Constant Current Supply Drive for Electrochromic Displays of the Segmented Type

The present invention relates to a driving circuit for an electrooptical display containing an electrochromic material held in two electrode carrying support plates to manifest reversible variations in the light absorption properties upon current supplied.

An electrochromic material is one in which the color is changed by the application of an electric field or current. See, for example, L. A. Goodman, "Passive Liquid Displays," RCA Report 613258.

The present inventors have discovered that the degree of the coloration of the ECD (electrochromic display) is dependent on the total amount of changes passed through a unit area. That is, the degree of coloration of the ECD increases as the total amount of charge per unit area is increased. Moreover, the present inventors have discovered that the degree of the coloration does not vary even when the temperature varies as long as the total amount of charge passed through a unit area is maintained at a predetermined value.

Generally, in the electro-chemical phenomenon, the electric current flowing through the system is dependent on the temperature when constant potential is supplied. That is, the electric current flowing through the system becomes small as the temperature becomes low. The ECD has a similar characteristic, that is, the response becomes slow as the temperature becomes low. The present invention is based on the above analysis, and is characterized in that a constant current drive is applied to the ECD, thereby eliminating the influence caused by the temperature variations.

Accordingly, an object of the present invention is to provide an improvement in a driving circuit for electrochromic displays which can enhance legibility of a visual display provided by the electrochromic displays.

Another object of the present invention is to provide a constant current supply drive circuit for the electrochromic displays.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

To achieve the above objects, pursuant to an embodiment of the present invention, constant current supply sources are provided for the respective segment electrodes included within the electrochromic display. A common electrode confronting the segment electrodes is maintained at the ground potential.

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawing which are given by way of illustration only, and thus are not limitative of the present invention and wherein,

FIG. 1 is a cross sectional view of a basic structure of a solid state ECD;

FIG. 2 is a cross sectional view of a basis structure of a liquid state ECD;

FIG. 3 is a layout of a typical seven-segment numeral display pattern;

FIG. 4 is a circuit diagram of a typical driver circuit of the constant potential type for ECD;

FIG. 5 is a circuit diagram of an embodiment of a driver circuit of the constant current type of the present invention;

FIG. 6 is a principal circuit diagram of the constant current type driver circuit of the present invention;

FIG. 7 is a circuit diagram of an embodiment of a constant current source employed within the driver circuit of the present invention;

FIG. 8 is a time chart for explaining operation of the constant current source of FIG. 7;

FIGS. 9 through 11 are circuit diagrams of other embodiments of the constant current source employed within the driver circuit of the present invention;

FIG. 12 a circuit diagram of an embodiment of a driver circuit of the present invention; and

FIG. 13 is a time chart for explaining operation of the driver circuit of FIG. 12.

Referring now in detail to the drawings, and to facilitate a more complete understanding of the present invention, basic structures of the ECD and the conventional driver circuit of the constant potential type will be first described with reference to FIGS. 1 through 4.

There are two types of electrochromic displays referred to as ECDs. In one kind, the color variations is produced by the change in the opacity of an inorganic solid film. A typical device structure is shown in FIG. 1, wherein a layer of carbon powder added with binder (registered trademark AQUADAG) is denoted as 1, a stainless plate is denoted as 2. Both the layer 1 and the stainless plate 2 constitute a back electrode. A spacer is denoted as 3; a transparent electrode is denoted as 4; a glass substrate is denoted as 5; an inorganic solid film which manifests the electrochromic phenomenon is denoted as 6; and an electrolyte is denoted as 7. The inorganic film 6 most commonly used for electrocoloration is WO.sub.3 with thickness of about 1 .mu.m. The electrolyte 7 is a mixture of sulfuric acid, an organic alcohol such as glycerol, and a fine white powder such as TiO.sub.2. The alcohol is added to dilute the acid and the pigment is used to provide a white reflective background for the coloration phenomenon. The thickness of the liquid is usually about 1 mm. The back electrode is properly selected for effective operation of the device.

The amorphous WO.sub.3 film is colored blue when the transparent electrode is made negative with respect to the back electrode. The applied voltage is several volts. Th color can remain for days when the voltage is removed. The blue color is diminished or bleached when the polarity of the applied voltage is reversed. This is termed bleaching.

The coloration of the film apparently is produced by the injection of electrons from the transparent electrode and hydrogen ions (protons) from the electrolyte. Bleaching occures because the electrons and protons are returned to their respective starting electrons when the polarity is reversed.

The second type of ECD utilizes an electrically-induced chemical reduction of a colorless liquid to produce a colored, insoluble film on the cathode surface. In the absence of oxygen, the colored film remains unchanged as long as no current flows. However, the coloration will disappear gradually in the presence of oxygen. This is termed fading. Reversing the voltage causes the film to dissolve into the liquid with the concurrent erasure of the color. The colorless liquid that has met with the most success so far is an aqueous solution of the conducting salt, KBr, and an organic material, heptylviologen bromide, which is the material that produces a purplish film upon electrochemical reduction. Typical voltages are about 1.0 VDC.

The basic cell structure is illustrated in FIG. 2. A glass substrate is denoted as 8; a back or counter electrode is denoted as 9; display electrodes are denoted as 10; a viologen mixture liquid is denoted as 11; a spacer is denoted as 12; and a sealing material is denoted as 13. The fluid thickness is normally about 1 mm thick. The viologen-based ECDs can be used in a transmissive mode if both electrodes are transparent or in a reflective mode if a white reflective substrate is mixed in with the clear electrochromic liquid.

Although the operating principle of ECDs has been discussed above, ECDs have the following characteristic features;

(1) the viewing angle is extremely wide

(2) a plurality of colors are selectable

(3) for a single cycle of coloration/bleaching the power dissipation is several through several tens mj/cm.sup.2

(4) memory effects are expected, which maintains the coloration state for several hours through several days after the coloration voltage is removed as long as ECDs are held in an electrically opened state. Of course, the memory effects require no externally supplied power.

By way of example, FIG. 4 illustrates a typical driver circuit of the constant potential type for a seven-segment numeral display utilizing the above constructed ECD of which the font is depicted in FIG. 3. Only three segments S.sub.1, S.sub.2 and S.sub.3 are illustrated in FIG. 4 for convenience sake. The driver circuit of FIG. 4 mainly comprises a power source B, polarity selection switches SW.sub.01 and SW.sub.02, the switches SW.sub.01 and SW.sub.02 being associated with each other, and segment switches SW.sub.1, SW.sub.2 and SW.sub.3.

When only a specific S.sub.1 is to be colored, the selection switches SW.sub.01, and SW.sub.02 are inclined toward the lower terminals, respectively, and only the segment switch SW.sub.1 connected to the segment S.sub.1 is closed. At this moment, the electric current flows from the counter electrode 9 to the segment electrode S.sub.1 through the electrolyte, thereby coloring the segment S.sub.1.

Once the segment S.sub.1 is sufficiently colored, at least one of the selection switches SW.sub.01 and SW.sub.02 is maintained at the intermediate position to terminate the flow of the electric current. The segment S.sub.1 is sustained in the coloration state. Alternatively, the segment S.sub.1 is also placed in the memory condition when the segment switch SW.sub.1 is opened even when the selection switches SW.sub.01 and SW.sub.02 are inclined toward the lower terminals. The coloration tone can be controlled by selectively varying the ON period of the respective segment switches SW.sub.1, SW.sub.2, and SW.sub.3.

Thereafter, when the segment S.sub.1 is to be bleached, the selection switches SW.sub.01 and SW.sub.02 are inclined toward the upper terminals, respectively, and only the segment switch SW.sub.1 connected to the segment S.sub.1 is closed. At this moment, the electric current flows from the segment electrode S.sub.1 to the counter electrode 9 through the electrolyte, thereby bleaching the segment S.sub.1. The degree of the bleaching is also controllable by varying the ON period of the segment switch SW.sub.1.

FIG. 5 shows an embodiment of a driver circuit of the constant current type of the present invention. The circuit of FIG. 5 mainly comprises the counter electrode 9, a segment electrode 14, an amplifier A, a power source V, a resistor R.sub.0, and polarity selection switches SW.sub.03 and SW.sub.04.

The coloration operation is performed when the selection switches SW.sub.03 and SW.sub.04 are inclined toward the lower terminals, respectively. At this moment, a constant current V/R.sub.0 flows through the ECD. After completion of the coloration operation, the selection switch SW.sub.03 is placed in the intermediate position, whereby the ECD is placed in the memory state. The bleaching operation is performed when the selection switches SW.sub.03 and SW.sub.04 are inclined toward the upper terminals, respectively. At this moment, the constant current V/R.sub.0 flows through the ECD in the direction counter to that in the coloration operation. After completion of the bleaching operation, the selection switch SW.sub.03 is placed in the intermediate position to terminate the flow of the electric current.

FIG. 6 shows a typical construction of the driver circuit of the present invention. A plurality of constant current sources 15 are provided in such a manner to correspond to the respective segments S.sub.1, S.sub.2 and S.sub.3. The segments S.sub.1, S.sub.2 and S.sub.3 are connected to the corresponding constant current sources 15 via the segment switches SW.sub.1, SW.sub.2 and SW.sub.3, respectively.

Now consider a particular condition where the segment S.sub.1 is desired to be colored, the segment S.sub.2 is desired to be bleached, and the segment S.sub.3 is desired to be maintained in the same state. The segment switches Sw.sub.1 and SW.sub.2 are closed while the segment switch SW.sub.2 is maintained open. The constant current source 15 connected to the segment switch SW.sub.1 is operated to draw out the constant current from the segment S.sub.1, and the constant current source 15 connected to the segment switch SW.sub.2 is operated to pour the constant current into the segment S.sub.2.

In this way the coloration of the segment S.sub.1 and the bleaching of the segment S.sub.2 are performed at the same time. This enhances legibility of a visual display even when the display pattern is changed from particular one to another. Moreover, this can minimize the deterioration of the counter electrode 9. When the total amount of the electric current to be drawn out from selected segments and the total amount of the electric current to be poured into selected segments are identical to each other, no current flows through the counter electrode 9. The electric current flowing through the counter electrode 9 corresponds to the difference between the total amount of the drawn out current and the total amount of the electric current poured into the segments.

Needless to say, the coloration and the bleaching can be performed at different moments through the use of the driver circuit of FIG. 6. However, in this case the above-mentioned merits are not expected.

FIG. 7 shows a typical construction of the constant current source employed within the driver circuit of FIG. 6. The constant current source of FIG. 7 mainly comprises power source terminals +V.sub.cc and -V.sub.ee, transistors Tr.sub.1 through Tr.sub.4, diodes D.sub.1 and D.sub.2 and a resistor R. The constant current source of FIG. 7 develops a constant current output I.sub.out in response to a control signal S.sub.c applied thereto. Operation of the constant current source of FIG. 7 will be described with reference to a time chart of FIG. 8, wherein M designates a memory period, W designates a coloration period, and E designates a bleaching period.

When the control signal S.sub.c bears a level "0," the transistors Tr.sub.1 and Tr.sub.2 are OFF and, hence, the transistors Tr.sub.3 and Tr.sub.4 are maintained OFF. Accordingly, the segment connected to the constant current source is placed in the memory state. When the control signal S.sub.c bears a positive level "+," the transistor Tr.sub.2 is turned ON and, therefore, the transistor Tr.sub.4 becomes ON via the diode D.sub.2. In the case where the diode D.sub.2 has a similar characteristic as the base junction of the transistor Tr.sub.4, the collector current of the transistor Tr.sub.4 becomes identical to that of the transistor Tr.sub.2. It will be clear from FIG. 7 that the collector current of the transistor Tr.sub.2 is controlled by the level of the control signal S.sub.c and the resistance value of the resistor R. Consequently, when the control signal S.sub.c takes the positive level, the transistor Tr.sub.4 functions to draw out the constant current, whereby the segment is colored.

After completion of the coloration to a desired tone, the control signal S.sub.c is returned to the level "0." The segment is placed in the memory state due to the backward biased collector junction of the transistores Tr.sub.3 and Tr.sub.4. When the control signal Sc bears a negative level "-," the transistors Tr.sub.1 and Tr.sub.3 and the diode D.sub.1 are ON. The transistor Tr.sub.3 functions to pour the constant current into the segment for bleaching purposes, since the collector current of the transistor Tr.sub.1 is controlled to take a predetermined value by the resistor R.

As discussed above, in accordance with the driver circuit of FIG. 7, the constant current controlled coloration, bleaching and sustaining are performed by controlling a potential to be applied to one terminal. The value of the constant current should be determined by taking account of the segment size and the preferred response.

When the ECD is driven by the constant current, a potential difference is created between the counter electrode and the segment electrode in a fashion dependent on the degree of the coloration. A large potential difference created between the counter electrode and the segment electrode will influence the life time of the ECD. Especially when the segment electrode is made of WO.sub.3, a high resistance is created during the bleaching. Therefore, the potential difference is considerably high near the end of the bleaching operation. In order to avoid undesirable influences, the voltages for the terminals +V.sub.cc and -V.sub.ee should be selected below four (4) volts. In this case, the transistors Tr.sub.3 and Tr.sub.4 are placed into the saturation states before the undesirable reaction occurs within the ECD, whereby the ECD is driven by the constant voltage basis rather than the constant current basis.

FIGS. 9 through 11 show other examples of the constant current source. The circuits of FIG. 10 functions in a same manner as that of FIG. 7. The circuit of FIG. 9 utilizes the current-amplification factors of the transistors Tr.sub.1 and Tr.sub.2. In addition, in FIG. 9, since the signal from S.sub.c is connected to the base of transistors Tr.sub.1 and Tr.sub.2, in lieu of the emitters as in FIGS. 10 and 11, when S.sub.c is high, the direction of flow of current I.sub.out in FIG. 9 will be opposite to the direction of flow of I.sub.out in FIGS. 10 and 11. Consequently, when the circuit of FIG. 9 is in a coloration cycle with a high S.sub.c, the circuits of FIGS. 10 and 11 will be experiencing a bleaching cycle for the same high S.sub.c. The circuit of FIG. 10 utilizes the current-amplification factors of the transistors Tr.sub.3 and Tr.sub.4. Therefore, the circuits of FIGS. 9 and 10 can minimize the current flowing through the resistor R. The constant current source of FIG. 11 includes a resistor R.sub.e which functions to increase the bleaching current as compared with the coloration current.

FIG. 12 shows a typical construction of the driver circuit of the present invention. Only one driver circuit connected to the segment S.sub.3 is illustrated in FIG. 12 for convenience sake.

The driver circuit of FIG. 12 mainly comprises an analogue switch 16, a D-type flip-flop 17 and the constant current source shown in FIG. 7. FIG. 13 shows various signals occurring within the driver circuit of FIG. 12. A segment selection signal S.sub.s3 is applied to the D-type flip-flop 17. Segment selection signals S.sub.s1 and S.sub.s2 are associated with the segments S.sub.1 and S.sub.2, respectively. H represents the colored state and L represents the bleached state. A clock signal CL is applied to the D-type flip-flop 17. A timing signal T functions to determine the period of time during which the current flows through the ECD. That is, the coloration current or the bleaching current flows during a time period when the timing signal T bears the level "H."

A signal Ch shows changes of the segment selection signal. The signal Ch bears a high level H during a time period when the clock signal CL continuously takes the low level L upon changing of the segment selection signal. A signal S.sub.EW is a product of the timing signal T and the signal Ch. The signal S.sub.EW takes the high level H during a time period when the coloration current or the bleaching current is forced to flow through the ECD. The signal S.sub.EW functions to turn on the analogue switch 16.

The analogue switch 16 functions to develop the control signal S.sub.c in response to the segement selection signal S.sub.s3 and the signal S.sub.EW. The control signal S.sub.c derived from the analogue switch 16 takes the high level H when the segment selection signal S.sub.s3 is changed to the high level H. The time period of the control signal S.sub.c is controlled by the signal S.sub.EW which is applied to the gate electrode of the analogue switch 16. The control signal S.sub.c takes the low level L when the segment selection signal S.sub.s3 is changed to the low level L. The control signal S.sub.c takes the level "0" when the signal S.sub.EW bears the low level L, whereby the segment is placed in the memory state since the output of the constant current source becomes the high impedance.

The trailing edge of the clock signal CL appears slightly before the leading edges of the remaining signals.

The invention being thus described, it will be obvious that the same way be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.