Self-Switching Power Supply

0I The proposed power supply switches itself off when no current is drawn by the load. How this is done is shown in the circuit diagram, Fig. 1. When a load current flows, the p.d. across Di is sufficient to cause D2 and T2 to conduct. T1 is then switched on and the relay is energized. When the load current ceases, T2 switches off. The base current of T1 will then charge C2 so that after a few seconds the relay is de- energized. The relay contact, re,, will then switch off the mains at the primary F1 Tr 1 B40C1500 SELF-SWITCHING POWER SUPPLY of the transformer. The supply is switched on again by reconnecting the load and pressing Si briefly. The output voltage depends on the re- sistance between A and B. A wire link there results in an output voltage of about 3.5 V. For each 100R increase, the output voltage will rise by about I V (the current from the regulator to ground is a nearly constant 10 mA). This makes it possible to obtain a variable output voltage with the aid of some r...