VIDEORECORDER PSUS
TV and monitor PSU systems operate at relatively high power, and so are more vulnerable to breakdown than other sections of the set. For this reason, and because of the great diversity of circuit designs encountered, much of this chapter has been devoted to them. In general the circuits of videorecorders require low-voltage supplies in the 9–12 V region, with some exceptions such as varicap tuning volt- age sources and screen/accelerating potentials for fluorescent status/ display panels, both low-energy devices.
To derive the low stabilised voltages required from mains power, two approaches are possible: a double-wound 50 Hz mains transformer with secondary windings feeding rectifier sets and series regulators as required; or a switch-mode PSU of one of the types already discussed in this chapter. The later the design of the VCR the more likely it is to have a switch-mode PSU.
Videorecorders have special requirements of their power supplies; in many cases regulators are switched by the system control section, and during standby mode power has to be maintained to clock- display, timer and system-control circuits. In portable equipment, power supplies are provided and withdrawn as the mode and user’s requirements dictate in order to minimise power consumption and prolong battery life.
Tiny d.c.–d.c. converters are often found in the midst of video- recorder operating circuits. To provide a low-current unstabilised sup- ply a postage-stamp sized screening can conceals a miniature ferrite transformer, blocking oscillator transistor and rectifier diode, working into an external reservoir capacitor. Apart from the absence of a stabilising circuit, these devices work on just the same principles as high-power SMPSUs, and have high conversion efficiency. They are particularly useful where a negative line is required, as where a fluorescent display panel is to be fed directly from a microprocessor or display-driver IC.