The production of automatic power-off degaussing circuit for color TV

As we all know, the traditional color TV degaussing circuit uses positive temperature coefficient thermistors (commonly known as PTC components) for degaussing. However, after degaussing, milliampere current still flows through the thermistor to maintain the high temperature of the thermistor. The disadvantage is that it consumes power and shortens the service life of the degaussing resistor. Many users often only press the "standby" button on the remote control to turn off the TV after watching TV. In this case, the degaussing circuit and the switching power supply continue to consume power, which is contrary to the energy-saving and green environmental protection concepts advocated now. Therefore, the new color TVs (including color displays) produced now all send a pulse level from the CPU degaussing control terminal, and the normally open contact of the relay is turned on by the switch transistor, so that the degaussing circuit degausses the color tube (color display). After the pulse level, the switch transistor is turned off, the relay is reset (the normally open contact is disconnected), the degaussing circuit is open, and the degaussing circuit no longer consumes power. So can the original color TV degaussing circuit be improved to achieve the above effect? ​​After the author's experiment, the answer is yes.

The actual circuit of this improvement is very simple (see the figure above). Its principle is: the DC 12V voltage generated by the TV after working (which can be provided by the switching power supply or the line output transformer) charges the capacitors Cl and C2 through Rl. When charging starts, the voltage drop across Rl is greater than the conduction voltage of the control tube Q (that is, Ube≥0.7V), so Q is saturated and turned on, relay 1 is energized, the normally open contact is closed, and the degaussing circuit is turned on to degauss the color tube (or color display j). After two or three seconds, the charging of capacitors C1 and C2 is basically completed. When the voltage drop across Rl is less than 0.6V, Q changes from on to off → relay J loses power → the normally open contact is disconnected → the power supply of the degaussing circuit is disconnected, the degaussing work is completed and no power is consumed. This is one of the advantages.

In the figure above, it is measured that the current flowing through R1 is 150mA when the capacitors Cl and C2 start to charge, and the instantaneous power is about 1W, so a 1W resistor is sufficient. Cl and C2 are two 470μF/16V capacitors. If there are 1000μF/16V capacitors, you can replace Cl and C2 with one capacitor. R2 is a capacitor charge discharge resistor to ensure that the charge on the plates of capacitors Cl and C2 is neutralized after shutdown. A power of 1W is sufficient. When the resistance value of R2 is 15kΩ, the machine can be turned on 5 minutes after shutdown to demagnetize again. The relay uses a DC12V20A specification, Q is Vce≥50v, ICm>lO0mA NPN silicon switch tube, such as 3DK4 and 3DK9.

The following figure is the printed circuit board diagram of the degaussing circuit. After the components are installed, they are attached to the switch transformer or the idle power supply with quick-drying glue, and they can work normally without debugging. The second advantage of this improvement is that the high power consumption of the degaussing circuit and the switching power supply startup is staggered, which reduces the impact on the power grid and the power input circuit.