Note: Descriptions are shown in the official language in which they were submitted.
5.3 ~
SYSTEM F'OR POWE~ING ~ T~BL~ WINDS~IELD FROM ~LT~JAT~ POWE~ SOURCES
BAC~GRO,UND OF D~_INVENTION
1. Field of thç,In,v,,ention
This invention relates to an electrically heatable transparency
and, in particular, to a heatable vehicular windshield powered by an
alternating current power source or by the electrical system of the
vehicle to defog or deice the windshield.
2. _escription of the Prior ~rt
Prior art heatable transparencies, e.g., of the type taught in
U.S. Patent No. 4~820~902~ use electroconductive members such as
transparent conductLve coatings to pass electrical current across the
transparency in order to raise the transparency's temperature. In
general, the electroconductive member is positioned along an internal,
non-exposed surface of one of the glass sheets of a transparency between
and in contact with a pair of bus bars to heat the windshield to defog
and deice same.
~ eatable windshields presently available are powered from an
internal automotive power source such as a car battery. U.S. Patent Nos.
4,668,270, h,743,741, 4,820,902 and 4,940,884 teach a heatable windshield
having a continuous electroconductive coating between and in contact with
top-to-bottom bus bars. U.S. Patent Nos. 3,794,809 and 3,752,348 teach
an electrically heated windshield having a top-to-bottom bus bar
confi~uration using isolation lines to divide the conductive material
into separa~e zones. European Patent No. 378~917 teaches a heat
reflecting sandwich plate having side-to-side bus bars for current to
flow through the heat reflecting film transversely across the plate.
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~ ~Ji ~ 4~
The heatable windslllelds of the abovc yublicatlons have been
deslgned to be powered by the electricaL system of the car and operate
when tbe car engille is running to protect agalnst total discharge of the
car's battery. However, when layers of ice and snow build up on the
car's windshield when it is stationary, e.g. parked overnight, heating
the windshield from the car's electrical sy6tem would require the car's
engine to run for long perlods of time in order to heat the windshield
sufficiently to remove the ice. When the car is stationary and idling
for long periods of time, undesired emissions from the car's exhaust
system may be put into the environment. This condition is accentuflted in
geographic areas where the climate i8 very cold such as Canada and the
northermnost states of the United States.
It would be advantageous tllereEore, to provide a heatable
windshleld which may be powered from a source other than the electrical
power system of the car to electrically heat the windshield to remove or
prevent fog, ice and snow from accumulating on the windshield when the
car is not in use.
SUMMARY OF THE lNVENTlQN
The present invention relates to a system for controlling
current to an electrically heatable transparency and a method for making
the same. The transparency is of the type having a substrate with first
and second sets of opposed edges, bus bars located along each opposed
edge of one of the sets of opposed edges, an electroconductive member of
a predetermined sur~ace resistivity between and in contact with the bus
bars and facilities for providing electrical access to the bus barsO The
system for controlling current to the transparency includes facilities
electrically connecting the bus bars to a power source and controls to
adjust the amount of power delivered from the utility supplied power
source to the transparency.
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g
The present invention also relates to an electrically heatable
transparency with facilities electrically interconnecting it with
multiple power sources, e.g., a car battery and normal household
current. Controls are provlded for switching between the power sources,
as required.
BRIEF ~ESCRIPTION OF THE DR~W~NG
Figure 1 is a plan view of a windshield incorporating the
features of the present lnvention.
Figure 2 ls an exploded cross-sectional vlew through line 2-2 of
Figure 1.
Figure 3 ls a plan view of a windshield incorporating the
features of an alternate embodiment of the invention which utilizes
multiple power sources.
Figure ~ is a schematic view of an electrical system for the
embodiment of the invention illustrated in Figure 3.
Figures 5-7 are schematic views oE alternate embodiments of the
invention .
DESCR~PTION OF THE INVENTION
This invention relates to a heatable windshield that may be
powered from an automobile in which the windshisld is mounted and/or from
a source other than the automobile. For e~ample and not limiting to the
invention, the heatable windshield may be powered by a utility supplied
power source or a car battery. In the following description of the
invention, the invention is discussed in connection with a heatable
automotive windshield. However, as will be appreciated, the invention is
not limited thereto and may be used with any type o~ heatable article
which includes an electroconductive member extending between spaced bus
bars.
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2 ~ 8
Wi~dshield.s of the type prese~tly available, for exampLe those
ta~lght in U.S. Patent No. 4,82~,902 which teachings are hereby
incorporated by reference, teach bus bars at the top and bottom of the
windshield as mounted in a vehicle wlth an electroconductive member, such
as but not limited to, arl electroconductlve coating extending between and
in contact with the bus bars. ln connecting a windshield of this design
to normal household voltage levels, excessive power would develop across
the coating filrn because the surface resistivity of these currently used
coatings is too low to accommodate the voLtage level supplied to the
windshield. The surface resistivity o the coating films may be
increased; however, in order to increase the surface resistivity, the
film thickness is decreased which could undesirably result in a thin
discontinuous film having hot spots or nonconductive areas. Although the
invention will be described in the following discussion using a heatable
windshield of a design different from the design of presently available
windshields described above, it will be appreciated that modifications to
the present heatable windshield designs may be made and the features of
the invention practiced therewith.
With reference to Figures 1 and 2, the discussion will now be
directed to features of the inventlon. A transparency 20 includes glass
sheets 22 and 24 lalninated together by a plastic interlayer 26 (shown
only in Flgure 2). The present invention is not limited to a particular
type of glass and the type of glass used in the practice of the invention
may be clear or colored. Still further, the invention is not limited to
the type of interlayer 26, which may be any of the types commonly used in
the art of laminating glass sheets, e.g., polyvinylbutyral.
An electroconductive member 28, which in the following
discussion is a transparent conductive coating but which is not limiting
to the practice of the invention, is preferably placed Oll an internal,
non-exposed surface of one of the glass sheets 22 and 24 of the
2 0 ~ 3 8
transparency 20 and most prefera~ly on the lnboard surface 30 oE the
outboard glass sheet 22. The electroconductive member 28, whether a
plurallty of wires or a coating, when used in an automotive transparency,
should provide the required light transmlttance when viewed therethrough.
Coatings which are presently available for use in heatable
windshields prefexably exhibit the combination of transparency and
electroconductivity required to serve as the heatlng element for the
transparency 20. Coatings of the type disclosed in U.S. Patent No.
4,610,771 and 5,028,759, which teachings are hereby incorporated by
reference, may be used in the practice of the invention. The coating of
the type disclosed in the patent includes a silver Eilm between a pair of
zinc stannate films with a copper primer between the film layers. The
coating exhibits a surface resistivity on the glass surface 30 of
approximately 7 to 8 ohms per square when the silver layer has a
thickness of approximately 110 Angstroms.
In the particular embodiment of the invention illustrated in
Figure 1, a pair of spaced bus bars 32 and 34 are provided at the right
and left hand sides of the transparency 20 as viewed in Figure 1, with
each bus bar 32 and 34 being in contact with the electroconductive member
28 in any convenient manner. External electrical access to the bus bars
32 and 34 is made by providing each of the bus bars with bus bar
extensions 36 and 38, respectively, connected to leads 40 and 42,
respectively, located along an edge of the transparency 20, preferably
the bottom edge 44, at a terminal area 46. As will be appreciated,
location of the bus bar extensions 36 and 38, leads 40 and 42~ and
terminal area 46 are not limiting to the present invention and may
include other configurations. If desired, the transparency 20 may also
include an opaque border 48 (shown only in Figure 2) which extends about
selected portions of the transparency's marginal edge. The opaque border
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2 ~
1~8 of the typë used in the art, conceals bus bars 32 and 34 from slght.
ln the embodlmerlt ~f the invention shown in Figure 2, the coatlng 28
extends over border 28 to the bus bars 32 and 3~.
The extensions 36 and 38 and leads 40 and 42 are electrically
insulated from the electroconducttve member 28 in any convenient manner
to ensure that power is delivered to the member 28 only through the bus
bars 32 and 34~ ~lthough not limiting in the present invention, this may
be accomplished by limiting the electroco~ductive member 28 on surface 30
to that area of the glass sheet 22 betweer~ bus bars 32 and 34. ln the
particular embodiment illustrated in Figures 1 and 2, the
electroconductive member 28 extends over the bus bars 32 and 3~ but is
spaced from the e~tensions 36 and 38 and leads 40 and 42 by the boundary
area 50 (shown only in Figure 1).
The bus bars 32 and 34 may be made of any metallic containing
material of the type normally used in the fabrication of bus bars for
automotive transparencies. The extensions 36 and 38 and leads 40 and 42
may be of any design and any configuration in order to provide external
electrical access to the bus bars 32 and 34. The bus bar material,
although not limited in the present inventlon, is preferably a silver
containing ceramic material of the type used in the art.
The lead 40 is electrically interconnected to one prong 52 of
plug 54 via wire 56. The lead 42 is electrically interconnect~d to the
other prong 58 of the plug 54 via wire 60, circuit 62 which includes
switch 64 and rectifying diode 66 connected in parallel, and wire 68.
The switch 64, as shown in Figure 1, is in the open position. If
desired, a controller 69, such as a thermostat or cycling timer, may be
included along wire 68 to control the temperature of the transparency 20
and/or the length of time that current is supplied to electroconductive
member 28 to prevent overheating~ As can be appreciated by one skilled
in the art, the switch 64, diode 66 and controller 69 may be combined
into a single unit.
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2 ~
When the plug 54 is connected to a utility supplled power source
70, e.g. a househo]cl plug WhiCtl provldes alternatlng current, and the
switch 64 is in the open position, electrical current passes erom the
power source 70 throu~h the cliode 66. The diode rectiEies and limita,
i.e. reduces, the voltage and current delivered to the transparency 20 by
permitting the current to flow in only one direction, thus reducing the
power delivered to the transparency by 50%. When the switch 64 is ln the
closed position, the full AC current flows through the closed 6witch. In
both instances, the windshield is heated.
After the electrical current passes through the switch 64 or the
diode 66, depending on if the switch 64 is in the open or closed
position, the current flows through wire 60, lead 42, bus bar e~tension
38, bus bar 34, electroconductive member 28, bus bar 32, extension 36,
lead 40, wire 56 to prong 58 of the plug 54 to complete the circuit. As
the current travels horizontally across the electroconductive member 28,
as viewed in Figure 1, from the bus bar 32 to the bus bar 34, the
transparency Z0 i9 heated to a temperature suEficient to remove fog, ice
and/or snow accumulated on the windshield 20, depending on the po61tion
of the switch 64, the voltage of the power source 70 and heating time.
This switch arrangement allows the transparency 20 to have a
high and low power operating mode, depending on temperature conditions.
More specifically, full power passes through the diode 66 when the switch
64 is in the open position. As a result, only half of the available
power will be delivered to the transparency. It is expected that ln the
"low" operating mode, this lower power level would be sufficient to defog
the windshield and provide some deicing capability on mildly cold nights
of between about 20 to 32F (-7 to 0C), but may not provide enough heat
to keep a car's windshield frost free at temperatures below about 20F
(-7C). With the switch 64 in the closed position, full power would be
supplied to the windshield. The "high" operating mode would operate more
-- 7 --
2~21~
~or provklillg increased delclng capability a8 Inay be requlred on colder
nights whe~ the telllperature ls below abo~lt 20F (-7C). By providillg the
swltch 64 and the diode 66 arrangement, power may be efficiently and
effectively delivered to the windshield. As can be appreciated, the
switching arrangement is not necessary if a single level of operation is
desired. Furthermore, the swltching arrangement may be modified to
provide variable control oE the current to the transparency 20, in a
manner well known to those skilled in the art, to provide a range of
operatillg modes rather only a high/low power system.
When the bus bars 32 aud 34 have a urliform cross section and are
made of the same material as the lead and the extensions throughout the
cross section, the current flow at the bottom of bus bar is greater than
the current flow at the top, as viewed in Figure 1, resultlng in uneven
heating of the electroconductive member 28. To minimize thte result, the
cross section of the bus bars may be varied, i.e., for a bus bar of
constant thickness. the bus bar may be tapered. While either constant
width or tapering width bus bars may be used, tapering width bus bars may
advantageously reduce costs by reducing materials. A bus bar design
contemplated by the invention includes silver ceramic leads 40 and 42 and
extensions 36 and 38 having a constant width of 0.375 inches (0.95 cm)
and thickness oE about O.OOOS inches (0.00127 cm), with the silver
ceramic bus bars 32 and 34 having a constant thickness of about 0.0005
inches (0.00127 cm) and a width at the bottom edge of 0.375 inches (0.95
cm) and a width at the top edge of 0.125 inches (0.32 cm) as viewed in
Figure 1.
As shown in Figure 1 and described above, the bus bars 32 and 34
are positioned along opposing edges of the windshield 20. Presen~ly
available heatable windshields typically have bus bars located along the
top and bottom edges and in contact with a heating film as disclosed in
Figures 5-7 and further disclosed in U.S. Patent Nos. 4,743,741 and
-- 8 --
4,820,902, and are usually heated from power supplled by the car's
battery. The following (ti9cUs9ion i8 directed towards a comparison
between this presently available heatable wlndshield configuration and
that shown in Figure 1. For the sake of illustration, it will be
pres~ed that the heating area of the windshield is 30 inches (76.2 cm)
high and 60 inches (152.4 cm ) across and the interconnecting
elec~roconductive fllm has a surface resistlvity oF 8 ohma per square.
The power developed across the heatable windshield's ~ilm ls
calculated by the formula:
P = (V2x)/(ry) ~equation (1)] where:
P is the power developed across the heating film
measured in watts;
V is the voltage supplied to the
wlndshield from the power source measured in volts;
x is the dimension of the heating film measurecl in
inches in the dlrection perpendicular to current
tra~el as the current flows from bus bar to bus bar
across the heating ilm;
r is the surface resistivity of the heating film
measured in ohms/square; and,
y is the dimension of the heating film measured in
inches bus bar to bus bar.
As will be appreciated by one skilled in the art, the aspect
ratio, x/y, of equation (1~ changes depending upon the dimensions of the
windshleld and the placement of the bus bars.
Using equation (1), the power developed across the film of the
presently a~ailable heatable windshield having top and bottom bus bars
that are 60 inches long and 30 inches apart (values x and y,
respectively) using a 72 volt car battery is calculated to be 1300
~21~
wAtts. The 1300 watts of power developed across the windshleld Eor
approxilllate1y 3 ~llnutes should De sufficiellt to heat the coating 28 to
defog or defrost the windshleld.
If the sa~le windshleld deslgn under dlscusslon were to be
powered from an alternating current power source such as a 120 volt
household electrical outlet, the power, P, developed across the heating
film would be 3600 watts, almost three times as much as the wlndshleld
powered by the car's battery. This would require a 120 volt9 30 amp
power line which is not a common household power source. The excessive
power developed in the windshield powered by alternating current may
result in shortening the usable llfe of the windshield by overheatlng and
damaglng to the plastic lnterlayer. Addltlonally, there would be a
higher cost to heat the wlndshield.
To eliminate potentlal damage to the windshield and economically
heat the windshleld, the desi~n of the windshield of the present
invention, as shown in Flgure 1, may be used. Having the bus bars along
the sidss, decreases the value of x and increases the value of y in
equation tl). More speciflcally, the aspect ratio changes from 2:1 to
1:2. With the surface resistivity remaining constant, the power9 P,
developed across the heating film decreases by a factor of 4 from tha~ of
a windshield confi~ur~tion having top and bottom bus bars as dlscu3sed
above so that in a w~ndshield of the design shown in Figure 1 having bus
bars about 30 inches in length and a distance between the bus bars of 60
inches and a film resistivity of 8 ohms per square, the power, P,
developed across the film when the windshield is powered by a 72 volt car
battery is 800 watts.
It should be appreciated that the excessive heating in a top and
bottom bus bar windshield configuration powered by a household current
may be reduced by reducing the surface resistivity of the heating film.
However, reducing surface resistivity will similarly reduce the
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2 ~
effectlveness of tlle wi~ldsllleld if it is to be powered by a car battery
as well as a utillty power source, as wlll be dlscussed later in more
detail.
As another alternative, a rectii.-ying diode (not Ghown) may be
added to the utility current input line of a top and bottom bus bar
windshield configuration to reduce the power in half. In the example
discussed above, the power would drop from 3600 watts to 1800 watts.
Though not excessive, this power level is believed to be at the upper
limit of the power that should be 8enerated by a windsbield having a 30
inch by 60 inch heating area to avoid overheating. More specifically,
although not limiting in the present invention, it is preferred that the
power generated by the heatin~ film be limited to a maximwn about 1 watt
per square inch (0.16 watt per square cm). In additlon, such a high
power requirement ~ill prevent the simultaneous use of other features
operating from a utility power source, such as an engine block heater, as
will be discussed later in more detail.
With reference to Figure 3, there is shown transparency 100
incorporating an alternate embodiment of the invention whereby the
transparency may be powered by elther a utility power source or a car
battery. The transparency 100 includes a bottom bus bar 102 extending
between the sides 104 and 106 and spaced from bottom edge 108 of the
transparency of the transparency 100, a pair of upper bus bars 110 and
112 adjacent the upper edge 114 of the transparency 100 and
electroconductive coatings 116 and 118. As shown in Figure 3, the
coating 116 extends between and is electrically interconnected to bus
bars 102 and 110 and the coating 118 extends between and is electricity
interconnected to bus bars 112 and 102, with coatings 116 and 118 and bus
bars 110 and 112 being spaced from one another by area 120, which may be
a coating deletion area.
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2 ~
The area 120 splits the coatl~g into two sections. Although not
li1niting in the present invention, in the embodiment illustrated in
Figure 3, the coating is divided into two areas of equal width. By
selectively applying current to the windshield 100, the windshield 100
can efEectively Eu~ction as a windshield of the design having
side-to-side bus bars or a windshleld of the deslgn having bus bars at
the top and bottom. More speciEically, with a current flow through
coatings 116 and 118 in series, i.e. from bus bar 112 through coating 116
to bus bar 102 through film 118 to bus bar 110, the resulting current
flow is effectively the same as if the transparency 100 had slde-to-slde
bus bars. With a current flow through coatings 116 and 118 in parallel,
i.e. through the bus bar 102, coatings 116 and 118 and bus bars 102 and
112, the resultin~ current flow is effectively the same as if the
transparency lU0 had top and bottom bus bars.
Clrcuit 122 lllustrated schematlcally i~ Flgure 4 and discussed
ln detail below, presents one possible arrangement whereby the power
source for heating the windshield 100 may be automatically switched from
alternating current power at a source external to the car to direct
current power from the car's battery; however, as will be appreciated by
those skilled in the art, the invention is not limlted thereto. With
contlnued re~erence to Figure 4, the circuit 122 has a utility supplied
power source 124, e.g. a household outlet, connected to one bus bar, for
example bus bar 110, through bus bar extension 126 and wires 128 and
130. ~he other bus bar llZ ls connected through bus bar extenslon 132,
wlres 134 and 136, controller 138 and wire 140 to the utility power
supply 124. Utility power source 124 may be a plug and outlet simllar to
that shown ln Flgure 1 and prevlously discussed. Wire 130 includes a
relay contact 142 and wire 136 includes a relay contact 144, both shown
in their normally closed position. A relay coll 146 ls connected in
parallel across an internal power source 148, e.g., a car battery, via
2~21,g~
wires 150 and 152. WLre lS2 :i8 connected to bus bar 110 through wire
151l, which includes relay contact 156 shown ln its normally open
posi~ion, wire 128 and bus bar extension 126. WLre 152 is also connected
to bus bar 112 through wire 158, which includes relay contact 160 shown
in its normally open position, wire 134 and bus bar extension 132.
Similarly, wire 150 connects power supply 148 to the lower bus bar 102
through bus bar extension 162 and includes a contact relay 164 shown in
its normally open position. Thls relay contact arrangement fuactions to
isolate the utility power source lZ4 from the car power source 148.
In operation, when relay coil 146 ls in a de-energized state,
e.g., when the windshield 100 is not boing powered by power supply 148,
tlle relay contacts 142 ad 144 are closed and relay contacts 156, 160 and
164 are open. This allows utility supplied current to be delivered to
the windshield 100 to heat the electroconductive film8 116 and 118 in
series as discussed above. The current passes through controller 138
which may be, for example, a thermostat or a cycling timer, to control
the heating cycle of the windshield 100. When relay coil is energi~ed 9
which occurs when the wlndshield 100 is powered by power supply 148,
relay contacts 142 and 144 open and relay contacts 156, 160 and 164
close. This allows electroconductive films 116 and 118 to be powered in
parallel in a manner as discussed above.
If desired, a "high-low" power switch arrangement may be
included in circuit 122 in a manner similar to that discussed in Figure
l. More speciically, a circuit 166, which includes a rectifying diode
168 and a switch 170, may be added along wire 136 so that when switch 170
is open, full current must pass through diode 168 which reduces the
current flow to the windshield 100 (low power mode) and when switch 170
is closed, full current by-passes the diode 168 and is delivered to the
windshield 100 (high power mode).
2 ~
It shoulcl be appreciated that relay contacts 156, 160 and 164,
wire 150 antl lead 162 may be eliminated and controller 146 and power
source 148 may be connected directly to bus bar 110 through extension 126
and wires 128 and 154 and directly to bus bar 112 through exte~sion 132
and wlres 134 and 158. Ilowever, it must further be appreciated that such
an arrangement will reduce the potential heating capabllity of the
transparency 100.
Figures 5 through 7 illustrate three of a multitude of alterrlate
control circuits by which a heatable windshield having top and bottom bus
bars may operate using either a utllity power source or a vehicle
battery. In Figure 5, the utility power source 200, which may include a
plug and outlet arrangement as discussed earlier, the windshield 202,
which includes upper bus bar 204, lower bus bar 206 and interconnecting
electroconductive member 208, and the vehicle power eource 210 are all
connected i~ parallel with rectifying diodes 212 and 214 positioned along
wires 216 and 218, respectively. The orientation of the diodes 212 and
214 prevents the power from going to the vehicle power source 210, as
well as reduces the power delivered to the windshield 202 to prevent
overheating as discussed earlier.
In Figure 6, diode 214 iæ eliminated and a switch 220 is
positioned along wire 222. When switch 220 is in position A, the
windshield 202 is powered by source 200 and it is in position B, the
windshield 202 is powered by source 210.
Figure 7 is similar to Figure 6 except that the switching
between the power sources is done automatically. More specifically,
switch 220 of Figure 6 is replaced by contact switches 224 and 226 and
relay coil 228. ln a manner similar to that discussed earlier with
respect to Figures 3 and 4, when coil 228 is de-energized, contact 224 i
closed and 226 is opened so that the windshield 202 is powered by utility
source 200. Whell relay coll 228 l~ energi~ed, contact 224 is opened and
contact 226 is closed so that the wlndshleld 202 i5 powered by vehicle
source 210.
As dlscussed earlier, the heatable windshield as disclosed in
the present inventlon may be used in combination with other devices that
are powered by a utility power source. Although not limiting in the
present invention, one possible device is an engine block heater 72 (in
Figure 1) and 172 ~in Figure 3) which is typically used to heat an engine
that i9 not operating to prevent the engine fluids from free~ing at
frigid temperature. A typical household current of 120 volts and 15 amps
will provide 1800 watts of power. Engine block heaters usually use
600-650 watts of power, leaving llS0 to 1200 watts for the windshield.
As discus6ed in the embodiment oE the invention presented earlier, 1200
watts of power should be sufficient to defog and de-ice the windshield.
The forms of the invention described in this disclosure
represent illustrative embodiments thereof. It is understood that
various changes may be made without departing from the teachings of the
invention defined by the claimed subject mattar which follows.
