DC power generation
The various sections of the TV receiver have different power requirements depending on their function and the level of the signal that is being handled. Large-amplitude signals from the video and the line output stages require a high or boost voltage of 150–250 V known as high tension (HT). On the other hand, a low voltage supply of 3–40 V, sometimes known as low tension (LT), is required for those sections such as the front end, video or sound-processing chips and drives and field and line timebase that handle small signal levels. Furthermore, extra high tension (e.h.t.) in the region of 30 kV is necessary for the final anode of the cathode-ray tube (CRT). In this chapter we will cover the principles of power generation and its applications to CRT television receivers. Power requirements for other display systems follow in later chapters.
CRT television power requirements
Apart from the a.c. supply to the tube’s heater, regulated power supplies are necessary. Protection measures have to be taken to ensure safe opera- tion of the receiver under normal conditions as well as under faulty con- ditions such as excessive load current. A great diversity of approaches and techniques are employed in the design of power supplies for TV receivers. The aim is to improve regulation and efficiency and to minimise power dissipation, thus reducing the cost and weight of the receiver.
The simplest power supply is the unregulated rectifier circuit shown in Figure 15.1, in which D1/D2 forms a full-wave rectifier, C1 a reservoir capacitor and R1/C2 a smoothing or low-pass filter to remove the 100 Hz ripple appearing at the output. For a more effective smoothing, the series resistor R1 may be replaced by a large inductor. In this simple circuit the d.c. output decreases as the load current increases. For a stable d.c. output, a regulator or a stabiliser is used. A regulator maintains the d.c. output level as constant against both changes in the mains supply and changes in the load current, known as mains regulation and load regulation respectively.