انواع التغذية .. كتاب التلفزيون الملون والعادي

Chapter Twelve:

Feeder circuits and voltage regulators in the television receiver

There are various feeding circuits in the TV receiver - all of them do the same work, which is feeding the different phases of the device, including valves in old devices - and transistors or integrated circuits in modern devices - and the required low voltages range from 5 to 250 (volts and from 100-400 volts) per The valve, and it may reach 750 volts, and these voltages do not include the high voltages needed to feed the screen valve, which we will explain in another chapter. As for the power of the devices, it was up to 250 watts, while in modern color it is (75-100 watts) and in black and white it is (35-45 watts).

1-12 Types of nutrition:

1- Feeding through a transformer that isolates the AC network current from the device’s circuits and secures it
necessary efforts.

2 - Feeding through a transformer, directly connected to the AC network - it has a voltage multiplier and does not guarantee circuit isolation from the network - and requires an isolation transformer when repairing operations.

3 - Direct feeding through a rectification circuit, and we obtain some voltages from the horizontal output stage after rectification - and it also requires an isolation transformer to ensure that there is no electrocution.

Feeding circuits can be classified according to the type of rectification circuit used, which are:

1 - Half wave rectifier circuit

2 - Full wave rectifier circuit
3- Bridge rectification circuit. .
4 - A double or triple voltage rectification circuit, especially the voltage connected to the screen valve elevator and the supply voltages may be regulated or unregulated - the old types of valves were not regulated voltages, but modern transistor devices and integrated circuits, their efforts are subject to precise regulation, to maintain image quality And measure and stabilize it - and also regulate the high voltage supplied to the screen
2-12 Silicon rectifiers and filter circuits:

The diode or rectifier is an element that changes the alternating current AC into direct current DC - and they differ in their types, shapes and sizes. The vacuum diodes used to perform the rectification task, followed by the diodes of the element selenium and germanium, and after 1951 the silicon diode was widespread in television rectifier circuits, for what It is distinguished in terms of cost, long operating life, tolerance to external factors and heat - its small reverse resistance, and its large current, and it is still and will remain in use during the coming years. Like the rest of the electronic elements with a silicon composition, such as transistors and integrated circuits, Figure (1-12) due to its tolerance to heat and other weather factors, and its reverse voltage is less than 1 volt in the forward direction. A small point of gold belonging to the opposite side of the silicon layer, and the binary has an anode and a cathode. Refer to the study of constituents in our book (simplified) in the principles of electrical and electronic engineering, part (2 + 1). The diode is connected with a series resistance to reduce the intensity of the current - as the capacitor connected after the silicon diode in the rectifier circuit (Fig. 2-12) seems to work as a short circuit at the moment the current is connected. If the reverse resistance of the diode is (1 ohm), the value of the series resistance with it (10 ohm), and the supply voltage is 220 volts alternating - then the maximum voltage reaches 220 x 1,414 = 311 volts, and the current intensity = 1311 (10) (1) 28 amperes, while the intensity reaches The current without resistance is 311: 1 = 311 amperes. The binary is damaged before reaching this high current value. And the current that comes out of the diode carries many zigzags that are not suitable for feeding the circuits of the television set, because they overlap in the phases of the picture and sound, so their quality decreases.

These aliasing can be removed by using filter circuits that include resistors, capacitors, and induction coils. It is known that the oscillations of the half-wave rectification circuit are 50 Hz and the full-wave oscillations are 100 Hz.

Filter circuits with RC resistance and capacitor: This method is suitable for low voltages and is small in size - low in cost, although the capacitor used is large in capacity and may reach thousands of microfarads. Increasing the value of resistance with the capacitance of the capacitor remains constant leads to a decrease in aliasing, but this weakens the rectified voltage and weakens the regulation process, and the value of R and C depends on the aliasing, current and degree of regulation required. Figure (3-12-8).

Filter circuit coil and capacitor L.C:
It is commonly used. And it is considered ideal in terms of voltage regulation - and it gives a rectifier voltage equal to (60) to (70%) of the peak voltage of its sine wave - and if there is a capacitor before the coil and another after it Fig. (12 - 3 - C), then the voltage reaches (90 %) of the peak voltage, but its organization is not as good as the circuit that has one capacitor after the coil, as in Figure (B-3-12).

The capacity of the used capacitors varies according to the type of circuit. If it is to feed a valve device, its capacity ranges from 25-200 microfarads. The self-induction of the coil ranges from (2 to 15 Hz), while the capacitance of the capacitor used in transistor circuits is (2000) microfarads or more, and the working voltage of the transistors ranges from (20 to 25 volts).