U.S. patent number 3,713,062 [Application Number 05/102,472] was granted by the patent office on 1973-01-23 for snap disc thermal sequencer.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Stuart L. Butler, Robert E. Crocker.
United States Patent |
3,713,062 |
Butler , et al. |
January 23, 1973 |
SNAP DISC THERMAL SEQUENCER
Abstract
Apparatus for sequencing the switching of electrical loads
comprises a plurality of switch cells built into an electrically
insulating housing. Each cell includes a bimetallic disc, motion
transfer pin, switch and terminal members. A disc plate is located
adjacent one side of the discs to retain them in position while a
control heater is positioned contiguous to the disc plate with
suitable electrical insulation interposed therebetween. A heater
grid cover and a terminal cover are located on opposite sides of
the housing to complete the assembly. The control heater can take
different forms including a printed heater on a ceramic substrate
or a semiconductive steep sloped positive temperature coefficient
of resistance (PTC) material and can be formed as individual heater
elements on a common heat sink. Line voltage compensation, as by
zener regulators, can also be provided.
Inventors: |
Butler; Stuart L. (Versailles,
KY), Crocker; Robert E. (Richardson, TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
22290035 |
Appl.
No.: |
05/102,472 |
Filed: |
December 29, 1970 |
Current U.S.
Class: |
337/107; 337/42;
337/102; 337/337; 219/486; 337/95; 337/104; 337/339 |
Current CPC
Class: |
H01H
61/02 (20130101) |
Current International
Class: |
H01H
61/02 (20060101); H01H 61/00 (20060101); H01h
071/16 () |
Field of
Search: |
;219/186,508,511
;337/42,43,95,96,102,104,107,112,337,338,339,354,370,371,377 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,391,692 |
|
Feb 1965 |
|
FR |
|
1,487,654 |
|
Jul 1967 |
|
FR |
|
Primary Examiner: Gilheany; Bernard A.
Assistant Examiner: Bell; F. E.
Claims
What is claimed is:
1. A thermal sequencer comprising an electrically insulative
housing containing a plurality of switch cells; each cell comprises
an electrical switch, switch terminals, a snap-acting disc, and
motion transfer means which extends between the snap-acting disc
and the electrical switch; an electrical resistance heater in heat
transfer relation with each of the snap-acting discs; a metallic
disc retaining plate located adjacent the discs to retain them in
the housing; a layer of insulation interposed between the heater
and the disc plate to electrically insulate the heater from the
metallic disc plate; a heater cover member mounting the heater in a
recess therein and formed with a grid construction which supports
the heater and which also provides apertures in the heater cover
member; and an electrically insulative cover having apertures
through which the switch terminals extend, the cover providing
electrical insulation among the terminals while permitting access
to them.
2. A thermal sequencer as set forth in claim 1 wherein the
snap-acting discs have high and low temperature snapping points
such that the time interval between respective electrical switches
will be approximately the same.
3. A thermal sequencer as set forth in claim 1 wherein the
snap-acting discs have high and low temperature snapping points
such that the time interval between respective electrical switches
will be different.
4. A thermal sequencer as set forth in claim 1 wherein said
electrical resistance heater is deposited in one continuous layer
on an electrically insulative substrate.
5. A thermal sequencer as set forth in claim 1 wherein said
electrical resistance heater is deposited in separate areas on an
insulative substrate whereby each heater area may be individually
trimmed for each switch cell and means is provided for connecting
the heater areas for electrical energization.
6. A thermal sequencer as set forth in claim 1 including means to
electrically connect one of the electrical switches in series with
the electrical resistance heater to serve to regulate maximum
temperature of the heater.
7. A thermal sequencer as set forth in claim 1 including another
cell and electric switch therein and means to electrically connect
said another electrical switche in series with the electrical
resistance heater, the cell containing the series connected switch
also containing a creep action bimetallic thermal element which
serves to regulate the maximum temperature of the heater.
8. A thermal sequencer as set forth in claim 1 further comprising a
thermal element of steep sloped positive temperature coefficient
(PTC) material and means to serially connect the material to the
electrical resistance heater to limit the maximum temperature
thereof to a predetermined value.
9. A thermal sequencer as set forth in claim 1 wherein said
electrical resistance heater is deposited in a plurality of
separated areas on an electrically insulative substrate and each
heater area is trimmed to a preferred characteristic for each
respective switch cell and means electrically interconnecting the
plurality of heater areas on the substrate so that they are
energized by connection to a single source of power.
10. Electrical sequencing apparatus comprising:
a housing of electrically insulative material, the housing having a
first side formed with a plurality of switch cavities and a second
side formed with a plurality of depending tubular walls defining a
disc mounting area, one for each switch cavity and aligned
therewith, a hub separating each switch cavity from each respective
disc mounting area, each tubular wall having a free distal end, an
annular groove formed in the free distal end of each tubular wall,
a snap-acting disc mounted in each annular groove, a switch
including a movable contact arm mounted in each switch cavity, a
bore provided in each hub, slidable motion transfer means slidably
mounted in each bore and adapted to transfer motion from each disc
to the respective movable contact arm;
a disc retaining plate provided with a plurality of apertures, each
aperture aligned with a respective disc, each aperture being
slightly smaller than each respective disc, the plate retaining the
discs in their respective annular grooves; and
a heater assembly mounted adjacent the discs and disc retaining
plate, the assembly comprising an electrically insulative
substrate, an electrically resistive coating located on the
substrate, spaced electrically conductive contact layers on the
resistive coating, means to electrically connect the heater for
energization, a heater cover member having a recessed portion
mounting the substrate, grid members defining apertures in the
heater cover member in communication with the recessed portion, the
heater cover mounted on the housing so that the resistive coating
is contiguous to and is facing the discs, and a layer of
electrically insulative material interposed between the resistive
coating and the discs and disc retaining plate.
11. Apparatus according to claim 10 including means to serially
connect one of the switches to the electrically resistive coating
and the discs are chosen to snap at different temperatures to
effect a sequential operation of the plurality of switches.
12. Apparatus according to claim 10 in which the resistive coating
is formed in separate areas, each area located in heat transfer
relation with a respective disc.
13. Apparatus according to claim 12 in which the discs are chosen
to snap at approximately the same temperature.
Description
This invention relates to a thermal sequencer to provide a time
interval between the switching of electrical loads and more
particularly to a sequencer with a plurality of bimetallic
snap-acting discs which are caused to be operated following the
energizing and deenergizing of a control heater. Among the several
objects of this invention may be noted the provision of a thermal
sequencer with a multiplicity of electrical circuits, with a
minimum packaged size, with a minimum complement of components,
with a simplicity in construction and economy in assembly. Another
object is the provision of simplified external control circuitry by
requiring one external circuit connection from the control heater
of the thermal sequencer. Another object is to provide thermal
sequencers with uniformly distributed control heaters and to be
designed so that they may be easily duplicated in production. A
further object is to provide a thermal sequencer in which the
number of sequencing stations can be easily varied and the time
interval in operation between stations can be varied. Still another
object is the limiting of the maximum temperature of the control
heater.
Other objects and features will be in part apparent and in part
pointed out hereinafter.
The invention accordingly comprises the elements and combination of
elements, features of construction, arrangement of parts and
planned sequence of operation which will be exemplified in the
structures hereinafter described, and the scope of the application
which will be indicated in the following claims.
In the accompanying drawings, in which several of the various
possible embodiments of the invention are illustrated:
FIG. 1 is an enlarged plan view of a thermal sequencer, with the
terminal cover broken away, according to the present invention.
FIG. 2 is a side elevation of FIG. 1.
FIG. 3 is a bottom view of FIG. 1 broken away to expose the various
parts.
FIG. 4 is an enlarged elevation of the FIG. 1 embodiment taken
along view line 4--4 of FIG. 1.
FIG. 5 is an enlarged horizontal cross-section along line 5--5 of
FIG. 1.
FIG. 6 is a perspective view of the disc plate.
FIG. 7 is a perspective view of a first embodiment of a heater
employed in the FIG. 1 sequencer.
FIG. 8 is an enlarged perspective view of a heater grid cover shown
broken away in FIG. 3.
FIG. 9 is an elevation of FIG. 8 partly broken away to show the
grid structure.
FIG. 10 is a perspective view of a second embodiment of a
heater.
FIG. 11 is a chart showing heating and cooling times for the switch
cells of the thermal sequencer.
FIG. 12 is a partial schematic showing a modified means for
limiting temperature of the heater.
FIG. 13 is a perspective view of a creep acting disc useful in the
thermal sequencer of the instant invention.
Corresponding reference characters indicate corresponding parts
throughout the several views of the drawings. Dimensions of certain
of the parts as shown in the drawings may have been exaggerated or
modified for the purposes of clarity of illustration.
Referring now to the drawings, the thermal sequencer, generally
indicated by reference numeral 2, is shown in FIGS. 1-5 with parts
broken away for clarity in illustration, in FIGS. 1 and 3 and
contains switch cells shown generally at numerals 4, 6, 8, 10 and
12. Housing 14, which may be made of conventional electrically
insulative molded phenolic, is provided with cavities on one side
generally indicated by numeral 16 as seen in FIGS. 1 and 5 and is
adapted on the other side to support bimetallic discs. As seen in
FIG. 5, an annular recessed surface is provided in the free distal
end of molded cylindrical walls 18 to support within the housing
snap-acting bimetallic discs 30 (removed from switch cell 4 and
thus not shown), 32, 34, 36 and 38 respectively for switch cells 4,
6, 8, 10 and 12. A hub 19 is formed in the housing for each cell
and contains a bore to slidably mount a motion transfer pin 50.
Additionally, housing 14 contains six spaced posts 21 for mounting
a heater assembly described below. The heater assembly comprises a
disc retaining plate 22, a strip of electrical insulation 24,
heater 26, and heater grid cover 28. On the side of housing 14 in
which cavity 16 is located switch cell 6, as seen in FIG. 5,
comprises terminal 40 electrically connected by cantilever mounted
electrically conductive flexible movable arm 42 with a movable
electrical contact 44 mounted thereon attached by means such as
rivet 45 to housing 14 and additionally terminal 46 which is made
of relatively stiff electrically conductive material, has mounted
thereon, as by welding, stationary electrical contact 48 is
attached to housing 14 by conventional means such as rivet 49.
Terminal cover 20, formed of a strip of electrical insulation and
provided with terminal receiving apertures, may conveniently be
placed on the housing to enclose the switch cells. Adjusting screw
47 located in tapped hole 51 of housing 14 bears against that
portion of stationary terminal 46 which mounts stationary contact
48 and serves as a calibrating screw to adjustably locate contact
48. Contact 44 is located generally opposite contact 48 and is
movable into and out of engagement therewith. Flexible arm 42 is
biased so that in its at rest position contact 44 engages contact
48 which is located as desired by means of adjusting screw 47 to
complete an electrical circuit from terminal 40 through flexible
arm 42, movable contact 44 to stationary contact 48 and terminal
46. Motion transfer pin 50 which is slidably mounted in a bore in
hub 19 for sliding motion in opposite directions is chosen in a
length for proper operation of contacts 44 and 48 in relation to
the configuration of snap-acting bimetallic disc 32. In FIG. 5 disc
32 is shown in its low temperature configuration or position and is
retained within support 18 of housing 14 by disc plate 22 made of
metal such as stainless steel. Plate 22 contains apertures 23 as
seen in FIG. 6, one for each disc, which are smaller in diameter
than the bimetallic discs. In the low temperature configuration the
discs bear against motion transfer pin 50 at end 54 so that it
bears against flexible arm 42 at end 56 to maintain contacts 44 and
48 out of engagement. When disc 32 is at its high temperature
position, its configuration or curvature is reversed from that
shown in FIG. 5 permitting pin 50 to slide in hub 19 from the force
transmitted by biased flexible contact arm 42 so that contacts 44
and 48 engage in electrical contact.
Switch cells 8, 10 and 12 are similar to switch cell 6 as
described.
Switch cell 4 is also similar to cell 6 except that the low
temperature configuration or position of snap-acting bimetallic
disc 30 is opposite to the position shown for disc 32 FIG. 5 so
that the contacts in the switch of cell 4 are engaged electrically.
Terminal 86 of cell 4, shown in FIG. 1, corresponds to terminal 40
of cell 6 and terminal 85, since it is internally connected to
heater 26 is cut off near the surface of housing 14, corresponds to
terminal 46. Thus, with snap-acting disc 30 (not shown) in its low
temperature position, an electrical circuit is provided from
terminal 86 through the switch of cell 4 to cut off terminal 85.
When snap-acting disc 30 is in its high temperature position, the
electrical circuit between terminals 86 and 85 is opened. Perhaps
it should be noted that construction can be made such that any or
all cells can be so constructed as to open electrical circuits on
temperature rise or any combination of circuit opening or closing
on temperature rise.
Heater 26, best seen in FIG. 7, comprises a ceramic substrate 57
such as alumina on which is deposited, such as by silk screen
printing, suitable conductive ink to form electrical resistance
heater surface 58 and electrically conductive silver or other
suitable material to form contact stripes 60 and 62. Insulation 24
seen in FIG. 2, comprises a strip of electrically insulating
material such as fluorinated ethylene propylene film or polyimide
film which serves to space apart and electrically insulate heater
26 from disc plate 22. Heater 26 is assembled in sequencer 2 so
that heater surface 58 contacts disc plate 22 through insulation 24
which prevents shorting out of the heater surface and provides
thermal coupling between heater 26 and plate 22. Additionally,
apertures 23 provide windows in disc plate 22 for heater surface 58
to be radiantly exposed to snap-acting bimetallic discs 30, 32, 34,
36 and 38 through insulation 24, so that the discs are heated in
part by conduction and in part by radiation from heater surface
58.
Heater grid cover 28, which may be made of conventional
electrically insulative molded phenolic is shown in FIGS. 8 and 9
to contain a recessed seating portion or ledge 64 which is adapted
to receive and locate heater 26. Grids 66 define heater apertures
68 exposing the surface of the substrate 57 of heater 26 to enhance
cooling.
Heater contact 72 shown in FIG. 4 is made of an electrically
conductive spring material such as phosphor bronze with free distal
end 74 formed to facilitate a sliding electrical contact and its
opposite end formed to facilitate attachment to housing 14.
Electrical connection to one end of heater 26 at contact stripe 60
is provided through aperture 70 in disc plate 22 (see FIG. 6) and a
similar aperture in insulation 24, (seen in FIG. 4) by free distal
end 74 of heater contact 72 which electrically engages contact
stripe 60. The other end of heater contact 72 is attached to
housing 14 by rivet 76. Heater terminal 78, formed of electrically
conductive material, is also conveniently attached to housing 14 by
rivet 76. This provides an electrical connection from terminal 78
through rivet 76 to heater contact 72 on to contact stripe 60 which
is in electrical connection with one end of electrical resistance
heater surface 58. The opposite end of electrical resistance heater
surface 58 is in electrical connection with contact stripe 62 (seen
in FIG. 7). Electrical connections to stripe 62 are made in a
similar manner by a second heater contact 80, see FIGS. 1 and 3.
End 82 of contact 80 bears against stripe 62 and is aligned in a
direction along the length of housing 14 compared to heater contact
72 aligned in a direction along the width of housing 14. The
opposite end of heater contact 80 is attached to housing 14 by
rivet 84 which also fastens cutoff terminal 85 to housing 14. The
electrical circuit for heater 26 thus is from heater terminal 78
through rivet 76, heater contact 72 which contacts heater stripe 60
on through the heater resistance layer 58, heater stripe 62 which
is contacted by heater contact 80 to rivet 84, cutoff terminal 85
and finally continuing through the switch of cell 4 to heater
terminal 86.
Switch cells 8, 10 and 12 which are similar to cell 6 can readily
be omitted or included as desired to vary the number of cells in
the sequencer. Also housing 14 can be increased in size by
providing additional cavities 16, supports 18 and hubs 19 for
switch cells in addition to those shown in FIGS. 1 and 3.
As described above, heater 26 in FIG. 7 is monolithically printed
or deposited on a single substrate. This achieves uniformity in
characteristics across the surface area 58 and provides a single
heater element for a plurality of switch cells.
Operation of thermal sequencer 2 will now be described with
reference to FIG. 11. Heating curve 120 represents the temperature
of snap-acting bimetallic disc 30 versus time after applying
voltage heater terminals 78 and 86 energizing heater 26 starting
from an ambient temperature, Ta. The temperature of bimetallic
snap-acting discs 32, 34, 36 and 38 are also represented closely by
curve 120 since the construction shown of sequencer 2 is
symmetrical and the heating is uniform across the heater surface 58
of heater 26. Cooling curve 122 represents the temperature of
bimetallic disc 30 and closely that of discs 32, 34, 36, 38 versus
time after heater 26 is deenergized starting from temperature T7.
T1, T2, T3, T4 and T5 represent the temperatures at which
bimetallic discs 38, 36, 34, 32 and 30 respectively snap to their
high temperature position and T6, T8, T9, T10 and T11 are the
respective temperatures at which the discs snap to their low
temperature position.
On applying power to heater terminals 78 and 86 which energizes
heater 26, when bimetallic discs of switch cells 4, 6, 8, 10 and 12
are at temperature Ta, switch cell 12 operates to close its
contacts after time delay t1, switch cell 10 closes its contacts
after time t2 for an additional delay after cell 12 of t2 less t1
and similarly for times t3, t3 less t2, t4 and t4 less t3 for
switch cells 8 and 6. Heating continues until time t5 when high
temperature snapping point T5 of bimetallic disc 30 is reached and
switch cell 4 operates to open its contacts deenergizing heater 26.
The heater cools until disc 30 reaches its low temperature snapping
point T6 at which time switch cell 4 will close its contacts
reenergizing the heater. Disc 30 temperature will cycle between T5
and T6 as long as power is supplied to heater terminals 78 and
86.
On removing power from heater terminals 78 and 86, when bimetallic
discs are at temperature T7, bimetallic disc 30 cools to its low
temperature snapping point T6 at which point bimetallic disc 30
resets. As cooling continues, bimetallic disc 32 reaches its low
temperature snapping point T8 opening the contacts of switch cell 6
after time delay t8; on further cooling to the low temperature
snapping point of disc 34, switch cell 8 opens its contacts after
time delay t9 or an additional delay after cell 6 of t9 less t8 and
similarly for times t10, t10 less t9, t11 and t11 less t10 for
cells 10 and 12.
One of the uses which sequencer 2 is particularly well suited is in
conjunction with forced air electric heaters or furnaces. In such
an application, operating requirements would typically call for the
fan motor and the first bank of heater elements to be turned on
approximately simultaneously after an initial time delay and the
remaining banks of elements to be turned on with a time delay of
approximately 10 seconds between stages. Assuming the furnace
comprises three banks of heater elements and a fan motor, switch
cells 10, 8 and 6 would be used for the banks of heater elements,
switch cell 12 for the fan motor and switch cell 4 to control the
maximum temperature of heater 26 of sequencer 2. Typical
temperature settings for the bimetallic discs would be as
follows:
Disc Snapping Point Switch Bimetallic High Low cell no. Disc No.
Temperature Temperature 4 30 245.degree. F. 225.degree.F. 6 32
225.degree.F. 205.degree.F. 8 34 200.degree.F. 180.degree.F. 10 36
170.degree.F. 150.degree.F. 12 38 170.degree.F. 150.degree.F.
heater 26 is designed to provide a rate of temperature rise which
results in operation of switch cells 12 and 10 typically in
approximately 30 seconds after the furnace control is turned on,
which applies power to heater terminals 78 and 87, to energize the
first bank of heater elements and the fan motor; in approximately
10 second intervals the second and third banks of heater elements
are energized as switch cells 8 and 6 operate and switch cell 4
operates subsequently in a cyclic mode as determined by the high
and low temperature snapping points of disc 30. For shut-down, the
room thermostat or similar sensor removes power from heater
terminals 78 and 86 which permits heater 26 to cool resulting in
switch cell 6 operating to turn off the third bank of heater
elements which is that bank last energized. Approximately 15
seconds later, switch cell 8 operates turning off the second bank
of heater elements and, in approximately an additional 15 seconds,
switch cells 10 and 12 operate to turn off the first bank of
elements and the fan motor. The functions of cells 12 and 10 can be
combined by using a single disc (combining discs 38 and 36) to
operate a double pin 50 to operate two sets of switch arms and
thereby control two electrical circuits with the operation of one
disc. Economies and timing advantages are apparent.
It should be noted that heater 26 may be formed by printing the
resistive coating 58 on the reverse side of substrate 57 thereby
eliminating the need of insulation 24 and, if desired, disc
retaining plate 22.
Another embodiment of the invention is shown in FIG. 10 in which
heater 90 comprises a substrate 95 such as alumina on which is
deposited conductive inks to form electrical resistance surface
areas 96, 98, 100, 102 and 104 which include supplemental areas
106, 108, 110, 112 and 114 respectively. Electrically conductive
silver is deposited in a continuous layer to form contact pad 92
and its stripe running across the center of the heater contacting
one side of each electrical resistance surface area. A second
continuous layer of conductive silver is deposited to form contact
pad 94 and its associated continuous stripes contacting the
opposite side of each resistance surface area. Thus all electrical
resistance surface areas are connected in parallel configuration
with contact pads 92 and 94. In this embodiment, each electrical
resistance surface area is individually trimmed by removing
portions of supplemental areas 106, 108, 110, 112 and 114 to make
all surface areas alike or different in characteristics as desired.
Heater 90 is assembled into thermal sequencer 2 in a similar manner
to heater 26 of FIG. 1 since contact pad 92 corresponds to contact
stripe 60 and pad 94 to stripe 62 for contacting heater contacts 72
and 80 respectively. Resistance surface areas 96, 98, 100, 102 and
104 provide individual heaters for switch cells 12, 10, 8, 6 and 4
respectively on a shared common heat sink. Simultaneous energizing
of all heater surface areas is achieved through a single pair of
electrical connections avoiding additional external circuitry.
Although the electrical resistance surface areas are shown
connected in parallel to contact pads and associated stripes 92 and
94, it is within the purview of this invention for the resistance
surface areas to be connected in series or series-parallel
combination with pads 92 and 94 to obtain particular desired heater
characteristics such as wattage, voltage, surface area and rate of
temperature rise.
Heater 90, when trimmed so that heater resistance surfaces areas
96, 98, 100, 102 and 104 are all approximately the same in
characteristics, such as resistance and temperature rise, provides
equivalent operation to that heretofore described with heater 26
when substituted for it.
Increased flexibility in design of sequencer 2 is obtained where
resistance surface areas 96, 98, 100, 102 and 104 of heater 90 are
trimmed to different characteristics of resistance and temperature
rise. Turn on time delay in operation of the switch cells is
determined by the snapping temperature point and rate of
temperature rise of the bimetallic discs. For example, a sequencer
incorporating a set of bimetallic discs having a set of snapping
temperature points tabulated above with reference to heater 26, and
with resistance surface areas 96 and 98 of heater 90 trimmed to
provide an increase in temperature rise, would have a decrease in
time for operation of switch cells 12 and 10, below the 30 seconds
previously indicated for turn on of the fan motor and the first
bank of heater elements while the additional time delay for
operating switch cells 8 and 6 for turning on the second and third
banks of heater elements would remain the same at approximately ten
second intervals. An increase or decrease in the additional time
delay for switch cells 8 and 6 would be obtained by trimming heater
surface areas 100 and 102 respectively, depending upon the extent
of removal of heater trim areas 110 and 112, to higher or lower
ohmic resistance values for lower or higher respective temperature
rise characteristics.
An additional advantage the FIG. 10 embodiment offers is the
opportunity to use bimetallic discs with approximately the same
snapping temperature point in all the switch cells and obtain the
desired time delay in operation between cells by trimming the
heater resistance surface areas to the desired characteristics.
Cooling time delay for turn off is adjusted as desired by selection
of low temperature snapping points of the bimetallic discs.
Switch cell 4, which is electrically in series with the circuit of
heater 26, can be varied from the construction previously described
within the purview of the invention. One modification would be the
replacement of snap-acting bimetallic disc 30 with a creep action
bimetallic disc 30' as shown in FIG. 13. Another variation would be
the employment of a bimetallic strip as the thermal element in
place of bimetallic disc 30. Yet another variation would be the
accommodation of a bimetallic strip with a movable contact
attached, to replace disc 30 and motion transfer pin 50, which
would electrically engage a suitably located stationary contact and
terminal replacing contact 48 and terminal 46. As shown in FIG. 12,
a further change would be the use of element 120 composed of a
steep sloped positive temperature coefficient of resistance (PTC)
material in series connection with heater 26 for sensing and
limiting the heater temperature. Element 120 is conveniently
located in switch cell 4 in place of its associated components.
Also when it is desired to limit variation in line voltage across
heater 26 so that variation in rate of rise in temperature of the
heater is correspondingly limited, a zener diode regulator may be
incorporated within switch cell 4. Drawings are not included for
all of the various modifications mentioned since the changes are
within the skill of the art.
As many changes could be made on the above constructions, such as
incorporating individual self contained switch cells in housing 14,
without departing from the scope of the invention, it is intended
that all matter contained in the above description or shown in the
accompanying drawings, shall be interpreted as illustrative and not
in a limiting sense, and it is also intended that the appended
claims shall cover all such equivalent variations as come within
the true spirit and scope of the invention.
It is to be understood that the invention is not limited to its
application to the details of construction and arrangement of parts
illustrated in the accompanying drawings, since the invention is
capable of other embodiments and of being practiced or carried out
in various ways.
* * * * *