CA2062188A1

CA2062188A1 - System for powering a heatable windshield from alternate power sources

Info

Publication number: CA2062188A1
Application number: CA002062188A
Authority: CA
Inventors: Harry S. Koontz; Nigel J. Thomas
Original assignee: PPG Industries Inc
Current assignee: PPG Industries Inc
Priority date: 1992-02-20
Filing date: 1992-03-03
Publication date: 1993-08-22
Also published as: JPH068798A; KR930019066A

Abstract

ABSTRACT OF THE DISCLOSURE

An electrically heatable transparency such as may be employed in a vehicle for defogging, defrosting or deicing the windows of the vehicle has the capability of being powered from either an internal or external power source. The transparency includes an electroconductive member in the form of a transparent conductive coating on a major surface of a glass sheet of the transparency. Bus bars contact the coating along opposed edge portions of the transparency and extend to an outer peripheral edge portion of the transparency via extensions of the bus bars for electrical connection to the power source. Connection to the external power source may be accomplished by use of a plug which is inserted into a household outlet connected to a utility supplied alternating current power source. A diode along the connecting wires acts to reduce the amount of power supplied to the transparency from the alternating current power source.

Description

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.

~ ~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.

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.

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 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.

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 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 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 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.

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.

Claims

1. A system for controlling current to an electrically heatable transparency of the type having a substrate with a first and second set of opposed edges, a bus bar located along each of said opposing edges of one of said sets of opposed edges, an electroconductive member of a predetermined surface resistivity between and in contact with said bus bars and means for providing electrical access to said bus bars, said system comprising:
means to electrically interconnect each, of said bus bars to an external power source; and means to control the amount of current directed from said external power source to said bus bars.

2. The system as in claim 1 wherein said current control means includes current reducing means and a switch means capable of directing at least a portion of said current through said current reducing means.

3. The system as in claim 2 wherein said electroconductive member is a transparent conductive coating.

4. The system as in claim 2 wherein said transparency is a windshield for a vehicle and said bus bars electrical connecting means includes a plug capable of being received by an alternating current power source outlet external to said vehicle.

5. The system as in claim 4 wherein said switch means includes a relay having an open position whereby current from said alternating current power source flows across said current reducing means and a closed position whereby current from said alternating current power source bypasses said current reducing means.

6. The system as in claim 5 wherein said current reducing means includes a rectifying diode capable of reducing the amount of current flowing from said alternating current power source to said bus bars.

7. The system as in claim 4 further including means to electrically interconnect said bus bars to an internal vehicle power source and means to selectively direct current from said external power source or said internal power source to said windshield.

8. The system as in claim 7 wherein the power generated by said electroconductive coating is no greater than about 1 watt per square inch.

9. The system as in claim 4 further including means to direct current from said external power source to additional devices of said vehicle.

10. The system as in claim 9 wherein said current directing means includes means to simultaneously direct current from said external source to said windshield and an engine block heater of said vehicle.

11. The system as in claim 4 wherein said first set of opposed edges has a distance between said opposed edges less than the distance between said opposed edges of said second set of opposed edges.

12. The system as in claim 11 wherein said bus bars are located along each of said second set of opposing edges of said transparency.

13. The system as in claim 12 wherein said bus bars taper from a predetermined width to a narrower width where said bus bars terminate.

14. The system as in claim 11 wherein said bus bars are located along each of said first set of opposing edges of said transparency.

15. The system as in claim 14 further including means to electrically interconnect said bus bars to an internal vehicle power source and means to selectively direct current from said external power source or said internal power source to said windshield.

16. The system as in claim 15 wherein the power generated by said electroconductive coating is no greater than about 1 watt per square inch.

17. The system as in claim 15 wherein one of said bus bars is a continuous bus bar which extends along one edge of said windshield, the other of said bus bars includes first and second linearly aligned, spaced apart bus bars which extend along the opposite edge of said windshield and said electroconductive coating includes a first coating portion extending between said continuous bus bar and said first aligned bus bar and a second coating portion spaced from said first coating portion and extending between said continuous bus bar and said second aligned bus bar.

18. The system as in claim 17 wherein said source selective means includes means to electrically interconnect said external power source to said first and second aligned bus bars such that current from said external power source flows through said first and second coating portions in series and means to electrically interconnect said first and second aligned bus bars and said continuous bus bar to said internal power source such that current from said internal power source flows through first and second coating portions in parallel.

19. A method for controlling current flow to an electrically heatable windshield for a vehicle of the type having a substrate with a first and second set of opposed edges, a bus bar located along each of said opposing edges of one of said sets of opposed edges, an electroconductive member of a predetermined surface resistivity between and in contact with said bus bars and means for providing electrical access to said bus bars, the method comprising the steps of:
providing electrical access to said bus bars from an external power source; and controlling the amount of electrical current from said external power source directed to said bus bars.

20. The method as in claim 19 further including the step of directing at least a portion of the current from said external power source through a current reducing means.

21. The method as in claim 21 further including the steps of electrically interconnecting said bus bars to an internal vehicle power source and selectively directing current from said external power source or said internal power source to said transparency.

22. A system for controlling current to an electrically heated transparency as herein described with reference to the accompanying drawings.