Note: Descriptions are shown in the official language in which they were submitted.
s
BACK~ OUNI) OF _HE INVENTION
Field of the Invention
- The present invention relates generally to a
cathode ray tube apparatus, and is directed more particularly
to a cathode ray tube apparatus in which when a discharge is
caused in a cathode ray tube, the discharging current is
suppressed so as to induce a voltage by the discharye only
in the tube thereby to reduce undesirable influences which
are exerted on the cathode and heater of the cathode ray
tube and the electrodes of an electron gun and so on as
much as possible and hence to reduce undesirable influences
on the circuit elements forming the cathode ray tube apparatus.
Description of the Prior Art
In the prior art cathode ray ~ube apparatus,
discharge is frequently generated in the tube or envelope.
For example, in a color cathode ray tube apparatus of the
Trinitron (registered Trade Mark) system, as shown in Fig. 1,
for a plurality of cathodes Kr, Kg and Kb in an envelope 1,
arranged are common first to fifth grids Gl to G5 on the
same axis, and a convergence device CD, which is formed of
four electrodes and disposed at the front end of fifth grid
G5 to form an electron gun 2. In this case, an anode voltage
is applied to the fifth and third grids G5 and G3, which are
electrically connected through a high voltage lead wire 6,
from the outside of the envelope 1 through, for example, an
anode button (not shown) t inner carbon layer 3 and conductive
contact spring 4, while a predetermined low voltage is
applied to the first, second and fourth grids ~,1, 52 and G4
through a stem pin 5. Though not shown, the inner deflection
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plate of convergence device CD is applied with the anode
voltage through the contact spring 4 and the outer deflection
plate of convergence device CD is applied with a middle high
voltage, which is somewhat lower than the anode voltage,
from the coaxial anode button through a high voltage supply
pipe (not shown).
With the above prior art color cathode ray tube
apparatus, there may occur a case where a discharge is
generated between the high voltage electrodes or fifth and
third grids G5, G3 and the lower voltage electrodes or
fourth and second grids G4, G2, in detail between the fifth
grid G5 and the fourth grid G4 or between third grid G3 and
fourth grid G4, or between the third grid G3 and second grid
G2, respectively. The above discharge is the discharge of
the electrical charges stored in a capacitor formed between
the inner carbon layer 3 and the outer carbon layer 7. In
this case, a current i flows into the outer carbon 7 through
the path indicated by the arrow in Fig. lo
Fig. 2 is an equivalent circuit of the above
discharging current path. In Fig. 2, 8 designates the
discharge gap, R a total resistance, C the above defined
capacitance and L a total inductance, respectively.
When the discharge occurs, it can be ascertained
by measurements of a current peak of several hundred amperes
which is reached in 100 to 200 nanoseconds and the time rate
of current change reaches about 10000 A/~s. It is obvious
that such large currents seriously affect the cathode ray
tube and the circuit elements disposed near the cathode ray
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tube.
In order to reduce damage caused by the above
discussed discharge, it is sufficient to introduce a high
impedance within the discharge path. To this end, in the
art such a system has been known, in which resistive particles
are mixed into the inner carbon layer 3 to give a predeter-
mined resistance value thereto to reduce the discharge
current. This is known as a so-called resistive carbon
coating method. In another method a resistive layer is
coated on the contact spring 4, contacting with the inner
carbon laver 3 and applying the high voltage to the electron
gun 2, to reduce the discharge current, which is known as the
resistive contact-spring system.
In any of the prior art systems, as will be clear
from the above description, the resistive material serving
to reduce the discharge current is provided within the
cathode ray tube for the following reasons. That is, in
order to remove the undesirable influences by the discharge
on the respective parts of the cathode ray tube and the
circuit elements thereabout, the following three points are
important.
(1) The discharge current is suppressed.
(2) The rising-up of the discharge current is made to
be low, i.e. the differentiated value of the discharge current
with respect to time ddt is made small, and
(3) The induction of high voltage by the discharge
is generated within the cathode rav tube.
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It is not necessary to explain the above point (1).
As to point (2), since a ground lead wire 9, which is connected
between the side of stem pin 5 and the outer carbon layer 7,
has a small coil or inductance component to induce a high
voltage at the side of stem pin 5 in accordance with the
equation E = L x di upon discharging and the magnetic material
near it is magnetized by the rising-up factor ~ or voltage is
induced in the deflection yoke to damage the circuit elements,
the rising-up factor -t must be made small. In order to
satisfy the points (1) and (2), it may be desirable to
introduce an impedance to the outside of the cathode ray tube,
for example, the ground lead wire 9. However, if the impedance
is provided at the outside of the cathode ray tube as set
forth above, upon discharging a high voltage is induced at
the side of the stem pin 5 to undesirably affect the circuit
near the same. For this reason, when the impedance for
preventing the discharge current is provided, it is selected
to be only within the cathode ray tube in the prior art.
However, in the prior art resistive carbon coating
system or resistive contact-spring system, are accompanied
with the following defects.
First, the resistive carbon coating system will
be described. In the resistive carbon coating system, as
shown in Fig. 3, the resistance value of the inner carbon
layer 3 in contact with the conductive contact spring 4 is
selected to be high. In this case, however, the contact
between the contact spring 4 and the carbon layer 3 results
in the peeling-off of carbon layer 3 to increase the
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possibility of discharge. Even if the discharge is caused
frequentIy, the discharge is feeble (soft). Therefore,
undesirable influences on the circuit near it will be caused,
but the picture flickers frequently during discharying which
is uncomfortable for a viewer. Further, due to the fact that
the contact of the spring 4 with the carbon layer 3 is a
point contact, the deterioration of the carbon layer 3
proposes a problem. That is, during a discharging or knocking
treatment, a current will flow through a very small portion,
10 which may deteriorate the carbon layer 3 due to the heat
generation and hence lower the reliability. In addition,
when a gas absorbing agent called a getter which is highly
conductive adheres to the carbon layer 3, the effect of
carbon layer 3 and its resistance disappears. Therefore, it
15 is necessary to shield an area A of the carbon layer 3, which
may make contact with the contact spring 4, so as to prevent
the adhesion of at least the getter to the carbon layer 3. In
fact, however, it is difficult to positively shield the area
A of the carbon layer 3. In this case, a device to shield
20 the area A is required, which results in the construction of
the cathode ray tube becoming complicated.
To avoid a problem, such a method may be considered
where a non-doped getter is used. This non-doped getter
has a property such that the scattering of barium Ba by
25 nitrogen gas N2 is not caused but barium is deposited
directly, so that the shield can be easily constructed.
According to the experiments, however, it was ascertained
that the non-doped getter deteriorated the uniformity of the
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color on the picture screen and the area deposited with
barium becomes small due to the non-porosity of the deposited
barium layer, which resulted in reduction of the gas absorp-
tion effect and consequently in a short life span of the
cathode ray tube. Further, even if the shield can be provided,
the shield effect fluctuates, which results in the scattering
of the discharge current and accordingly positive shielding
effect is not assured. Also, the getter is generally attached
to the electron gun 2 with a metal plate. In the resistive
carbon coating system, however, if the electron gun and the
getter are not isolated electrically, the discharge current
will flow through the metal plate. To avoid this, a technique
is necessary where the getter is attached to a color selecting
mask, anode button or the like, which results in difficulties
during the manufacturing process.
Further, the maintenance or control of the static
resistance and the dynamic resistance is difficult. The
static resistance means the quotient of several volts
difference set between the carbon layers by the current
flowing to the carbon layers by the voltage difference, and
the dynamic resistance means the quotient of the high anode
voltage divided by the peak value of the discharge current.
Upon comparing the static resistance with the dynamic
resistance, the dynamic resistance has a tendency to decrease.
This difference is determined mainly by the surface condition
and the inner construction of the carbon layer 3, because the
discharge along the surface of the carbon layer 3 (surface
discharge) is determined by its surface condition and the
impedance ~the resistance to AC) is determined by the inner
construction of the carbon layer 3. The maintenance or control
thereof becomes important to maintain the results certain.
The control of the grain size of the carbon powders or the
5 coating method thereof may be considered, but the resistive
carbon coating system lacks the ability to prevent fluctuations
of the shield effect by the getter.
Secondly, the resistive contact spring system will
be now described. According to this system as shown in Fig. 4,
10 a resistive layer 10 is coated on the surface of the contact
spring 4, which contacts with inner carbon layer 3, to reduce
the discharge. This system, however, is not practical. That
is, even if the resistive layer 10 is coated on the surface
of the spring 4 or disposed between the spring 4 and carbon
15 layer 3, the thickness of the resistive layer 10 is small so
that discharge along the surface will be caused. In other
words, the voltage across the resistive layer 10 becomes a
high voltage due to the discharge which will cause the
secondary discharge along the surface due to the short length,
20 so that the discharge can not be suppressed. Further, it is
necessary to shield the resistive layer 10 so as to prevent
it from being coated by the back flash of the conductive
getter material. In addition to the surface condition and
the inner construction of the resistive layer 10, there exists
25 a problem due to the capacitance since, due to the short
distance between the inner carbon layer 3 and the contact
spring 4, a capacitance Cl exists therehetween. The equivalent
circuit thereof can be as shown in Fig. 5. Thus, the high
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frequency current such as the discharge current flows through
the capaGitance Cl. As a result, the impedance (AC resistance)
thereof becomes low, and accordingly the effect by the pro-
vision of the resistive layer 10 is limited even if the static
resistance is selected to be relatively high.
OBJECT AMD SUr~1ARY OF THE INVENTION
Accordingly, an object of the present invention is
to provide a novel cathode ray tube apparatus free of the
defects encountered in the prior art.
Another object of the invention îs to provide a
cathode ray tube which is simple in construction but can
positively prevent the generation of discharge current
without damaging the heater, cathodes and so on within the
cathode ray tube.
A further object of the invention is to provide a
cathode ray tube in which a discharge current suppressing
means such as a resistor or coil is inserted into the
discharge current flowing path between the final electrode
of an electron gun supplied with high voltage and a stem
pin for supplying low voltage to suppress the discharge
current and hence to avoid undesirable affects on the
cathode ray tube and circuit elements thereof.
According to an aspect of the present invention,
a cathode ray tube apparatus is provided, which comprises a
cathode r~y tube having an electron gun wherein a discharge
current suppressing means is inserted into the discharge
current flow path between the final electrode of said gun
to which an anode voltage is supplied and a stem pin to
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which a low voltage is supplied.
- The other objects, features and advantages of the
present invention will become apparent from the following
description taken in conjunction with the accompanying
drawings through which the like references designate the same
elements and parts.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view showing a prior art
cathode ray tube;
Fig. 2 is an equivalent circuit diagram of the
discharge path of the cathode ray tube shown in Fig. l;
Figs. 3 and 4 are cross-sectional views
respectively showing essential parts of discharge current
suppressing devices employed in the prior art cathode ray
tube;
Fig. 5 is an equivalent circuit diagram used to
explain the device of Fig. 4;
Fig. 6 is a side view showing the essential part
of a cathode ray tube apparatus according to the present
invention;
Fig. 7 is a side view showing the essential part
of another example of the invention and
Fig. 8 is a cross-sectional view showing an
example of a resistor useable inthe invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be hereinafter
described with reference to the attached drawings by way
of example.
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Fig. 6 shows an example of the invention, which
is applied to a color cathode ray tube apparatus of the
Trinitron (registered Trade Mark) system by way of example.
In this case, Fig. 6 shows only an electron gun consisting
of first to fifth grids Gl to G5 and convergence device CD
of the color cathode ray tube apparatus. In this example,
the discharge will take place between the high voltage
electrodes such as the fifth and third grids G5, G3 and the
low voltage electrodes such as the fourth and second grids
G4, G2 as described above. Upon the discharge between the
grids G5 and G4, the discharge current flows through a lead
member or wire 11 connected from the fourth grid G4 to the
stem pin 5 (refer to Fig. l); upon the discharge between the
grids G3 and G4, the discharge current flows through the high
voltage lead member or wire 6 connecting the fifth grid G5
to the third grid G3 and through the lead wire 11 of fourth
grid G4; and upon the discharge between the third and second
grids G3 and G2, the discharge current flows through the
high voltage lead wire 6, respectively.
In the example of the invention shown in Fig. 6,
resistors 12, each serving as a discharge current suppressing
means are respectively inserted into the high voltage lead
wire 6, which connects the third grid G3 with the fifth grid
G5 of the electron gun and into the lead wire 11 connected
from the fourth grid G4 of the electron gun. The resistor
12 must satisfy such a condition that it is not damaged upon
discharge and is not changed especially during the knocking
process carried out at the final step of manufacturing the
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cathode ray tube apparatus in which process, a voltage about
twice as-high as the operating voltage is applied so as to
increase the voltage at which discharge begins and also to
remove burrs and so on between adjacent grids by discharge.
The resistive material used in the cathode ray tube is
limited due to its effect on the cathode and so on. As the
specific resistor 12, which will satisfy the above conditions,
it was noted that a resistor, whose resistance variation is
within 50~ for the absolute resistance value of 3 K~, can be
practically used without any problem. In practical use, two
kinds of materials can be considered for making the resistor
12. One is a ceramic resistor which is made such that
conductive metal oxide, silicon, carbon and so on are mixed
with material, which is necessary for making ceramic, and
the mixture is sintered at a high temperature. The other
method is a resistor manufactured such that carbon, which is
used in the cathode ray tube as the inner conductive layer,
iron oxide, silicon carbide SiC or the like is formed into a
thin layer resistor with the form of a lead wire shape.
As a practical example of the latter resistor,
a resistor shown in Fig. 8 may be used. This resistor
consists of a hollow ceramic cylinder 13, a thin resistive
layer 14 made of the above carbon or the like and coated on
the inner surface of ceramic cylinder 13, contact terminals
15 are inserted into the pipe 13 for contacting the
resistive layer 14 at both ends thereof, and lead wires
16 respectively led out from terminals 15.
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Since the above specific resistor is made of ceramic,
carbon, silicon, metal oxide and so on, which are generally
used in the cathode ray tube, this resistor has less effect on
the cathode. It was ascertained that such resistor was not
changed even at temperatures of 400~C to 450C used for the
knocking process and were necessary to manufacture the cathode
ray tube. It was further noted that if the length of the
resistor 12 was not selected more than a certain value, there
was caused a secondary discharge along the surface thereof.
Since the lead wire 6, which connects the grid G3 to grid G5,
and the lead wire 11 of grid G4 both have sufficient length,
the above resistor 12 can be easily inserted into each of the
lead wires 6 and 11 with good results.
By the way, it was ascertained that, in an ordinary
cathode ray tube without the present invention, a current of
400 A (peak value) flowed and its rising-up time arrived at
10000 A/~s, while in the cathode ray tube of the invention
with the resistors 12 of 3 kQ respectively inserted into the
lead wires 6 and 11, currents passing therethrough were both
less than lOA and their rising-up was 25 A/~s, which is
remarkably smaller than that of the prior art.
The above description is given for the case where
the present invention is applied to the cathode ray tube which
employs the uni-potential type electron gun. However, the
present invention or resistor 12 can be applied to a bi-
potential type electron gun with the same effect and also
to an electric field lens type electron gun with substantially
the same effect.
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In case of the bi-potential type electron gun,
as shown in Fig. 7, the resistor 12 can be inserted into a
lead member or wire 17 led out from the third grid G3.
The present invention can be applied not only to
the cathode ray tube of a television receiver but also to the
cathode ray tube of an oscilloscope with the same effect.
As the discharge current suppressing means, in
place of the above specific resistor, a coil may be used
with the same effect.
According to the present invention described above,
into the discharge current flowing path between the final
electrode of the electron gun of the cathode ray tube to which
electrode the high voltage is applied and the stem pin which
applies the low voltage to the cathode ray tube i.e. the lead
wire between the electrodes or between the electrode and the
stem pin, there is inserted as the discharge current
suppressing means a resistor 12. Thus, the current during
the discharge is suppressed, and hence the undesirable
influence on the external circuit elements such as the
transistors of a horizontal output circuit and video output
circuit and on the heater and cathode of the cathode ray
tube can be positively avoided. In this case, since the
length of resistor 12 can be selected to be long due to its
insertion position, no surface discharge is caused.
Further, since the resistor 12 is not affected by
the getter back flash, there is no fear that barium Ba will
adhere to the resistor 12 and hence the resistance value
thereof will not fluctuate.
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Also, when the ceramic resistor is employed as the
resistor 12, it is a so-called bulk resistor. Thus, its
frequency characteristic is good (the resistance value thereof
does not vary with frequency), the static resistance thereof
is 2 KQ which is sufficient, and the difference between the
static and dynamic resistances is very small. Therefore,
the discharge current is determined by only the static
resistance, so that the control thereof is positive and
superior.
The ceramic resistor is not changed in its resistance
value during the manufacturing process of the cathode ray tube,
especially during the heat and knocking processes and the
material of the ceramic resistor is the same as that used in
the cathode ray tube, so that it does not affect the cathode.
Further, according to the invention, the discharge
current suppressing means is merely inserted into the lead wire,
so that no change is required in the manufacturing process of
the cathode ray tube, and accordingly the cathode ray tube can
be easily manufactured.
It will be apparent that many modifications and
variations could be effected by one skilled in the art without
departing from the spirits or scope of the novel concepts of
the present invention, so that the spirits or scope of the
invention should be determined by the appended claims only.
