MT1 of the triac, reaches
approximately 7 to 8 volts will the diac 16 forced into conduction through the
gate G and the triac switched on. The triac remains conductive during the
remainder of the half cycle of the ~C mains until the current reaches next zero
crossing and then switches off. It w~ll not switch on again until the junction
voltage is again high enough ~ith respect to MT2 during the next half cycle.
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Thus, by firstly sett1ng switch SW3 to connect 17 to the reslstance 18 and then
varying the resistance 18, the timing of the point in the mains AC cycle at
which the junction voltage reaches the prescribed value can be chosen and thus
the conductive duty cycle of the triac be controlled. Lower values of the
resistance 18, mean that the triac is switthed earlier in the AC cycle and
therefore high average current is applied to the load with subsequent strong
heating~ Larger values of resistance give lower heating.
Reference to Figure 2 shows a novel control devLce for effecting the
circuit action described for Figure 1 and illustrates the ~erminals L1, I.2, Hl
and H2 to which the external AC source and load are respectively connected.
The entire assembly is carried in a box 20 with a cover or lid 21 from which
protrudes a shaft 22 for the variable resistor 18. The triac is attached to
one side of the lid by fastener 28 so that the lid, constructed of a heat
conductive material such as aluminium, forms a heat sink for the triac.
Figure 3 shows a sectional view through an embodiment of the
invention and illustrates the device in the OFF position with the switch
contacts SWl and S~2 in open conditionO The connect actuator 25 is pressed
upwards by the upper contact springs of SWl and SW2. Figure 3 also illustrates
the wipers 29 and 30 of variable resistance 18 in the position in which the
wipers are also lifted from the printed circuit board 31 on which the elements
of resistance 18 are deposited (of which further details are g~ven later). It
should be observed that the wipers do not form part of the triac and load
series circuit, but are part of the timing switch circuit for the triac. They
do not therefore carry the load current.
In the sectiona] view shown in Figure ~ the apparatus of Figure 3 is
in lts ON pOSitiOI1. The connect actuator 25 is shown bearing upon the movable
members of contacts SW1 and SW2 so that they are closed. The sliders 29 and 30
which are moved against the circuLt board are then Ln position to contact the
elemen~s of resistance 18 on the circuit board 31 when shaft 22 is subsequently
rotated.
Discrete contacts are illustrated on the spring arms forming SWl and
SW2. These contacts can be dispensed wlth iQ general however since, as
explained above, SWl and SW2 do not break or make on load current, all of which
is effected by the triac. As a practical matter and in the event of failure of
the triac the switches SWl and SW2 would be required to make and break load
current for a small number of operations (until the operating person noted the
failure). Typically such switches without discrete contacts could accommodate
of the order of 50 operations without undue erosion.
It will also be noted that both the upper and lower contact arms of
S~1 and SW2 are springs. This ensures a wiping action between the contacting
faces, and thus good low resistance contact. When discrete contacts are not
used it is desirable to dimple at least one of the contact arms at the region
where contact will be made with the other arm for effective localising of the
contacting areas.
Figure 5 shows a plan view of the upper surface of the circuit
board 31, which is an inexpensive epoxy glass paper composite board on which
the resistance 18 is formed and comprises an inner conductor 40 and an outer
resistive element 41. Element 41 is profiled to obtain the appropriate
required variation of resistance of element 18 with respect to rotation of the
bridging sliders 29 and 30 between sections 40 and 41. Other interconnections
on the circuit board complete those illustrated in Figure 1 for the triac, the
choke, the capacitors and diac. Surprisingly, it is found that the resistive
element 41 of resistive ink can be printed on the board such as by a screen
printing process, without special preparation or the need to make the board of
a high cost cera[nic substrate. Typically used is a polyimide resistance ink.
The printed materlal is baked to harden and condition it, and finished wlth a
lubricant such as Lubriplate*. The sliders 29 and 30 are formed so that ~len
the device 22 is in the ~N condltion the pressure exerted on the elements 40
and 41 does no~ exceed 40 grams each. A pressure of not less than 10 grams i5
required for rellability and allowing for tolerances therefore the figure of
40 grams is taken as a design figure. Test points 80, 81, 82, 83 and ~4 are
provided, if desired, connecting to selected regions of the perimeter of
element 41 so that the resistance at various points along the arc of travel of
the slider 30 can be checked at assembly and for quality control purposes.
Resistance can be trimmed where needed using a laser knife. The temperature to
which the board is raised to bake the resistive coating is chosen to be above
any temperature llkely to be experienced by the device in use since it was
found that the resistance may change if the baking temperature is exceeded.
Typlcally baking temperatures of about 100C to 115C have been found to be
sufficient, though higher temperatures are envisaged dependent UpOtl the
temperature which the board can tolerate without changing of its properties.
Typically it is considered desirable to maintain a "guard band" of about 70 C
between the tolerance temperature of the board and the ink baking temperature.
~h Should a ceramic board be used in place of the cheaper composite7then much
higher temperatures can be used without difficulty. Ceramics in general are
brittle and thus present other problems in manufacture and assembly.
Figure 6 illustrates the mounting of the triac 10 to a heat sink 24
(as discussed for Figure 2) and to the circuit board 31. It is found that a
discrete heat sink is not usually required, and Figure 6A illustrates the
mounting of the triac 10 directly to the cover 21, suitably of aluminium, as
the heat sink. When electric isolation between the triac and its mounting is
needed, a mica insulation washer 62 can be introduced between the triac and the
heat sink and a non~conductive nylon fastener 28 used in place of the screw
* Trade Mark
illustrated in Figure 2. Instead of the mica washer a composite material now
on the market which is heat conductive, requires a minimum of heat sink grease,
and is a good electrical insulator and is known as CH0-THERM* sold by Chomerics
of 77 Dragon Court, Woburn9 Mass. 018011 U.S.~., may be used. Insulated triacs
are also avallable with good isolation for voltages of approxlmately 240 volts
AC to ground, thus avolding the need for insulation washers. A declsion in any
particular instance will depend upon costs lnvolved. Isolated triacs are
typically more expensive than unlsolated ones of the aame voltage and current
ratlng. It is also found that heavier curren~ rated trlacs tend to be operable
at higher temperatures than lower current rated triacs for the same load
current.
Figure 7 illustrates the mounting of the neon lamp 1 below the
connect actuator 25. Flgure 7A illustrates a vlew at 90 to thls. The
actuator 25 is hollow so that light from the neon can fall on the base 55 of
the shaft 22 J Whicll iS made of polycarbonate and is transparent so that the
light from the neon can pass up the shaft, to a lens 60 moulded as part of the
shaft above a knob 61 (as illustrated in Figure 8). The lower end of the
shaft 22 and the actuator 25 are profiled, and the end of the shaft spli, so
that on assembly they snap to one another to form a single operative unit,
wherein the shaft rotates in the actuator but the actuatr is captured on the
end of the shaft. Particularly suited to the manufacture of the shaft 22 are
the fluorescent dyed LISA~plastics available fron Bayer A.G., Leverkusen,
Federal Republic of Germany. These will accept illumination incident on the
side and contain dyes which fluoresce from the illumination. Fluorescent light
1s then transmltted by total internal reflection to the end of the shaft 22.
This will permlt considerable leeway in the placing of the neon. Various
colours of fluorescence are available, which allows different indication
* Trade ~ark
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colours for the control device.
With further reference to Figures 3 and 4 a locklng tab 65 extends
from the lid 21 towards insulated mount 70 for the variable resistor
contacts 29 and 30 and engages in a slot 64 in the mount only when the control
is in the OFF position. To move from this position the shaf t 22 must be
depressed against the springing action of the contacts SW1 and SW2 and of
slider contacts 29 and 30 and the knob turned 9 SO that the lower face of tab 65
then rides against the underside of the mount 70 with the slider contacts 29
and 30 pressed into connection with the elements of variable resistor 18, and
SW1 and SW2 are in ON condition.
Flgure 8 sho~/s an embodiment in which the mount 35 for the sliders 29
and 30 is modified to be received in the board 31 for rotation. A keyway is
formed in the un~ and is engaged by a spline 34 on the shaf t 22. The upper
side 36 of the spline contacts the underside of the lid 21 only in the OFF
position of the control to allow the shaft 22 to move upwards and allow the
actuator to move to the position in which the switches SWl and SW2 are open
(such as shown in Figure 7A). When shaf t 22 is pushed down against the
springing action of the moveable contacts of SWl and SW2 and rotated, upper
side 36 of the spline engages against the under-surface of land 37. In all
embodiments in the OFF position of the control, sliders 29 and 30 do not bridge
the elements 40 and 41 of resistance 18, even though they may be resting
against the board. Reference also to Flgure 5 shows that whereas slider 29
continuously engages track 40, there is an angle of about 30 of rotation oE
the shaft over which slider 30 does not engage track 41. It is this which
constitutes the open circuit position of SW3 depicted in Figure 1. The
requirement in the structures of Figures 3, 4 and 3 for both depression of the
knob 60, and its turning, to place the control in ON condition therefore
provides a double safety arrangement, neil:her one actLon alone being e~fective
~o turn it OM. In Flgure 8, the continuous engagement of the mount 35 in che
board retalned by ears 38 ensures that the préssure exerted by the sliders 29
and 30 on the board (or the resistance elements) can be precalculated and
remain at that value.
Figure 8 also illustrates a variation ln which shaft 22 engages an
acetal bullet 39 urged by spring 43. This bullet acts as a detent to allow a
positive "~eel" when the control is in ~he maximum ON position. Other detent
arrangements to achieve this effect may be used.
Figure 9 illustrates in plan view the structure of base 20 of a
typical embodiment. Vertical risers 51 and 52 from L3 and H2 respectively
connect to7and support, the circuit board 31 in a manner similar to risers 53
and 54 shown in Figures 3 and 4. Additional support is provided by upper
faces 55, 56, 57 and 58 of corner buttresses moulded in the base 20. Web 59
steadies and supports the rear face,of the mounting flange 60 of triac 10 (see
also Figure 6A).
It is found that since complete control can be had over the shaping
of element 41, control of the phase angle of the switching signal applied to
the triac gate and power output as shown in Figure 10 can be obtained. Thus
the initial ON condition is achieved at about 5% rotation oi the control at a
power output of 5% of total, at 25% rotation this is increased to 10% output
and at 50% rotation to 20% output. 50% output is obtained at some
75% rotation, rising to 100% output at 95% rotation. Such po~er increments
give a practically linear relationship of temperature to shaft angle (or
percentage rotation) for the heating element load 6. Because all heat
regulating switching is done by the triac, heavy duty contacts of the kind
required in prior art controls described above are not required for SWl and
SW2.
If the lug L3 is brought to a termina~ it can pro~ide an additional
connection to the circuit so that the control can be used as a commercial
replacement uni~ for appliances ln which present controls have a separate
indicator ligh~, one side of which is permanen~ly connected to the mains L1.
The other terminal of the light is then connected to L3, when the present
device is substituted.
It will be appreciated by those skilled in the art that, with minor
modificatione to the circuit board, a temperature probe resistance can be
included in series with reslstance 18 to operate as a temperature sensor
located near the burner element or in an oven if the unit ls used a6 an oven
control. In such cases, the dial of the unit (forming part of knob 61 for
instance) could be calibrated in degrees of temperature.
In practice some radio frequency interference is generated by the
switched wave form produced in the triac if the device is not suppressed. The
combination of choke 3 and capacitor 4 has been found fully effective as
suppressors to remove any problem in the frequency spectrum above 450 KHz.
The device is also inherently protected against any unintended
switch-on by transients occurring in the AC mains (a problem encountered
sometimes in some microwave ovens), since when the control is off, SWl and SW2
are open and the unit completely isolated from the mainsO
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