Controlled and energization circuit for electrically heated vehicle windows

A control and energization circuit for applying the vehicle alternator output directly to the resistive window heaters is disclosed. By disconnecting the alternator output from the vehicle battery, thereby temporarily interrupting the charging cycle of the battery, substantially the full alternator output may be applied to the window heaters to deice the vehicle windows.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to circuits for controlling the energization of 
electrically heated vehicle windows. More specifically, this invention 
relates to a control and energization circuit for temporarily applying 
large quantities of heat to vehicle windows to deice the windows. More 
specifically still, this invention relates to a circuit for temporarily 
interrupting the charging cycle of a vehicle battery to apply 
substantially the entire vehicle alternator output to the electrical 
resistance window heating elements. More specifically still, the present 
invention relates to such a circuit which includes protective features to 
assure that the duration of actuation is brief and subject to available 
electrical energy for battery operation of other vehicle circuits, that 
unnecessary actuation is avoided, and that too rapid reactuation is 
avoided. 
2. Description of the Prior Art 
Electrical heated rear windows are in common use, and it has been proposed 
to use electrically heated windscreens. The latter would have the 
advantage of allowing hot air ducts between the heater matrix and the 
screen to be dispensed with. Such heated vehicle windows as are in use and 
as are proposed receive electric energy at an essentially fixed and 
constant rate. A constant heating rate as presently used is adequate to 
demist a vehicle window but is not adequate, owing to limitations in the 
vehicle electrical system, to remove any substantial ice accumulations (to 
deice) the vehicle windows. However, if an electrically heated window is 
to be capable of deicing as opposed to mere demisting, a large amount of 
power is required. Such a requirement would place a heavy strain on the 
vehicle electrical system. Alternatively, if such a strain on the vehicle 
electrical system is to be avoided, the time required for electrical 
deicing would be so great as to render the system of little practical 
value. 
An object of this invention is to provide a circuit which will permit an 
improved method of electrically deicing vehicle windows. 
SUMMARY OF THE INVENTION 
The invention accordingly provides a deicing and demisting circuit for use 
in a vehicle having a generator with a field winding fed from a voltage 
regulator, at least one window provided with an electrical resistance 
heating element, and a battery; the circuit comprising: first switching 
means operable to connect the heating element to the generator output; 
second switching means normally occupying a first state connecting the 
generator output to the battery and to the voltage regulator, and 
switchable to a second state in which the battery is disconnected from the 
generator output and the field winding is connected to the battery; and a 
control circuit responsive to a manual actuation to switch the second 
switching means to said second state for a limited period of time and 
thereafter to return the second switching means to said first state. The 
invention provides further for battery sensing means responsive to battery 
voltage operative to return the second switching means to its first state 
in the event battery voltage drops below a predetermined value. The 
invention provides further for deice repeat demand inhibit means to 
prevent too rapid reactuation of the second switching means. The invention 
further provides ambient temperature responsive means operative to return 
the second switching means to its first state in the event ambient 
temperature is above a selected temperature above the freezing point.

DETAILED DESCRIPTION 
In the known system of FIG. 1, an alternator 10 with integral rectifying 
circuit has a main electrical output A, ground or common terminal E, an 
ignition sense terminal I, and a field winding terminal F. The main 
electrical output A is connected to charge a battery 12. The ignition 
sense output I is also connected to the battery 12 via an ignition warning 
light 14 and ignition switch 16. A voltage regulator 18 is connected 
between the ignition sense output I and the field winding terminal F. 
As seen in FIG. 2, the alternator 10 comprises Y-connected stator windings 
20, field winding 22, diode rectifier bridge 24, and protection diodes 26. 
It will be noted that in this conventional arrangement the end of the 
field winding 22 opposite the terminal F is connected internally of the 
alternator to the ignition sense output I. Thus, referring to FIGS. 1 and 
2, when the ignition switch 16 is closed, current flows from the battery 
12 via the warning lamp 14, which is thereby illuminated, and the voltage 
regulator 18 to energize the field winding 22. When the vehicle engine is 
started, the voltages at alternator outputs A and I will equalize and the 
lamp 14 will be extinguished. 
Turning to FIG. 3, a first embodiment of the invention has a deice/demist 
control unit generally indicated at 28 interposed between the alternator 
10 and a window heating element 30. While heating element 30 is shown 
symbolically as a resistance, it will be appreciated that this resistance 
could take the form of a resistive film or of a grid of resistive wires. 
The control unit 28 includes a first switch 51 which when closed, for 
example by a vehicle operator, connects the element 30 to the alternator 
main output A to give either deicing or demisting, as described below. A 
second switching means comprises switches 52a and 52b ganged together for 
operation by a control circuit 32. Switch 52a is normally closed to 
connect the alternator main electrical output A to the battery, while the 
normal position of switch 52b connects the alternator ignition sense 
output I to the vehicle ignition switch 16 via the ignition warning lamp 
14. In its other position, switch 52b permits a connection of the voltage 
regulator 18 to the ignition switch 16 by-passing the ignition warning 
lamp 14. 
A difference between the alternator electrical connections of the known 
arrangement of FIGS. 1 and 2 and the embodiment of FIG. 3 lies in the fact 
that the field winding 22 has both ends brought out to terminals F, F' the 
internal connection of the latter end to the terminal I being dispensed 
with. 
The control circuit 32 includes a manually operable deice switch 53. 
Control circuit 32 is connected to receive battery voltage via the 
ignition switch 16 when the latter is closed. 
In use, the driver may close the switch 51 to connect the window heating 
element 30 to the alternator main electrical output A, which results in 
sufficient heating for demisting. Demisting can occur while normal vehicle 
battery charging current is being applied to battery 12 through normally 
closed switch 52a. If deicing is desired, the switch 53 may be closed 
briefly to bring the control circuit 32 into operation. 
The control circuit 32 acts to switch the switches 52a, 52b from their 
normal state to their second state for a predetermined time period. The 
time period should be selected to be sufficient for deicing. After passage 
of the selected time period, the switches 52a, 52b will revert to their 
normal state. When the deicing mode is in operation, switching of switch 
52a from its normal state to the second state disconnects the battery 12 
from the alternator 10, thereby permitting the full alternator output to 
be delivered to the heating element 30. Also, the switching of switch 52b 
disconnects the voltage regulator 18 and battery 12 from the ignition 
sense terminal I so that the voltage regulator 18 is receiving only 
battery voltage. This consequently ensures a high field winding current, 
giving a high alternator output which is being fed solely to the heating 
element 30. The switch 52b also shunts the ignition warning lamp 14 so 
that the current flowing from the battery 12 to the voltage regulator 18 
does not cause a warning indication. 
The embodiment of FIG. 4 is broadly similar to the FIG. 3 embodiment. 
However, the window heater 30 is divided into three elements each of which 
is connected to receive the output of one of the alternator a.c. phase 
windings. The first switch 51 is arranged to complete the Y-connection of 
the three heater resistances 30. The operation of this embodiment is 
otherwise the same as for FIG. 3, and like references denote like parts. 
FIG. 5 illustrates in block form the control circuit 32. Depression of the 
switch 53 generates an output from a deice initiate and inhibit circuit 34 
to set a control gate and power buffer 36, which in turn operates the 
switches 52 to place them in their off-normal, or other, state. The 
initiate and inhibit circuit 34 also starts a timer circuit 38. After the 
predetermined time has elapsed, the timer circuit 38 triggers the initiate 
and inhibit circuit 34 to reset the gate 36 and thus causes the switches 
52 to revert to their original or normal state. 
Apart from these basic functions, it is desirable that the control circuit 
32 provide various safeguards since undue use of the deice made will 
severely load the battery, especially if other vehicle electrical 
equipment such as headlights and windshield wipers are in use. FIG. 5 
therefore includes a battery voltage sensor 40 which is arranged to 
receive a signal indicative of battery voltage and is preferably arranged 
to respond to the battery voltage falling below a selected predetermined 
level to actuate the inhibit function of the circuit 34. This sensor 40 
may have a long time constant, on the order of about 30 seconds, to allow 
the vehicle starter motor to be operated without terminating deicing. A 
deice repeat demand inhibit circuit 42 acts to disable the gate 36 for a 
fixed time period after each deicing cycle, and may for instance comprise 
a flip-flop set by the output of the timer 38. The timer circuit 38 may 
also include a temperature sensor, such as a thermistor, positioned to 
determine the interior temperature of the vehicle and connected to modify 
the time period of the deice cycle. Preferably the relationship between 
the temperature sensor and timer circuit 38 is such that when the interior 
temperature is more than a few degrees above the freezing point, the 
deicing mode is terminated a brief time period after initiation. 
The switches 52a, 52b are preferably semiconductor switching devices such 
as power transistors or SCR's. Alternatively, electromagnetic relays may 
be used. If SCR's are used, the repeat demand inhibit circuit 42 may be 
dispensed with since the deice mode can only begin with the engine and 
alternator stopped; if the alternator is producing an output, the forward 
voltage across the SCR's will prevent turn-off. 
The first switch 51 may also include a semiconductor device. In this case, 
the semiconductor device portion of switch 51 may be operated also by the 
output of the control gate and power buffer 36. Thus, manual operation of 
the switch will cause the deice mode to be selected each time, to be 
followed by demist when the timer 38 or an inhibit operates. 
The screen heating element may be an embedded element or a resistance thin 
film. Although an alternator has been described, a d.c. generator with a 
field winding may also be used. Delta connections, in place of the 
described Y-connections are comtemplated. 
The invention is particularly applicable to car and truck windshields, but 
may also be used with other windows in such vehicles or other vehicles 
such as aircraft.