This circuit was designed to ensure that an amplifier circuit containing a TDA1516Q would not exceed its maximum supply voltage when the load is small. This amplifier is used in a PC to increase the audio power somewhat. The PC power supply, however, created so much interference that an additional power supply was required. The power supply has its own power transformer (US: substitute 115 V primary) with a secondary voltage of 12 V AC. After rectification and filtering this results in a DC voltage of about 16 V. The regulator consists of a P channel MOSFET SJ117, the gate of which is driven via a voltage divider connected to T2. The base of T2 is held at a constant voltage by LED D2 so that the voltage across emitter resistor R2 is also constant and therefore carries a constant current.
When the output voltage is higher than about 13.5 V, zener diode D1 will start to conduct and supply part of the current through R2 — as a result the MOSFET will be turned on a little less. In this way there is a balance point, where the output voltage will be a little over 13.5 V (1.5 V across R2 plus the 12 V zener voltage). The regulator is capable of delivering up to about 2 A — in any case, it is a good idea to fit the MOSFET with a heatsink.
It is possible to add an optional potentiometer in series with the 12-V zener diode, which will allow a small amount of adjustment of the output voltage. The relay at the AC power line input ensures that the power supply is only turned on when the PC is turned on. This relay is driven by a 4-way power supply connector from the PC.