economical power supply

The power supply described here uses a silicon-controlled rectifier (SCR) that, depending on the load current, selects taps on the secondary of the mains transformer. The output voltage of around 9 V is eminently suitable as input voltage for a 5 V regulator, which consequently works with the absolutely minimum power dissi- pation. With low to medium load currents, the SCR is in the blocking state. Recti- fication of the secondary transformer voltage then takes place in D1, D 2, D5, and D6only. The load current flows during the positive half cycle via D1, load, and D5; during the negative half wave it flows through D2, load, and D6. The tapped secondary voltage amounts to 8 V in either case, while a 2 V section remains unused. With increasing load current, the out- put voltage drops until no current flows any more through the zener diode. Transistor T7switches off which removes the short circuit from the gate of the SCR, which then con- ducts. As soon as that happens, the full sec...