This power supply circuit brings together a dual polarity power supply and soft power to a power transformer (to supply a large amplifier for example) on the same board.
The regulator circuit is performed by a pair of 79xx and 78XX (XX = 12 to 24), if the current requirement does not exceed 200 mA, the heatsink is not needed here (but available in the printed circuit board). Tthis circuit part provides power to devices such as amplifiers (preamplifier, fans, the protection module etc. …).
Soft power for a large transformer is made by taking the series resistance of 100 ohms/10 watt or two resistors 47 ohm / 5 watts in the primary circuit, the resistance is shorted by the relay after ± 1.5 seconds. This avoids a sudden use of the current during power up. A Zener diode can be adjusted to allow the use of various voltage relay.
Parts list : BR1 = bridge rectifier 50V/1A or more D1, D2 = 1N4002 D3 = zener diode 12V 1W (depending on the voltage of the relay) D4 = 1N4148 C1, C2 = 1000 uF 40V C3, C4, C5, C6 = 100 nF MKT C7, C8 = 10 uF 40V C9 = 470 uF 40V C10 = 100 uF 40V IC1 = 7812 IC2 = 7912 SW1 = 2 switch circuits T1 = BC547 R1 = 10 K 1 / 4 watt R2, R3 = 2 * 47 1 * 100 or ohms/5W ohms/10W REL = 2 relay circuits 12V (10A 250V) (for other voltage - change Zener) TR1 = 2 * 15V 250 mA TR2 = amp transformer (eg 2 * 50V 6A) F1 = 200 mA slow fuse F2 = 6 A slow fuse (depending on transformer)