Temperature controlled thin film circular heater

A thin film tin-oxide heater (10) including an annular inner heat region (12), an annular outer heat region (14), a first silver buss bar (16), and a second silver buss bar (18). The radius (r.sub.2) between the inner and outer heat regions is selected so that the resistance per unit square and power per unit area for the inner heat region approximates the resistance per unit square and power per unit area for the outer heat region.

TECHNICAL FIELD
 The present invention relates to the use of thin conductive films in
 resistance heating applications and, more particularly, to printed heating
 elements for surface heating applications, such as counter top stoves,
 which are constructed with large-area circular heating panels that provide
 even, low-power density, efficient heating.
 BACKGROUND ART
 U.S. Pat. No. 5,616,266, entitled "Resistance Heating Element with Large
 Area, Thin Film and Method," issued Apr. 1, 1997 and co-pending patent
 application, Ser. No. 08/874,524, entitled "Method and Apparatus for Edge
 Heating of Thin Film Heating Element," filed Jun. 13, 1997, both assigned
 to assignee of the present patent application, disclose thin film
 resistance heating elements for use in a variety of oven and space heater
 applications. The present invention improves upon the design of the thin
 film heaters disclosed in these patents.
 The '266 patent discloses a thin film heater having a metal substrate with
 a ceramic layer thermally bonded across one side of the metal substrate.
 An electrically conductive, large area thin metallic film is deposited on
 the ceramic layer, isolated from the metal substrate. A pair of spaced
 apart electrical terminals are provided at the ends of the conductive
 film. Preferably, the conductive film is stannic oxide (tin-oxide) and is
 deposited onto the ceramic layer as a very thin film of, for example, 2
 microns or less. Large area heaters constructed in this manner have been
 found to be capable of temperatures in excess of 500.degree. F. while
 allowing operation at high power levels, but lower power densities. Low
 power densities produce an extremely even heat at lower temperatures
 without significant hot spots or excessive thermal gradients over the area
 of the panel.
 Co-pending application Ser. No. 08/874,524 discloses a method and apparatus
 for controlling heat loss at the peripheral edges of heaters of the type
 disclosed in the '266 patent. A thin film conductive edge heater strip is
 formed around the peripheral edges of a large area thin film heater and is
 separately controlled to adjust its heat loss in order to compensate for
 heat loss at the outer edges of the large area heater. This design can be
 used in combination with the improved thin film heater of the present
 invention.
 Circular heating elements are conventionally made in the form of a spiral,
 such as the spiral heating elements of electric stove-top heaters.
 Circular heaters are employed because most cooking utensils are circular
 and because a round shape improves the efficiency of the heater by
 matching more closely the geometry of the load. Thick film heaters,
 typically 0.001" thick, provide a relatively uniform, low-temperature
 operating surface. However, a more uniform and lower operating temperature
 heater is the thin film heater, such as disclosed in the '266 patent. Thin
 film heaters made from metal oxides, such as tin-oxide, provide long term
 durability and stability up to approximately 500.degree. C., which is hot
 enough not only for cooking purposes but for many other purposes as well.
 DISCLOSURE OF INVENTION
 Briefly described, a first embodiment of the present invention comprises a
 circular resistance heating element that includes an annular, electrically
 conductive, thin film outer heat region and an annular, electrically
 conductive, thin film inner heat region within the outer heat region. A
 first buss bar separates and electrically connects the inner and outer
 heat regions and a second buss bar electrically connects to and extends
 around the outer peripheral edge of the outer heat region and electrically
 connects to and extends around the inner edge of the inner heat region.
 With this design, a voltage applied across the first and second buss bars
 applies the same voltage across the outer heat region and across the inner
 heat region. In addition, the relative widths of the inner heat region and
 the outer heat region are determined so that the power dissipated per unit
 area for the inner and outer heat region film are approximately equal. In
 this manner, the resistance heating of the circular heater is kept
 relatively uniform across the surface of the heater and thus the
 temperature is more uniform than for a single region film.
 The invention contemplates the provision of at least two annular heat
 regions, but provision of additional annular regions increases the
 uniformity of the heat gradient radially of the heating surface and
 reduces the current density at the inner diameter of any region.
 A second embodiment of the resistance heater of the present invention
 comprises annular outer and inner heat regions that do not necessarily
 have to be circular as with the first embodiment, but which are each
 divided into at least two radially divided sections. The inner and outer
 heat regions are electrically connected in parallel and each include a
 first buss bar extending around outer edge of a first section of the heat
 region (or alternatively around the inner edge of the first section of the
 heat region), one or more intermediate buss bars electrically
 interconnecting the first section with subsequent sections, and a final
 buss bar extending around one of the inner and outer edges of the
 preceding section and the inner and outer edges of the last subsequent
 section. The initial intermediate buss bar extends around the edge of the
 first section that the first buss bar does not extend around, i.e. if the
 first buss bar extends around the outer edge, then the initial
 intermediate buss bar extends around the inner edge. The initial
 intermediate buss bar also extends around the same edge of the next
 subsequent section, and any additional intermediate buss bars extend
 around the inner or outer edge of a preceding section not occupied by a
 preceding buss bar and extend around one of the inner and outer edges of a
 subsequent section. In other words, the buss bars alternate from inner
 edges to outer edges of each section so that all sections making up a
 region are electrically connected in series. A voltage applied across the
 first and final buss bars applies a fraction of the total voltage across
 each section of the inner and outer heat regions, first through the first
 section of each region, and then through subsequent sections. This has the
 advantage of uniform heat distribution of the first embodiment and also
 the advantage of lower voltage and resistance per unit square for each
 section of heating element.
 These and other features, objects, and advantages of the present invention
 will become apparent from the following description of the best mode for
 carrying out the invention, when read in conjunction with the accompanying
 drawings, and the claims, which are all incorporated herein as part of the
 disclosure of the invention.

BEST MODE OF CARRYING OUT THE INVENTION
 Reference will now be made in detail to the preferred embodiments of the
 invention, examples of which are illustrated in the accompanying drawings.
 While the invention will be described in conjunction with the preferred
 embodiments, it will be understood that the described embodiments are not
 intended to limit the invention specifically to those embodiments. On the
 contrary, the invention is intended to cover alternatives, modifications
 and equivalents, which may be included within the spirit and scope of the
 invention as defined by the appended claims.
 Referring to FIG. 1, a first embodiment is shown for a thin film heater 10
 of the present invention. Heater 10 is in the form of a circular heater
 element that is suitable, for example, for a stove top cooking appliance.
 Heater 10 includes an inner annular thin film heat region 12 and an outer,
 concentric, annular thin film heat region 14. Inner heat region 12 and
 outer heat region 14 comprise the heating surface area of heater 10 and
 both are thin film heating elements formed in the manner discussed in U.S.
 Pat. No. 5,616,266 and in co-pending patent application Ser. No.
 08/874,524. Exemplary manufacturing techniques include spray pyrolysis,
 chemical vapor deposition, vacuum deposition, sputtering, silk screening,
 and extrusion techniques.
 A first silver buss bar 16 separates inner heat region 12 from outer heat
 region 14 and is adapted for connection to electrical terminal L1. A
 radial slot or gap region 20 provides an electrically isolated access path
 for buss bar 16 to the exterior of heater 10 for connection to terminal
 L1.
 A second silver buss bar 18 surrounds outer heat region 14 and is adapted
 for connection to electrical terminal L2. Radial slot region 20 also
 provides an access path for buss bar 18 to extend to the center of inner
 heat region 12, where a portion of buss bar 18 forms an inner buss bar
 18'. The formation of buss bars 16, 18 is also discussed in the
 forementioned '266 patent and patent application Ser. No. 08/874,524.
 Typically, the substrate for the heater is masked where the buss bars are
 to be located, and then the thin film heater material is deposited or
 printed onto the substrate. A buss bar material, such as ceramic silver
 consisting of silver flakes, glass frit and a thixotropic screening medium
 that is burned off in the process of firing the bus bars, is silk screened
 in place in a manner where the material slightly overlaps the edges of the
 thin film heater material where electrical contact need be made.
 A voltage applied across terminals L1 and L2 applies the same voltage
 across inner heat region 12, from buss bar 18' to buss bar 16, and also
 applies the same voltage across outer heat region 14 from buss bar 18 to
 buss bar 16. However, it is a unique feature of the invention for the
 resistance per unit square for heat regions be equal so, with properly
 positioned buss bars, the power per square unit is equal. As a result, the
 heating across the inner and outer regions is sufficiently uniform.
 Resistance per unit square is a concept derived from bulk resistivity and
 is a surface resistivity term for conductive thin films that are uniform
 in thickness.
 To achieve uniform heating, the radial widths of the inner and outer heat
 regions are determined as follows. The radius r.sub.1 of buss bar 18' and
 the radius r.sub.3 of buss bar 18 are selected based on application design
 criteria. For example, cooktop stove heating elements have diameters
 ranging from six to twelve inches. Radius r.sub.1 can be as minimal as
 possible given the space requirements for the design of buss bar 18'. For
 example, buss bar 18' can be reduced to an enclosed circular cul-de-sac,
 with sufficient space reserved for slot region 20. Because buss bar 18'
 will always have some radial dimension, r.sub.1 can never be zero,
 although it may approach zero for practical purposes. Radius r.sub.3,
 theoretically, has no limit to its length, although in general the greater
 the radial width of a heat region, the greater the potential for
 generating a heating gradient.
 Radius r.sub.2 of buss bar 16 is selected so that the power per unit area
 is the same for both the inner and outer heat regions, which ensures
 generally uniform heating across the inner and outer heat regions. Radius
 r.sub.2 can be calculated as follows:
 Let: N.sub.12 =# of squares for the inner heat region
 A.sub.12 =Area of the inner heat region
 N.sub.23 =# of squares for the outer heat region
 A.sub.23 =Area of the outer heat region
 Then r.sub.2 is selected such that:
EQU V.sup.2 /R.sub.12 A.sub.12 =V.sup.2 /R.sub.23 A.sub.23 (1)
 where V=voltage; R.sub.12 =resistance of inner heat region; and R.sub.23
 =resistance of outer heat region. This is the equation for equal power
 density for each region.
 If .gamma.=resistance per square unit, and R=.gamma.N, then
EQU V.sup.2 /.gamma.N.sub.12 A.sub.12 =V.sup.2 /.gamma.N.sub.23 A.sub.23
 Therefore:
EQU N.sub.12 A.sub.12 =N.sub.23 A.sub.23 (2)
 must be satisfied because a principle feature of the invention is that both
 the voltage and the resistance per unit square are approximately the same
 for both the inner and outer heat regions.
 Applying basic geometry principles:
 N.sub.12 =ln(r.sub.2 /r.sub.1)/2.pi.
 N.sub.23 =ln(r.sub.3 /r.sub.2)/2.pi.
 A.sub.12 =.pi.(r.sub.2.sup.2 -r.sub.1.sup.2)
 A.sub.23 =.pi.(r.sub.3.sup.2 -r.sub.2.sup.2)
 Substituting into equation (2)
EQU ln(r.sub.2 /r.sub.1)(r.sub.2.sup.2 -r.sub.1.sup.2)=ln(r.sub.3
 /r.sub.2)(r.sub.3.sup.2 -r.sub.2.sup.2) (3)
 There is always an r.sub.2 that satisfies equation (3) for practical heat
 region designs. Although the example that follows does not take into
 account the width and spacing of the silver buss bars, this can be
 accomplished readily and although equation (3) may not be readily
 solvable, analytically an interactive computer program yields a solution
 in general.
 The following example provides an illustration:
 assume r.sub.1 =1" r.sub.2 =1.995" r.sub.3 3"
 N.sub.12 =ln(r.sub.2 /r.sub.1)=0.6906
 N.sub.23 =ln(r.sub.3 /r.sub.2)=0.408
 A.sub.12 =(r.sub.2.sup.2 -r.sub.1.sup.2)=(1.995.sup.2 -1)=2.98
 A.sub.23 =(r.sub.3.sup.2 -r.sub.2.sup.2)=(9-1.995.sup.2)=5.02
 N.sub.12 A.sub.12 =(0.6906)(2.98)=N.sub.23 A.sub.23 =(0.408)(5.02)=2.05
 For an application requiring 1500 watts:
 the power across inner heat region: 1500(A.sub.12)/(A.sub.12 +A.sub.23)=559
 watts, and
 the power across outer heat region: (1500.times.5.02)/8=941 watts
 Resistance per unit square .gamma. for the inner heat region:
 (230.sup.2.times.2.pi.)/(0.6906.times.559)=861
 Resistance per unit square .gamma. for the outer heat region:
 (230.sup.2.times.2.pi.)/(0.408.times.941)=866
 For some applications, 115 volts and 216 ohms/square approach the upper
 limit of stable operation of some thin films. One potential solution to
 this problem is dropping the voltage via a gated triac and fusing the
 circuit. The insulation provided by the insulating substrate between the
 user and the voltage source should satisfy electrical codes. Protection
 against a broken cooktop, for example, which could expose a user to
 voltage, can be provided by a GFI. The lower voltage Ground Fault
 Interrupter also prevents leakage and dielectric breakdown.
 FIG. 2 shows an alternative embodiment for a thin film heater that achieves
 more uniform heat distribution radially across the heater element. The hot
 regions of a heat region form along the inner areas of the heating
 element, where current densities are greater. Provision of three or more
 heat regions improves uniform heat distribution, but for many
 applications, however, two regions may be sufficient.
 Heater 30 of FIG. 2 includes an inner heat region 32, an intermediate heat
 region 34, and an outer heat region 36. A first buss bar 38 extends
 through a radial gap 42 and includes an outer ring 44 and an inner ring
 43. A second buss bar 40 extends through a radial gap 41 and includes an
 inner ring 45 and an outer ring 46.
 As an example of the heating efficiency of heater 30, the following is
 provided:
 Let A.sub.1, A.sub.2, and A.sub.3 be the areas of the inner, intermediate
 and outer heat regions, respectively, P.sub.1-3 be the power of each
 region, and .gamma..sub.1-3 be the resistance per unit square for each
 region.
 Then, for the same inner and outer radii of FIG. 1,
 A.sub.1 =1.5 in.sup.2 (32)
 A.sub.2 =2.325 in.sup.2 (34)
 A.sub.3 =3.177 in.sup.2 (36)
 P.sub.1 =1.5(1500)/7=321 watts
 P.sub.2 =2.235(1500)/7=498 watts
 P.sub.3 =3.007(1500)/7=681 watts
 Ln.sub.1 (1.581)=0.458
 Ln.sub.2 (2.288/1.706)=0.2935
 Ln.sub.3 (3/2.413)=0.2177
 .gamma..sub.1 =230.sup.2 (2.pi.)/(0.4581)(321)=2260 ohms/sq.
 .gamma..sub.2 =2274
 .gamma..sub.3 =2242
 R.sub.1 =1.0
 R.sub.2 =1.581
 R.sub.3 =1.706
 R.sub.4 =2.288
 R.sub.5 =2.413
 R.sub.6 =3.0
 Where R.sub.1-6 are in inches and each bus bar is 1/8 inch in width.
 Thus, .gamma..sub.1 =.gamma..sub.2 =.gamma..sub.3, within less than 1% of
 one another due to rounding errors in the above example.
 Another alternative embodiment for reducing the required resistance per
 unit square of a thin film heating element is shown in FIG. 3. A thin film
 heater 50 has an inner heat region 52 and an outer heat region 54 and is
 divided into 4 sections or quadrants A, B, C, and D by narrow radial slots
 or gaps 60, 62, 64, and 66. Electrical terminal L2 connects to a first
 silver buss bar 70 along the outer edge of outer heat region 54 of
 quadrant A and to a first buss bar 72 along the outer edge of inner heat
 region 52 of quadrant A. The choice of buss bars 72, 74 extending
 initially along the outer edges, rather than the inner edges, of the inner
 and outer heat regions of quadrant A is arbitrary and can be reversed.
 A second buss bar 74 extends along the inner edge of outer heat region 54
 in both quadrants A and B and, thus, electrically connects the outer
 region heating elements of both quadrants A and B. Another second buss bar
 76 extends along the inner edge of inner heat region 52 in both quadrants
 A and B. Third buss bars 78, 80 electrically connect the heating elements
 of quadrant B to quadrant C, and fourth buss bars 82, 84 electrically
 connect the heating elements of quadrant C to quadrant D. Finally, fifth
 buss bars 86, 88 connect both the inner and outer heat regions to terminal
 L1.
 The alternating inner-edge/outer-edge positions of subsequent buss bars,
 for example buss bars 70, 74, 78, 82, and 86, creates a voltage drop
 across each heating element section for both the inner and outer heat
 regions. Thus, for example, a voltage of 230V applied across terminals L1
 and L2 applies approximately 57.5V across each heating element section.
 Reducing the voltage by a factor of four reduces the required resistance
 per unit square by a factor of 16 and allows for improved stable operation
 of many types of thin film heating elements since the resistance per unit
 square can be lower.
 For the embodiment illustrated by FIG. 3, since each section A, B, C, D
 receives the same voltage, the relative widths of the heating elements of
 the inner and outer heat regions can be selected, in a manner similar to
 selecting r.sub.2 of the heating element of the first embodiment of FIG.
 1, to ensure that the power/area of any section and region is the same.
 For this, equation (1) above is modified to:
EQU V.sup.2 /R.sub.12 A.sub.12 =V.sup.2 /R.sub.34 A.sub.34 (4)
 and equation (2) above is modified to:
EQU N.sub.12 A.sub.12 =N.sub.34 A.sub.34 (5)
 The geometry of the heating element of each sections changes slightly:
EQU N.sub.12 =2.times.ln(r.sub.2 /r.sub.1)/.pi.
EQU N.sub.23 =2.times.ln(r.sub.4 /r.sub.3)/.pi.
EQU A.sub.12 =.pi.(r.sub.2.sup.2 -r.sub.1.sup.2)
EQU A.sub.23 =.pi.(r.sub.4.sup.2 -r.sub.3.sup.2)
 where radius r.sub.1 is the radius of the circular path defined by buss
 bars 76, 84; radius r.sub.2 is the radius of the circular path defined by
 buss bars 72, 80, 88; radius r.sub.3 is the radius of the circular path
 defined by buss bars 74, 82; and radius r.sub.4 is the radius of the
 circular path defined by buss bars 70, 78, 86.
 Modified equation (3) then becomes:
EQU ln(r.sub.2 /r.sub.1)(r.sub.2.sup.2 -r.sub.1.sup.2)=ln(r.sub.4
 /r.sub.3)(r.sub.4.sup.2 -r.sub.3.sup.2) (6)
 Depending on the width of the buss bars and the width of the gaps between
 adjacent buss bars, radius r.sub.3 can vary relative to radius r.sub.2,
 but is proportional thereto. Thus, r.sub.3 =K+r.sub.2, where K is equal to
 the widths of, for example, buss bars 82, 88 added together plus the width
 of the gap therebetween. As an example, buss bars 82, 88 may each have a
 width of approximately 1/8 inch and the gap therebetween may have a width
 of 1/8-1/4 inch.
 Again, there is always an r.sub.2 (and an r.sub.3) that satisfies equation
 (6) for practical heater designs, and these radii can be calculated via a
 cut and try computer program.
 Since the voltage across the heating elements is cut by a factor of 4, the
 resistance per unit square is cut by a factor of 16, since resistance is
 inversely proportional to voltage squared.
 FIG. 4 shows an additional buss bar configuration 100 with a similar
 3-region design as the heater of FIG. 2. Circular, thin film heater 102
 includes three circular regions 104, 106, 108, and is divided into four
 quadrants 110, 112, 114, 116. Leads L.sub.1 includes buss bars 120, 122
 and lead L.sub.2 includes buss bars 124, 126. Intermediate buss bars 128,
 130, 132, 134, 138, 140, 142, 144, 146, 148, 150, 152. Circular heater 102
 is broken up into radial sectors or quadrants and the buss bars are
 connected in series in a manner similar to that shown in FIG. 3.
 The foregoing descriptions of specific embodiments of the present invention
 have been presented for purposes of illustration and description. They are
 not intended to be exhaustive or to limit the invention to the precise
 forms disclosed, and obviously many modifications and variations are
 possible in light of the above teaching. The embodiments were chosen and
 described in order to best explain the principles of the invention and its
 practical application, to thereby enable others skilled in the art to best
 utilize the invention and various embodiments with various modifications
 as are suited to the particular use contemplated. It is intended that the
 scope of the invention be defined by the Claims appended hereto when read
 and interpreted according to accepted legal principles such as the
 doctrine of equivalents and reversal of parts.