Display shutdown highlight elimination circuit

Single chip microcomputer STM32L151CCU6

Industrial Router Crystal 3.2*2.5mm 3225 26M (26.000MHZ) 12PF 10PPM 20PPM 30PPM

Manufacturers produce direct sales LED lamp bead patch lighting products 0805 white light super bright indicator light dedicated

Photocoupler

When the TV is turned off, the line and field scanning circuit will stop working immediately, but the cathode temperature of the picture tube will not drop suddenly, and the hot electrons will still be emitted, and the anode high voltage of the high-voltage filter capacitor formed by the graphite layer inside and outside the tube cone still exists. A beam of electrons will be generated. At this time, because there is no deflection, the electron beam will concentrate on the center of the screen, causing a bright spot in the center of the screen, which will gradually disappear after several tens of seconds. Since the electron beam continues to bombard the phosphor in the center of the screen, the phosphor will be overheated and damaged to form a black spot, so it is necessary to try to eliminate the shutdown highlight.
There are two types of commonly used shutdown and brightening point circuits: the first type keeps the picture tube between the cathode and the cathode for a period of time after the shutdown, so that the cathode tube is turned off until the cathode is cooled. The second type is to make the grid-yin have a positive voltage at the moment of shutdown. , thereby generating a large beam current, quickly neutralizing (bleeding) the charge on the high voltage capacitor.
1. The cut-off type keeps the gate at ground potential after shutdown, maintains a high positive voltage on the cathode, or maintains a very negative voltage on the gate to achieve the purpose of cutting off the picture tube.
Figure 10-22 shows an example of a circuit for a cut-off type brightening point. The working process is as follows. After starting up, the reverse pulse of the transformer is self-returned by VD1 rectification, C1 filtering and sent to the three viewing tubes as their working power.
At the same time, this voltage is charged to C2 via C2 and VD2, VD2 is turned on to make the gate potential approximately ground, and C2 is charged with approximately 180 V. When the power is turned off, the voltage on C1 is quickly discharged, and the positive terminal is at zero potential, and C2 is only discharged through R. The discharge time constant is large, so the voltage of the gate is more than one hundred volts, which is enough to make the picture tube Deadline. This negative voltage discharge is slow and can be maintained until the cathode cools down to stop emitting electrons. The above process achieves the effect of eliminating the shutdown highlights.
The shortcoming of the cut-off type bright-bright point circuit is that the charge discharge on the high-voltage filter capacitor is very slow, and there is still a high voltage for a long time after the shutdown, and the maintenance personnel must pay attention.
At the moment of shutdown, a large beam current is provided, so that the high-voltage capacitor can be quickly discharged, so that it is impossible to attract the cathode hot electron, and the purpose of eliminating the bright spot is achieved.
The second figure is a schematic diagram of a high-voltage bleed-type erasing point circuit. After the power is turned on, the +120V power supply charges the C358 through the R378, and the polarity is as shown. During normal operation, C358 blocks DC, so the +120 V power supply has no effect on point A. At the moment of shutdown, C358 is discharged through R37, +120V power supply + 12V power supply R355, R354, etc., and its discharge time constant is greater than 0.15 s, so a negative potential is formed at point B, and the potential at point A is also negative. After 10V and 11 two-stage injection, the DC potential of the output terminals of the three final stage discharge tubes is greatly reduced, that is, the cathode potential of the picture tube is greatly reduced. This produces a large beam current, which quickly neutralizes the charge on the high voltage filter capacitor at the moment the scan grating is reduced, completing the brightness reduction point.
This method is widely used in color television sets, but it is not intuitive because it is generally done in the pre-stage circuit. 
In addition, a method of increasing the time constant of the power supply decoupling circuit in the line and field scanning circuits can also be employed. After the shutdown, the decoupling capacitor still maintains a certain voltage, and can still perform the shortening scan for a short time. When the scanning amplitude is gradually attenuated, the charge on the high-voltage filter capacitor is consumed, and the purpose of eliminating the shutdown highlight can also be achieved. This method is often used in combination with the high pressure venting method to obtain better results.