U.S. patent application number 09/775633 was filed with the patent office on 2002-04-25 for crt display apparatus.
Invention is credited to Heishi, Akinori, Yasui, Hironobu.
Application Number | 20020047671 09/775633 |
Document ID | / |
Family ID | 18760124 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020047671 |
Kind Code |
A1 |
Heishi, Akinori ; et
al. |
April 25, 2002 |
CRT display apparatus
Abstract
A CRT display apparatus is disclosed. The CRT display apparatus
includes a CRT having an electron gun whose electrodes for
controlling an electron beam are applied with voltages from their
respective voltage sources specifically designed to supply an
electrode current, and is capable of automatically ceasing a
discharge that has occurred under fault conditions between any
electrode within the electron gun and the cathode or an anode of
the CRT.
Inventors: |
Heishi, Akinori; (Tokyo,
JP) ; Yasui, Hironobu; (Tokyo, JP) |
Correspondence
Address: |
BIRCH, STEWAR, KOLASCH & BIRCH, LLP
P.O. BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
18760124 |
Appl. No.: |
09/775633 |
Filed: |
February 5, 2001 |
Current U.S.
Class: |
315/387 ;
348/E3.035; 348/E5.134 |
Current CPC
Class: |
H04N 5/68 20130101; H04N
3/185 20130101 |
Class at
Publication: |
315/387 |
International
Class: |
G09G 001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 11, 2000 |
JP |
274354/00 |
Claims
1. A CRT display apparatus comprising: a CRT having an electron
gun; said electron gun including: a cathode; a G1 electrode, a G2
electrode, and a G3 electrode disposed in that order for drawing
electrons from said cathode; and a modulating Gm electrode disposed
between said G2 electrode and said G3 electrode, a transformer
producing a high tension to be applied to an anode of said CRT at a
secondary coil thereof; a voltage-dividing resistor dividing said
high-tension to produce a voltage to be applied to said G3
electrode; a voltage generator generating a voltage having a value
depending on a current flowing into said dividing resistor as a
feedback voltage; a high-tension control circuit for controlling,
on the basis of said feedback voltage, a voltage at a primary coil
of said transformer so as to keep said high-tension at a
predetermined value; a G2 electrode voltage source for outputting a
predetermined voltage to be applied to said G2 electrode; a Gm
electrode voltage source for outputting a predetermined voltage to
be applied to said Gm electrode; and a comparator for comparing
said feedback voltage with one of an output voltage of said Gm
electrode voltage source and an output voltage of said G2 electrode
voltage source, said comparator supplying a detection signal to
said high-tension control circuit upon detecting that only said
feedback voltage has fallen from a normal value, said high-tension
control circuit controlling said voltage at said primary coil of
said transformer so as to lower said high-tension in response to
said detection signal.
2. A CRT display apparatus according to claim 1, in which said
comparator is provided with a timer circuit to enable said
high-tension control circuit to lower said high-tension over a
predetermined period of time preset in said timer circuit, and
thereafter return said high-tension to a normal value.
3. A CRT display apparatus comprising: a CRT having an electron
gun; said electron gun including: a cathode; and a G1 electrode, a
G2 electrode, and a G3 electrode disposed in that order for drawing
electrons from said cathode, a transformer producing a high tension
to be applied to an anode of said CRT at a secondary coil thereof;
a voltage-dividing resistor dividing said high-tension to produce a
voltage to be applied to said G3 electrode; a voltage generator
generating a voltage having a value depending on a current flowing
into said dividing resistor as a feedback voltage; a high-tension
control circuit for controlling, on the basis of said feedback
voltage, a voltage at a primary coil of said transformer so as to
keep said high-tension at a predetermined value; a G2 electrode
voltage source for outputting a predetermined voltage to be applied
to said G2 electrode; and a comparator for comparing said feedback
voltage with an output voltage of said G2 electrode voltage source,
said comparator supplying a detection signal to said high-tension
control circuit upon detecting that only said feedback voltage has
fallen from a normal value, said high-tension control circuit
controlling said voltage at said primary coil of said transformer
so as to lower said high-tension in response to said detection
signal.
4. A CRT display apparatus according to claim 3, in which said
comparator is provided with a timer circuit to enable said
high-tension control circuit to lower said high-tension over a
predetermined period of time preset in said timer circuit, and
thereafter return said high-tension to a normal value.
5. A CRT display apparatus comprising: a CRT having an electron
gun; said electron gun including: a cathode; a G1 electrode, a G2
electrode, and a G3 electrode disposed in that order for drawing
electrons from said cathode; and a modulating Gm electrode disposed
between said G2 electrode and said G3 electrode, a transformer
producing a high tension to be applied to an anode of said CRT at a
secondary coil thereof; a voltage-dividing resistor dividing said
high-tension to produce a voltage to be applied to said G3
electrode; a first voltage generator generating a voltage having a
value depending on a current flowing into said dividing resistor as
a feedback voltage; a high-tension control circuit for controlling,
on the basis of said feedback voltage, a voltage at a primary coil
of said transformer so as to keep said high-tension at a
predetermined value; a G2 electrode voltage source for outputting a
predetermined voltage to be applied to said G2 electrode; a Gm
electrode voltage source for outputting a predetermined voltage to
be applied to said Gm electrode; a second voltage generator for
outputting a voltage of a value equal to a value of said feedback
voltage when said CRT display apparatus is operating normally; and
a comparator for comparing said feedback voltage with an output
voltage of said second voltage generator, said comparator supplying
a detection signal to said high-tension control circuit upon
detecting that only said feedback voltage has fallen from a normal
value, said high-tension control circuit controlling said voltage
at said primary coil of said transformer so as to lower said
high-tension in response to said detection signal.
6. A CRT display apparatus according to claim 5, in which said
comparator is provided with a timer circuit to enable said
high-tension control circuit to lower said high-tension over a
predetermined period of time preset in said timer circuit, and
thereafter return said high-tension to a normal value.
7. A CRT display apparatus comprising: a CRT having an electron
gun, said electron gun including: a cathode; a G1 electrode, a G2
electrode, and a G3 electrode disposed in that order for drawing
electrons from said cathode; and a modulating Gm electrode disposed
between said G2 electrode and said G3 electrode, a transformer
producing a high tension to be applied to an anode of said CRT at a
secondary coil thereof; a first voltage-dividing resistor dividing
said high-tension to produce a voltage to be applied to said G3
electrode; a second voltage-dividing resistor connected in series
to said first voltage-dividing resistor to produce a screen bias
voltage used to adjust a black level of a screen of said CRT; a
voltage generator generating a voltage having a value depending on
a current flowing into said first and second voltage-dividing
resistors as a feedback voltage; a high-tension control circuit for
controlling, on the basis of said feedback voltage, a voltage at
said primary coil of said transformer so as to keep said
high-tension at a predetermined value; a G2 electrode voltage
source for outputting a predetermined voltage to be applied to said
G2 electrode; a Gm electrode voltage source for outputting a
predetermined voltage to be applied to said Gm electrode; and a
comparator for comparing said screen bias voltage with one of an
output voltage of said Gm electrode voltage source and an output
voltage of said G2 electrode voltage source, said comparator
supplying a detection signal to said high-tension control circuit
upon detecting that only said screen bias voltage has fallen from a
normal value, said high-tension control circuit controlling said
voltage at said primary coil of said transformer so as to lower
said high-tension in response to said detection signal.
8. A CRT display apparatus according to claim 7, in which said
comparator is provided with a timer circuit to enable said
high-tension control circuit to lower said high-tension over a
predetermined period of time preset in said timer circuit, and
thereafter return said high-tension to a normal value.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a CRT display apparatus
including a CRT, to be more specific, relates to a CRT display
apparatus including a CRT having an electron gun whose electrodes
for controlling an electron beam are applied with voltages from
their respective voltage sources specifically designed to supply an
electrode current.
BACKGROUND OF THE INVENTION
[0002] FIG. 8 shows the structure of a display apparatus disclosed
in Japanese Unexamined Patent Publication No. 11-224618 as an
example of a CRT display apparatus including a CRT having such an
electron gun. This apparatus features a CRT of high intensity and
high resolution called "Hi-Gm tube" that has an electron gun within
which an additional electrode called "Gm electrode" is disposed
between a G2 electrode and a G3 electrode for modulating an
electron beam.
[0003] In the figure, there is shown a CRT 1, an anode 2, a flyback
transformer 3, and a secondary coil 4 disposed within the flyback
transformer 3. The secondary coil 4 is connected to the anode 2 of
the CRT 1 through a high-tension line 5. Reference numeral 6
denotes a high-impedance resistor disposed within the flyback
transformer 3, one end of which is connected to the secondary coil
4 and the other end of which is connected to one end of a variable
resistor 7 for a G3 electrode 11. The other end of the variable
resistor 7 is connected to one end of a high-tension detecting
resistor 10 disposed outside the flyback transformer 3, and a
slidable terminal of the variable resistor 7 is connected to the G3
electrode 11. The node of the variable resistor 7 and the
high-tension detecting resistor 10 makes a high-tension level
feedback point 9. A voltage at the high-tension level feedback
point 9 is supplied to a high-tension control circuit 22. Reference
numeral 12 denotes a Gm electrode, and this Gm electrode 12 is
applied with a voltage from a Gm electrode voltage source 17.
Reference numeral 13 denotes a G2 electrode, and this G2 electrode
13 is applied with a voltage from a G2 electrode voltage source 16.
Reference numeral 14 denotes a G1 electrode, and 15 denotes a
cathode. Reference numeral 18 denotes a current source for
supplying a current of an electron beam (referred to as "beam
current" hereinafter) to the anode 2. Reference numeral 20 denotes
a primary coil of the flyback transformer 3, one end of which is
connected to the high-tension control circuit 22 and the other end
of which is connected to a power supply circuit 23 supplying power
for producing the high-tension. In FIG. 8, the structure of the
electron gun is the same as that of the conventional electron gun
for the part following the G3 electrode, and therefore, that part
is omitted from the drawing to simplify explanation.
[0004] In the display apparatus described above, currents in
proportion to the beam current flowing from the cathode to the
screen flow through the G2 electrode 13 and the Gm electrode 12
peculiar to the Hi-Gm tube respectively. Therefore, in order to
make the voltage drops as small as possible, the voltages to be
applied to these electrodes are supplied from the voltage sources
16 and 17 of low output impedance respectively. In FIG. 8, the G2
electrode 13 is applied with about 500V and the Gm electrode 12 is
applied with about 80V. When the potential of the cathode 15 is
lower than the potential of the Gm electrode 12, electrons pass
through the Gm electrode 12 and flow to the screen. The slope of
the potential after the Gm electrode 12 is of the order of 10.sup.6
(V/m). Compared with the potential slope between the cathode 15 and
the G1 electrode 14, it is greater by an order of magnitude.
Accordingly, after electrons pass through the Gm electrode 12, most
of them can move towards the screen without being affected by
spatial charges.
[0005] For this reason, variation in the intensity of the electron
beam in the Hi-Gm tube when the cathode potential is varied by a
certain value is about twice as much as that in the conventional
CRT. That is, the variation of the cathode potential required to
vary the intensity of the electron beam by a certain value is less
than half the variation required in the conventional CRT. In other
words, with the Hi-Gm tube, the variation in the intensity of the
electron beam can be doubled for the same variation of the cathode
potential. Consequently, with the Hi-Gm tube, it is possible to
easily adapt to video signals of high frequency, and therefore to
easily provide a display apparatus of high intensity and high
resolution.
[0006] As the voltage of the cathode 15 decreases, the beam current
increases and brightness of the screen goes up. At this time, the
currents flowing through the Gm electrode 12 and the G2 electrode
13 increase in proportion to the beam current. The anode 2 is
applied with the voltage stepped up to about 25 KV by the flyback
transformer 3, and the beam current is supplied from the beam
current source 18 connected to the secondary coil 4. The
high-tension of about 25 KV applied to the anode 2 is divided by
the high-impedance resistor 6 (about 100 M.OMEGA.), the variable
resistor 7 and the high-tension detecting resistor 10, to produce a
voltage of about 7 KV at the slidable terminal of the variable
resistor 7. This voltage is supplied to the G3 electrode 11
functioning to converge the beam current. Since almost no current
flows through the G3 electrode 11, there occurs no voltage drop.
Accordingly, the voltage applied to the G3 electrode 11 does not
fluctuate. The voltage at the high-tension level feedback point 9
is input to the high-tension control circuit 22. The high-tension
control circuit 22 controls a switching frequency or duty ratio of
the voltage applied to the primary coil 20 such that the optimum
voltage of about 25 KV always appears along the high-tension line
5.
OBJECT AND SUMMARY OF THE INVENTION
[0007] In the display apparatus described above, if sparking occurs
between the anode 2 and the G3 electrode 11, since the potential of
the G3 electrode 11 goes up, a discharge occurs between the G3
electrode 11 and the Gm electrode 12 or the G2 electrode 13.
Furthermore, a discharge between the G3 electrode 11 and the
cathode 15 can occur through the Gm electrode and the G2 electrode.
Subsequently, a current starts to flow through the G3 electrode 11,
whereby the potential of the G3 electrode 11 falls due to a voltage
drop caused by the current flowing through the high-impedance
resistor 6. If the potential of the G3 electrode 11 falls, since
the effect of converging the beam current is weakened, the beam
current directly hits the G3 electrode 11, and consequently, the
current flowing to the screen is interrupted and the screen is
blanked out. At this time, since the voltage at the high-tension
level feedback point 9 as well falls, the high-tension control
circuit 22 operates to raise the voltage of the high-tension line
5, whereby the voltage of the high-tension line 5 goes up to an
extraordinary value. As a result of the rise of the high-tension
line 5, the voltage of the G3 electrode 11 does not go down
completely, and therefore the abnormal discharge continues. In such
a state, each electrode within the electron gun is applied with an
uncontrollable high voltage continuously, and extraordinary
electrode currents flow, so the apparatus cannot recover from the
state in which the screen remains blanked out. As a result, the
performance characteristics of the electron gun is deteriorated.
Such a dangerous state can occur also in a case where an initial
sparking has occurred between the G3 electrode 11 and the Gm
electrode 12 or G2 electrode 13 due to impurities etc. If this
state in which the high-tension has been raised continues further,
not only the CRT but also other electric components within the
apparatus can undergo large stress.
[0008] An object of the present invention is to provide a display
apparatus, which includes a CRT having an electron gun whose
electrodes for controlling an electron beam are applied with
voltages from their respective voltage sources specifically
designed to supply an electrode current, and is capable of
automatically ceasing a discharge that has occurred between any
electrode within the electron gun and the cathode or the anode of
the CRT.
[0009] This object is achieved by a CRT display apparatus
comprising:
[0010] a CRT having an electron gun;
[0011] said electron gun including:
[0012] a cathode;
[0013] a G1 electrode, a G2 electrode, and a G3 electrode disposed
in that order for drawing electrons from said cathode; and
[0014] a modulating Gm electrode disposed between said G2 electrode
and said G3 electrode,
[0015] a transformer producing a high tension to be applied to an
anode of said CRT at a secondary coil thereof;
[0016] a voltage-dividing resistor dividing said high-tension to
produce a voltage to be applied to said G3 electrode;
[0017] a voltage generator generating a voltage having a value
depending on a current flowing into said dividing resistor as a
feedback voltage;
[0018] a high-tension control circuit for controlling, on the basis
of said feedback voltage, a voltage at a primary coil of said
transformer so as to keep said high-tension at a predetermined
value;
[0019] a G2 electrode voltage source for outputting a predetermined
voltage to be applied to said G2 electrode;
[0020] a Gm electrode voltage source for outputting a predetermined
voltage to be applied to said Gm electrode; and
[0021] a comparator for comparing said feedback voltage with one of
an output voltage of said Gm electrode voltage source and an output
voltage of said G2 electrode voltage source, said comparator
supplying a detection signal to said high-tension control circuit
upon detecting that only said feedback voltage has fallen from a
normal value,
[0022] said high-tension control circuit controlling said voltage
at said primary coil of said transformer so as to lower said
high-tension in response to said detection signal.
[0023] This object is achieved also by a CRT display apparatus
comprising:
[0024] a CRT having an electron gun;
[0025] said electron gun including:
[0026] a cathode; and
[0027] a G1 electrode, a G2 electrode, and a G3 electrode disposed
in that order for drawing electrons from said cathode,
[0028] a transformer producing a high tension to be applied to an
anode of said CRT at a secondary coil thereof;
[0029] a voltage-dividing resistor dividing said high-tension to
produce a voltage to be applied to said G3 electrode;
[0030] a voltage generator generating a voltage having a value
depending on a current flowing into said dividing resistor as a
feedback voltage;
[0031] a high-tension control circuit for controlling, on the basis
of said feedback voltage, a voltage at a primary coil of said
transformer so as to keep said high-tension at a predetermined
value;
[0032] a G2 electrode voltage source for outputting a predetermined
voltage to be applied to said G2 electrode; and
[0033] a comparator for comparing said feedback voltage with an
output voltage of said G2 electrode voltage source, said comparator
supplying a detection signal to said high-tension control circuit
upon detecting that only said feedback voltage has fallen from a
normal value,
[0034] said high-tension control circuit controlling said voltage
at said primary coil of said transformer so as to lower said
high-tension in response to said detection signal.
[0035] This object is achieved also by a CRT display apparatus
comprising:
[0036] a CRT having an electron gun;
[0037] said electron gun including:
[0038] a cathode;
[0039] a G1 electrode, a G2 electrode, and a G3 electrode disposed
in that order for drawing electrons from said cathode; and
[0040] a modulating Gm electrode disposed between said G2 electrode
and said G3 electrode,
[0041] a transformer producing a high tension to be applied to an
anode of said CRT at a secondary coil thereof;
[0042] a voltage-dividing resistor dividing said high-tension to
produce a voltage to be applied to said G3 electrode;
[0043] a first voltage generator generating a voltage having a
value depending on a current flowing into said dividing resistor as
a feedback voltage;
[0044] a high-tension control circuit for controlling, on the basis
of said feedback voltage, a voltage at a primary coil of said
transformer so as to keep said high-tension at a predetermined
value;
[0045] a G2 electrode voltage source for outputting a predetermined
voltage to be applied to said G2 electrode;
[0046] a Gm electrode voltage source for outputting a predetermined
voltage to be applied to said Gm electrode;
[0047] a second voltage generator for outputting a voltage of a
value equal to a value of said feedback voltage when said CRT
display apparatus is operating normally; and
[0048] a comparator for comparing said feedback voltage with an
output voltage of said second voltage generator, said comparator
supplying a detection signal to said high-tension control circuit
upon detecting that only said feedback voltage has fallen from a
normal value,
[0049] said high-tension control circuit controlling said voltage
at said primary coil of said transformer so as to lower said
high-tension in response to said detection signal.
[0050] This object is achieved also by a CRT display apparatus
comprising:
[0051] a CRT having an electron gun,
[0052] said electron gun including:
[0053] a cathode;
[0054] a G1 electrode, a G2 electrode, and a G3 electrode disposed
in that order for drawing electrons from said cathode; and
[0055] a modulating Gm electrode disposed between said G2 electrode
and said G3 electrode,
[0056] a transformer producing a high tension to be applied to an
anode of said CRT at a secondary coil thereof;
[0057] a first voltage-dividing resistor dividing said high-tension
to produce a voltage to be applied to said G3 electrode;
[0058] a second voltage-dividing resistor connected in series to
said first voltage-dividing resistor to produce a screen bias
voltage used to adjust a black level of a screen of said CRT;
[0059] a voltage generator generating a voltage having a value
depending on a current flowing into said first and second
voltage-dividing resistors as a feedback voltage;
[0060] a high-tension control circuit for controlling, on the basis
of said feedback voltage, a voltage at said primary coil of said
transformer so as to keep said high-tension at a predetermined
value;
[0061] a G2 electrode voltage source for outputting a predetermined
voltage to be applied to said G2 electrode;
[0062] a Gm electrode voltage source for outputting a predetermined
voltage to be applied to said Gm electrode; and
[0063] a comparator for comparing said screen bias voltage with one
of an output voltage of said Gm electrode voltage source and an
output voltage of said G2 electrode voltage source, said comparator
supplying a detection signal to said high-tension control circuit
upon detecting that only said screen bias voltage has fallen from a
normal value,
[0064] said high-tension control circuit controlling said voltage
at said primary coil of said transformer so as to lower said
high-tension in response to said detection signal.
[0065] In any of the CRT display apparatuses described above, the
comparator may be provided with a timer circuit to enable the
high-tension control circuit to lower the high-tension over a
predetermined period of time preset in the timer circuit, and
thereafter return the high-tension to a normal value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0066] Embodiments of the invention will now be described by way of
example and with reference to the accompanying drawings in
which:
[0067] FIG. 1 is a block diagram showing a structure of a first
example of the CRT display apparatus according to the
invention;
[0068] FIG. 2 is a block diagram showing a structure of a second
example of the CRT display apparatus according to the
invention;
[0069] FIG. 3 is a block diagram showing a structure of a third
example of the CRT display apparatus according to the
invention;
[0070] FIG. 4 is a block diagram showing a structure of a fourth
example of the CRT display apparatus according to the
invention;
[0071] FIG. 5 is a block diagram showing a structure of a fifth
example of the CRT display apparatus according to the
invention;
[0072] FIG. 6 is a block diagram showing a structure of a sixth
example of the CRT display apparatus according to the
invention;
[0073] FIG. 7 is a block diagram showing a structure of a seventh
example of the CRT display apparatus according to the invention;
and
[0074] FIG. 8 is a block diagram showing a structure of a
conventional CRT display apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0075] FIG. 1 is a block diagram showing a structure of a first
example of the CRT display apparatus according to the invention. In
FIG. 1, reference numerals identical to those in FIG. 8 represent
the same elements. In the figure, there is shown a CRT 1, an anode
2, a flyback transformer 3, and a secondary coil 4 disposed within
the flyback transformer 3. The secondary coil 4 is connected to the
anode 2 through a high-tension line 5. Reference numeral 6 denotes
a high-impedance resistor disposed within the flyback transformer
3, one end of which is connected to the secondary coil 4 and the
other end of which is connected to one end of a variable resistor 7
for a G3 electrode 11. The other end of the variable resistor 7 is
connected to one end of a high-tension detecting resistor 10
disposed outside the flyback transformer 3, and a slidable terminal
of the variable resistor 7 is connected to the G3 electrode 11. The
node of the variable resistor 7 and the high-tension detecting
resistor 10 makes a high-tension level feedback point 9. A voltage
at this high-tension level feedback point 9 is supplied to a
high-tension control circuit 22. Reference numeral 12 denotes a Gm
electrode, and this Gm electrode 12 is applied with a voltage from
a Gm electrode voltage source 17. Reference numeral 13 denotes a G2
electrode, and this G2 electrode 13 is applied with a voltage from
a G2 electrode voltage source 16. Reference numeral 14 denotes a G1
electrode, and 15 denotes a cathode. Reference numeral 18 denotes a
current source supplying a beam current to the anode 2. Reference
numeral 20 denotes a primary coil of the flyback transformer 3, one
end of which is connected to the high-tension control circuit 22
and the other end of which is connected to a power supply circuit
23 supplying power for producing the high-tension.
[0076] The apparatus of FIG. 1 further includes a comparison
circuit 19 that compares the voltage at the high-tension level
feedback point 9 with the output voltage of the Gm electrode
voltage source 17. The output of the comparison circuit 19 is input
into the high-tension control circuit 22. In FIG. 1, the structure
of the electron gun is the same as that of the conventional
electron gun for the part following the G3 electrode, and
therefore, that part is omitted from the drawing to simplify
explanation.
[0077] The operation of the apparatus of the first example will be
explained below. The anode 2 is applied with the voltage stepped up
to about 25 KV by the flyback transformer 3, and the beam current
is supplied from the beam current source 18 connected to the
secondary coil 4. The high-tension of about 25 KV applied to the
anode 2 is divided by the high-impedance resistor 6 (about 100
M.OMEGA.), the variable resistor 7 and the high-tension detecting
resistor 10, to produce a voltage of about 7 KV at the slidable
terminal of the variable resistor 7. This voltage is supplied to
the G3 electrode 11 functioning to converge the beam current. Since
almost no current flows through the G3 electrode 11, there occurs
no voltage drop. Accordingly, the voltage applied to the G3
electrode 11 does not fluctuate. The voltage at the high-tension
level feedback point 9 is input to the high-tension control circuit
22. The high-tension control circuit 22 controls a switching
frequency or duty ratio of the voltage applied to the primary coil
20 such that the optimum voltage of about 25 KV always appears
along the high-tension line 5.
[0078] On the other hand, the voltage at the high-tension level
feedback point 9 and the output voltage of the Gm electrode voltage
source 17 are also input into the comparison circuit 19. In a case
where, although the output voltage of the Gm electrode voltage
source is at a normal value, only the voltage at the high-tension
level feedback point 9 falls from its normal value, the comparison
circuit 19 determines that a discharge has occurred between any
electrode within the electron gun and the anode or the cathode, and
delivers a signal indicative of occurrence of the discharge. In
response to this signal, the high-tension control circuit 22
reduces the effective value of the voltage applied to the primary
coil 20 by controlling the switching frequency or duty ratio of
this voltage. Thus, the voltage of the high-tension line 5 and the
voltage applied to the G3 electrode 11 go down, and the discharge
ceases. It is also possible to make a microcomputer (not shown)
performing other functions of the display apparatus carry out the
function of this comparison circuit 19.
[0079] FIG. 2 is a block diagram showing a structure of a second
example of the CRT display apparatus according to the invention. In
FIG. 2, the elements that are the same as those in FIG. 1 are given
the same reference numerals, and explanation thereof will be
omitted. The apparatus of the second example uses a conventional
CRT 100 instead of the Hi-Gm tube 1, and therefore, the Gm
electrode voltage source 17 used in the first example is not
provided.
[0080] The operation of the apparatus of the second example will be
explained below. The anode 2 is applied with the voltage stepped up
to about 25 KV by the flyback transformer 3, and the beam current
is supplied from the beam current source 18 connected to the
secondary coil 4. The high-tension of about 25 KV applied to the
anode 2 is divided by the high-impedance resistor 6 (about 100
M.OMEGA.), the variable resistor 7 and the high-tension detecting
resistor 10, to produce a voltage of about 7 KV at the slidable
terminal of the variable resistor 7. This voltage is supplied to
the G3 electrode 11 functioning to converge the beam current. Since
almost no current flows through the G3 electrode 11, there occurs
no voltage drop. Accordingly, the voltage applied to the G3
electrode 11 does not fluctuate. The voltage at the high-tension
level feedback point 9 is input to the high-tension control circuit
22. The high-tension control circuit 22 controls a switching
frequency or duty ratio of the voltage applied to the primary coil
20 such that the optimum voltage of about 25 KV always appears
along the high-tension line 5.
[0081] In the second example, the voltage at the high-tension level
feedback point 9 and the output voltage of the G2 electrode voltage
source 16 are input into the comparison circuit 19. In a case
where, although the output voltage of the G2 electrode voltage
source is at a normal value, only the voltage at the high-tension
level feedback point 9 falls from its normal value, the comparison
circuit 19 determines that a discharge has occurred between any
electrode within the electron gun and the anode or the cathode, and
delivers a signal indicative of occurrence of the discharge. In
response to this signal, the high-tension control circuit 22
reduces the effective value of the voltage applied to the primary
coil 20 by controlling the switching frequency or duty ratio of
this voltage. Thus, the voltage of the high-tension line 5 and the
voltage applied to the G3 electrode 11 go down, and the discharge
ceases. It is also possible to make a microcomputer (not shown)
performing other functions of the display apparatus carry out the
function of this comparison circuit 19.
[0082] FIG. 3 is a block diagram showing a structure of a third
example of the CRT display apparatus according to the invention. In
FIG. 3, the elements that are the same as those in FIG. 1 are given
the same reference numerals, and explanation thereof will be
omitted. The third example differs from the first example in that
the comparison circuit 19 is supplied with the output voltage of
the G2 electrode voltage source 16 instead of the output voltage of
the Gm electrode voltage source 17. That is, in the third example,
the voltage at the high-tension level feedback point 9 and the
output voltage of the G2 electrode voltage source 16 are input into
the comparison circuit 19. In a case where, although the output
voltage of the G2 electrode voltage source is at a normal value,
only the voltage at the high-tension level feedback point 9 falls
from its normal value, the comparison circuit 19 determines that a
discharge has occurred between any electrode within the electron
gun and the anode or the cathode, and delivers a signal indicative
of occurrence of the discharge. In response to this signal, the
high-tension control circuit 22 reduces the effective value of the
voltage applied to the primary coil 20 by controlling the switching
frequency or duty ratio of this voltage. Thus, the voltage of the
high-tension line 5 and the voltage applied to the G3 electrode 11
go down, and the discharge ceases. It is also possible to make a
microcomputer (not shown) performing other functions of the display
apparatus carry out the function of the comparison circuit 19.
[0083] FIG. 4 is a block diagram showing a structure of a fourth
example of the CRT display apparatus according to the invention. In
FIG. 4, the elements that are the same as those in FIG. 1 are given
the same reference numerals, and explanation thereof will be
omitted. The apparatus of the fourth example has a
normal-level-high-tension source 24 which generates a voltage
equivalent to the voltage at the high-tension level feedback point
9 when there is no discharge occurring between any electrode within
the electron gun and the anode or the cathode, and the apparatus is
functioning normally.
[0084] The operation of the apparatus of the fourth example will be
explained below. The anode 2 is applied with the voltage stepped up
to about 25 KV by the flyback transformer 3, and the beam current
is supplied from the beam current source 18 connected to the
secondary coil 4. The high-tension of about 25 KV applied to the
anode 2 is divided by the high-impedance resistor 6 (about 100
M.OMEGA.), the variable resistor 7 and the high-tension detecting
resistor 10, to produce a voltage of about 7 KV at the slidable
terminal of the variable resistor 7. This voltage is supplied to
the G3 electrode 11 functioning to converge the beam current. Since
almost no current flows through the G3 electrode 11, there occurs
no voltage drop. Accordingly, the voltage applied to the G3
electrode 11 does not fluctuate. The voltage at the high-tension
level feedback point 9 is input to the high-tension control circuit
22. The high-tension control circuit 22 controls a switching
frequency or duty ratio of the voltage applied to the primary coil
20 such that the optimum voltage of about 25 KV always appears
along the high-tension line 5. In the fourth example, the voltage
at the high-tension level feedback point 9 and the output voltage
of the normal-level-high-tension source 24 are input into the
comparison circuit 19. In a case where, although the output voltage
of the normal-level-high-tension source 24 is at a normal value,
only the voltage at the high-tension level feedback point 9 falls
from its normal value, the comparison circuit 19 determines that a
discharge has occurred between any electrode within the electron
gun and the anode or the cathode, and delivers a signal indicative
of occurrence of the discharge. In response to this signal, the
high-tension control circuit 22 reduces the effective value of the
voltage applied to the primary coil 20 by controlling the switching
frequency or duty ratio of this voltage. Thus, the voltage of the
high-tension line 5 and the voltage applied to the G3 electrode 11
go down, and the discharge ceases. It is also possible to make a
microcomputer (not shown) performing other functions of the display
apparatus carry out the function of this comparison circuit 19.
[0085] FIG. 5 is a block diagram showing a structure of a fifth
example of the CRT display apparatus according to the invention. In
FIG. 5, the elements that are the same as those in FIG. 1 are given
the same reference numerals, and explanation thereof will be
omitted. Generally, a flyback transformer for a CRT includes a
variable resistor used for producing a screen bias voltage which is
applied to a G2 electrode to adjust a black level of the CRT. The
fifth example utilize the screen bias voltage produced by this
variable resistor for detecting occurrence of a discharge between
any electrode within the electron gun and the anode or the cathode.
In FIG. 5, a variable resistor 8 for producing the screen bias
voltage is connected between the variable resistor 7 for the G3
electrode and the high-tension detecting resistor 10. The screen
bias voltage obtained at the slidable terminal of the variable
resistor 8 is supplied to the comparison circuit 19.
[0086] The operation of the apparatus of the fifth example will be
explained below. The anode 2 is applied with the voltage stepped up
to about 25 KV by the flyback transformer 3, and the beam current
is supplied from the beam current source 18 connected to the
secondary coil 4. The high-tension of about 25 KV applied to the
anode 2 is divided by the high-impedance resistor 6 (about 100
M.OMEGA.), the variable resistor 7 and the high-tension detecting
resistor 10, to produce a voltage of about 7 KV at the slidable
terminal of the variable resistor 7. This voltage is supplied to
the G3 electrode 11 functioning to converge the beam current. Since
almost no current flows through the G3 electrode 11 as long as
there is no discharge occurring between any electrode within the
electron gun and the anode or the cathode, there occurs no voltage
drop. Accordingly, the voltage applied to the G3 electrode 11 does
not fluctuate, and the voltage obtained at the slidable terminal of
the variable resistor 8 connected to the variable resistor 7 in
series does not fluctuate as well. The voltage at the high-tension
level feedback point 9 is input to the high-tension control circuit
22. The high-tension control circuit 22 controls a switching
frequency or duty ratio of the voltage applied to the primary coil
20 such that the optimum voltage of about 25 KV always appears
along the high-tension line 5.
[0087] In the fifth example, the output voltage of the Gm electrode
voltage source 17 and the screen bias voltage from the variable
resistor 8 are input into the comparison circuit 19. In a case
where, although the output voltage of the Gm electrode voltage
source 17 is at a normal value, only the screen bias voltage falls
from its normal value, the comparison circuit 19 determines that a
discharge has occurred between any electrode within the electron
gun and the anode or the cathode, and delivers a signal indicative
of occurrence of the discharge. In response to this signal, the
high-tension control circuit 22 reduces the effective value of the
voltage applied to the primary coil 20 by controlling the switching
frequency or duty ratio of this voltage. Thus, the voltage of the
high-tension line 5 and the voltage applied to the G3 electrode 11
go down, and the discharge ceases. It is also possible to make a
microcomputer (not shown) performing other functions of the display
apparatus carry out the function of the comparison circuit 19.
[0088] FIG. 6 is a block diagram showing a structure of a sixth
example of the CRT display apparatus according to the invention. In
FIG. 6, the elements that are the same as those in FIG. 1 are given
the same reference numerals, and explanation thereof will be
omitted. The sixth example as well as the fifth example utilizes,
for detecting occurrence of a discharge between any electrode
within the electron gun and the anode or the cathode, the output of
the variable resistor 8 which produces the screen bias voltage to
be applied to the G2 electrode for adjusting the black level.
However, it differs from the fifth example in that the comparison
circuit 19 is supplied with the output voltage of the G2 electrode
voltage source 16 instead of the output voltage of the Gm electrode
voltage source 17.
[0089] In the sixth example, in a case where, although the output
voltage of the G2 electrode voltage source 16 is at a normal value,
only the screen bias voltage falls from its normal value, the
comparison circuit 19 determines that a discharge has occurred
between any electrode within the electron gun and the anode or the
cathode, and delivers a signal indicative of occurrence of the
discharge. In response to this signal, the high-tension control
circuit 22 reduces the effective value of the voltage applied to
the primary coil 20 by controlling the switching frequency or duty
ratio of this voltage. Thus, the voltage of the high-tension line 5
and the voltage applied to the G3 electrode 11 go down, and the
discharge ceases. It is also possible to make a microcomputer (not
shown) performing other functions of the display apparatus carry
out the function of the comparison circuit 19.
[0090] FIG. 7 is a block diagram showing a structure of a seventh
example of the CRT display apparatus according to the invention. In
FIG. 7, the elements that are the same as those in FIG. 1 are given
the same reference numerals, and explanation thereof will be
omitted. The seventh example differs from the first example in that
the output of the comparison circuit 19 is supplied to the
high-tension control circuit 22 through a timer circuit 25.
[0091] In the seventh example as well as the first example, the
voltage at the high-tension level feedback point 9 and the output
voltage of the Gm electrode voltage source 17 are input into the
comparison circuit 19. In a case where, although the output voltage
of the Gm electrode voltage source 17 is at a normal value, only
the voltage at the high-tension level feedback point 9 falls from
its normal value, the comparison circuit 19 determines that a
discharge has occurred between any electrode within the electron
gun and the anode or the cathode, and delivers a signal indicative
of occurrence of the discharge. This signal is input to the
high-tension control circuit 22 through the timer circuit 25. In
response to this signal, the high-tension control circuit 22
reduces the effective value of the voltage applied to the primary
coil 20 by controlling the switching frequency or duty ratio of
this voltage over a period of time preset in the timer circuit 25.
When the period has elapsed, the high-tension control 22 ceases its
control operation. Thus, the apparatus can automatically recover
from the state in which the screen is blanked out. The period of
time preset in the timer circuit 25 may be variable. It is possible
to provide all the examples described above with the timer circuit
25. It is also possible to make a microcomputer (not shown)
performing other functions of the display apparatus carry out the
function of the comparison circuit 19.
[0092] The above explained preferred embodiments are exemplary of
the invention of the present application which is described solely
by the claims appended below. It should be understood that
modifications of the preferred embodiments may be made as would
occur to one of skill in the art.
* * * * *