Electric heating – Heating devices – With power supply and voltage or current regulation or...
Reexamination Certificate
2000-01-12
2001-10-16
Hoang, Tu Ba (Department: 3742)
Electric heating
Heating devices
With power supply and voltage or current regulation or...
C219S483000, C219S486000
Reexamination Certificate
active
06303911
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a thin film resistive heater and more particularly to a circuit and method for controlling the temperature of a thin film resistive heater used to heat a liquid crystal display.
BACKGROUND OF INVENTION
FIG. 1
shows an exploded view of a conventional module
10
which is used for housing a Liquid Crystal Display (LCD)
14
. The module
10
includes a front frame
12
, an LCD
14
, a thin film resistive heater
16
, and a back frame
18
. The device
10
is held together by interlocking tabs and recesses.
As is known in the art, if the temperature of the LCD
14
becomes too cold, the liquid crystal material within the LCD
14
becomes increasingly viscous. If such a result occurs, the LCD
14
does not work properly.
In view of this problem, the thin film resistive heater
16
is provided to maintain the temperature of the LCD
14
within a certain temperature range. To perform its heating function, the thin film resistive heater
16
includes a glass substrate
20
which contains a thin film coating of, for example, indium tin oxide (ITO). The substrate
20
is attached to a plastic frame
22
. As is known in the art, when a current is passed through the thin film coating, heat is transferred to the glass substrate
20
thus creating a heater.
When the module
10
is assembled, glass substrate
20
of the thin film heater
16
is placed in thermal contact with the LCD
14
. This configuration allows for the transfer of heat from the heater
16
to the LCD
14
.
Typically, a thermistor
24
is placed on an outer edge of the glass substrate
20
to monitor the temperature of the thin film heater
16
. As is known in the art, the thermistor is a device whose resistance is a function of temperature.
Based on the design of the device shown in
FIG. 1
, the thermistor
24
is required to be placed on an outer edge of the thin film heater
16
to ensure that the thermistor
24
does not interrupt the viewing area of the LCD
14
. In particular, if the thermistor
24
is positioned at the center of the glass substrate
20
, a shadow will appear on the LCD
14
. This is obviously undesirable given that such a shadow would impact the usefulness and desirability of the LCD
14
.
FIG. 2
shows a conventional circuit that is used to control and measure the temperature of the thin film heater
16
shown in FIG.
1
. The circuit shown in
FIG. 2
contains a power supply
26
which provides a supply voltage V
SUPP
to bus bars
21
of the glass substrate
20
contained within the thin film heater
16
. As is known in the art, the application of the voltage V
SUPP
creates a current I which in turn heats the substrate
20
.
The circuit shown in
FIG. 2
also contains a temperature feedback circuit
28
which is connected to the power supply
26
and the thermistor
24
. The temperature feed back circuit
28
continuously measures the temperature of the thermistor
24
.
If the temperature of the thermistor
24
falls below a certain temperature To , the feedback circuit
28
instructs the power supply
26
to apply the voltage V
SUPP
to the bus bars
21
to heat the glass substrate
20
. Conversely, if the temperature of the thermistor
24
reaches temperature To, the feedback circuit
24
then instructs the power supply
26
to remove the voltage V
SUPP
.
While the conventional device discussed above and shown in
FIGS. 1 and 2
allows for the temperature of the thermistor
24
to be measured, the device still has significant drawbacks. In particular, the design of the device shown in
FIGS. 1 and 2
results in a large thermal mass at the edge of the thin film heater
16
where the thermistor
24
is located. As is known in the art, large thermal masses resist changes in temperature. That is, large thermal masses either maintain temperature for prolonged periods of time or require an inordinately large amount of heat to achieve an increase in temperature.
Given that the edge of the thin film heater
16
is in contact with a large thermal mass and that the thermistor
24
is required to be positioned at this location, it is difficult to accurately monitor and maintain the temperature of thin film heater
16
. In view of this problem, there currently exists a need for a device which can accurately measure the temperature of the center of a thin film heater in a manner that is minimally impacted by the thermal mass proximate to the heater and does not affect, in any way, the viewing area of the heater.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a device which can accurately measure the temperature of a thin film heater which is minimally impacted by a surrounding thermal mass.
It is another object of the invention to provide a device which can accurately measure the temperature of a thin film heater that does not, in any way, impact the viewing are of the thin film heater.
In accordance with the invention, a device and method are disclosed which regulate the temperature of a thin film heating element by using a modeling technique which assumes that the thin film coating of the heater functions as a single electrical resistor.
In accordance with one embodiment of the invention, a circuit for controlling the temperature of a heating device is disclosed where the circuit comprises: a heater containing a thin film coating; and a control circuit for applying a voltage to the heater; wherein the control circuit regulates the temperature of the heater by using a modeling technique which assumes that the thin film coating functions as a single resistor.
In accordance with another aspect of this embodiment of the invention, the control circuit includes: a power supply for applying a voltage to the heater; a resistor that is connected in series with the power supply and the heater; a voltage sensing device which measures voltage drops occurring across the resistor; and, a temperature control circuit which monitors the voltage sensing means and the power supply to regulate the temperature of the heater.
In accordance with another aspect of this embodiment of the invention, the temperature control circuit regulates the temperature of the heater by: (i) calculating the resistance of the thin film coating and,(ii) calculating the temperature of the thin film coating based on the calculated resistance.
In accordance with yet another aspect of this embodiment of the invention, the resistance of the thin film coating is calculated by using the equation R
2
=R
1
(V
SUPP
/V
1
−1).
In accordance with even yet another aspect of this embodiment of the invention, the temperature of the thin film coating is calculated by using the equation T
2
=T
0
+(R
2
−R
0
)/a.
In accordance with another embodiment of the invention, a method for calculating the temperature of a heater containing a thin film coating is disclosed, where the method comprises the steps of: (i) calculating a resistance of the thin film coating, and, (ii) calculating a temperature of the thin film coating based on the resistance calculated in step (i).
In accordance with another aspect of this embodiment of the invention, the resistance of the thin film coating is calculated by using the equation R
2
=R
1
(V
SUPP
/V
1
−1).
In accordance with still another aspect of this embodiment of the invention, the temperature of the thin film coating is calculated by using the equation T
2
=T
0
+(R
2
−R
0
)/a. dr
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide an understanding of the invention and constitute a part of the specification.
FIG. 1
illustrates an exploded view of a conventional module which is used for housing an LCD;
FIG. 2
illustrates a conventional circuit which is used to control and measure the temperature of a thin film heater shown in
FIG. 1
; and,
FIG. 3
is a schematic representation of an illustrative embodiment of the present invention that is used to control and measure the temperature of a thin film heater.
REFERENCES:
patent: 4523084 (1985-06
Hoang Tu Ba
Honeywell International , Inc.
Yeadon Loria B.
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