Controlling unit for A.C. driving an electrostatic device

A controlling unit for a.c. driving an electrostatic device such as an electrostatic display device having a fixed electrode, a resilient sheet electrode and a dielectric layer interposed between the electrodes wherein the sheet electrode is drawn to the outer surface of the fixed electrode to cover the same upon the application of a voltage between the fixed and the sheet electrodes whereas the sheet electrode is kept apart from the fixed electrode upon the removal of the voltage between the electrodes. The controlling unit of this invention includes a resistance in series with an a.c. voltage supply and the electrostatic display device, a triode thyristor switch with a first terminal and a second terminal connected across the fixed and the sheet electrodes, and a switching means for controlling the gate of the thyristor so that the thyristor has an on-off switching function at its leakage current region.

This invention relates to a controlling unit for an electrostatically 
operated device such as an electrostatic display device which operates 
essentially according to the absolute value of the voltage difference 
between the electrodes of a capacitor. 
The controlling unit of the invention is particularly suitable for use in 
controlling the operation of an electrostatic display device which, as is 
shown in FIGS. 1A and 1B, comprises a fixed electrode 1 fixed at one end 
thereof to a base (not shown) and having a cylindrical or flat outer 
surface, a resilient sheet electrode 2 fixed at one end thereof to the 
base adjacent to the fixed electrode 1, and a dielectric layer 3 
interposed between the fixed and the sheet electrodes. The sheet electrode 
2 is made of, for example, a thin polymer film typically a polyethylene 
terephthalate film 8 microns in thickness as a core with an electrically 
conductive metal such as aluminum vacuum deposited thereon. 
Hence, when no voltage is applied between the fixed and the sheet 
electrodes, the sheet electrode 2 is kept away from the outer surface of 
the fixed electrode 1 as illustrated in FIGS. 1A and 1B by solid lines due 
to the resilience of the sheet electrode 2. On the other hand, when a 
voltage is applied between the electrodes 1 and 2, the sheet electrode 2 
is drawn to the outer surface of the fixed electrode 1 to cover this outer 
surface due to the electrostatic force generated between the electrodes 1 
and 2, as shown in FIGS. 1A and 1B by the dashed lines. Therefore, when 
the appearances of the outer surfaces of the fixed and the sheet 
electrodes are different from each other in their reflectivity, color, 
pattern, or the letters and the like they carry, a wide variety of 
displays can be realized through the application of a voltage between the 
fixed and the sheet electrodes to cause the appearance of the device to 
change. 
Conventionally, a d.c. driving method has been extensively used for driving 
such an electrostatic display device as described above since on-off 
control of the device can be easily performed by d.c. driving. An example 
of a d.c. driving method is shown in FIG. 2, in which the reference 
numeral 1 designates the fixed electrode, 2 the sheet electrode, 3 a d.c. 
power source, 4 a high resistance of 5 megaohms for example, 5 a switching 
transistor, 6 a biasing power source for the transistor 5, and 7 an on-off 
switch, respectively. An a.c. driving method, however, has been proposed. 
The method has disadvantages in that the sheet electrode will possibly 
vibrate when the a.c. power has a low frequency or when the response of 
the sheet electrode is rapid, and the method needs a driving power source 
because of the use of a bidirectional triode thyristor, typically a TRIAC, 
results in complicated circuits. 
Another difficulty encountered with a.c. driving the electrostatic display 
device resides in the switching means even when the a.c. voltage used as 
the power source has a sufficiently high frequency so as not to cause 
vibration of the sheet electrode. For example, when the fixed electrode 1 
and the sheet electrode 2 are short-circuited through a transistor switch, 
a complicated controlling circuit is required which includes, as is shown 
in FIG. 3, an n-p-n transistor 9 for short-circuiting the electrodes 1 and 
2 during each half cycle when the fixed electrode 1 is positive with 
respect to the sheet electrode 2, a p-n-p transistor 10 for 
short-circuiting the electrodes during each half cycle when the sheet 
electrode is positive with respect to the fixed electrode, and a 
transistor 11 forming an inverter circuit for energizing the transistor 10 
from biasing power source 6. 
It is also to be noted that a bidirectional triode such as a TRIAC has been 
used as a bidirectional switching element, making use of the switching 
characteristics in its negative resistance region. However, only a 
relatively large current can be switched an extremely small current such 
as 1 mA or less, as is the case with the invention, cannot be switched by 
a bidirectional triode thyristor. 
An object of the invention is, therefore, to provide a controlling unit for 
a.c. driving an electrostatically operated device, simple in construction 
and inexpensive in manufacture. More particularly, the invention is to 
provide a controlling unit suitable for a.c. driving an electrostatic 
display device which includes capacitance as essential operating feature.

The present invention makes use of notable characteristics in the leakage 
current region of a triode thyristor. 
The voltage-current characteristic curve of an SCR, one of the triode 
thyristors used in the invention, is shown in FIG. 4. A negative 
resistance region C is in the first quadrant between the off-state A and 
the on-state B, and a reverse blocking state D and a reverse voltage 
breakdown E are in the third quadrant. Such an SCR has been used, 
therefore, as a unidirectional switching element between the off-state A 
and the on-state B. 
However, an examination of the voltage-current characteristics of the SCR 
at a very small current reveals that the SCR has a clear bidirectionality 
in the leakage current region, and that no negative resistance exists in 
that region, thereby permitting the anode-cathode conductance both in the 
forward and reverse directions through the application of a gate current. 
FIG. 5 shows a detailed dynamic characteristic curve of the SCR of FIG. 4 
in the leakage current region, in which the curve a-o-a' is the 
voltage-current characteristic curve for a small gate current I.sub.G, for 
example, 0.4 mA, and the curve b-o-b' is the voltage-current 
characteristic curve for an I.sub.G of zero. Now, assume that the a.c. 
power voltage has maximum positive and negative voltages of X and X', 
respectively, and the load resistance is fixed, and then load lines X-Y 
and X'-Y' are established. Therefore, when I.sub.G is zero, the 
anode-cathode voltage V.sub.AC is an a.c. voltage with an amplitude of 
Q.sub.2 Q.sub.2 ', and when I.sub.G is 0.4 mA, V.sub.AC is an a.c. voltage 
with an amplitude of Q.sub.1 Q.sub.1 '. According to actual experiments, 
the point P.sub.1 the intersection between the load line XY and the 
voltage-current characteristic curve for an I.sub.G of 0.4 mA a-o-a' has a 
voltage smaller than 0.1 volts and a current flow smaller than 0.2 mA. 
That is, Q.sub.2 Q.sub.2 ' is substantialy comparable with the power 
voltage XX' and Q.sub.1 Q.sub.1 ' is substantially zero. 
In the embodiment of the electrostatic display device previously mentioned, 
the capacitance between the fixed electrode and the sheet electrode is 
about 200 pF when the outer surface of the fixed electrode is covered by 
the sheet electrode. When an a.c. voltage of 180 volts (RMS) is applied 
between the electrodes, the current flow therethrough is smaller than 0.1 
mA. Such a small current flow is apparently regarded as leakage current 
flows. Now, the leakage current in this specification includes the 
off-state current flow in the first quadrant of the voltage-current 
characteristic diagram of the SCR as well as the reverse current flow in 
the third quadrant, and usually means a very small current flow of less 
than or equal to a few hundredths of the rated current. 
FIG. 6 shows an embodiment of the a.c. driving unit of the invention 
coupled to the electrostatic display device. The a.c. driving unit 
comprises a high resistance of, for example, about 2 megaohms in series 
with an a.c. voltage supply 8 and the electrostatic display device 
including the fixed electrode 1 and the sheet electrode 2, and an SCR 
designated by the reference numeral 12 having an anode T.sub.A and a 
cathode T.sub.C connected to the fixed electrode and the sheet electrode, 
respectively, and having a gate G connected to a pulse signal generator 
shown equivalently as a d.c. supply 6 coupled with a switch 7. 
In operation, when an a.c. voltage is applied between the fixed and the 
sheet electrodes from the a.c. power supply 8, as shown in FIG. 7, closing 
the switch 7 changes the gate-cathode voltage V.sub.GC from 0 to +E, 
causing the cathode-anode current I.sub.GC to change from 0 to +I. This in 
turn causes the cathode and the anode to be substantially short-circuited 
as explained hereinbefore with reference to FIG. 4, therefore the voltage 
between the fixed electrode and the sheet electrode is removed so that the 
sheet electrode is restored to its original position. 
As mentioned above, the invention makes the best use of the characteristics 
in the leakage current region of a thyristor, which have not been noted 
even the manufactures of thyristors, and naturally have not been 
previously used for any purpose. However, since there exists a 
substantially symmetric voltage-current characteristic curve with no 
negative resistance over the first quadrant and the third quadrant in the 
leakage current region of a thyristor, as mentioned before, the operating 
points can be controlled through the gate current only and a very simple 
circuit construction for a.c. driving an electrostatic device can be 
realized. The a.c. driving of the invention naturally permits the use of a 
commercial a.c. supply, which therefore makes the supply device simple. 
The a.c. driving of the invention has a further advantage over the 
conventional d.c. driving. In the conventional d.c. driving the 
electrostatic display device, residual electric charges on the electrodes 
are often apt to cause the unstable flapping movement of the sheet 
electrode after the electrodes are short-circuited. But according to the 
a.c. driving of the invention, no electric remains on the electrodes when 
short-circuited through a thyristor, ensuring the stable movement of the 
sheet electrode.