Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2001-02-05
2002-04-02
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S030000, C315S388000, C315S389000
Reexamination Certificate
active
06366035
ABSTRACT:
FIELD OF THE INVENTION
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
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 G
2
electrode and a G
3
electrode for modulating an electron beam.
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 G
3
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 G
3
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 G
2
electrode, and this G
2
electrode
13
is applied with a voltage from a G
2
electrode voltage source
16
. Reference numeral
14
denotes a G
1
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 G
3
electrode, and therefore, that part is omitted from the drawing to simplify explanation.
In the display apparatus described above, currents in proportion to the beam current flowing from the cathode to the screen flow through the G
2
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 G
2
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
6
(V/m). Compared with the potential slope between the cathode
15
and the G
1
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.
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.
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 G
2
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&OHgr;), 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 G
3
electrode
11
functioning to converge the beam current. Since almost no current flows through the G
3
electrode
11
, there occurs no voltage drop. Accordingly, the voltage applied to the G
3
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 25KV always appears along the high-tension line
5
.
OBJECT AND SUMMARY OF THE INVENTION
In the display apparatus described above, if sparking occurs between the anode
2
and the G
3
electrode
11
, since the potential of the G
3
electrode
11
goes up, a discharge occurs between the G
3
electrode
11
and the Gm electrode
12
or the G
2
electrode
13
. Furthermore, a discharge between the G
3
electrode
11
and the cathode
15
can occur through the Gm electrode and the G
2
electrode. Subsequently, a current starts to flow through the G
3
electrode
11
, whereby the potential of the G
3
electrode
11
falls due to a voltage drop caused by the current flowing through the high-impedance resistor
6
. If the potential of the G
3
electrode
11
falls, since the effect of converging the beam current is weakened, the beam current directly hits the G
3
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 G
3
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 G
3
electrode
11
and the Gm electrode
12
or G
2
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.
An object of the present invention is to provide a display
Heishi Akinori
Yasui Hironobu
Vo Tuyet T.
Wong Don
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