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Dual power supply



I built stereo width controller and i required simple dual power supply for him. Supply has symmetrical voltage output +12V and -12V with max. current 100mA.

Circuit description

Circuit has on the primary side only fuse. I couldn't find smaller than 50mA. We can connect power cord directly to the X1 connector or via power switch on the chassis. On the secondary side of transformer are connected two fuses 100mA and after them is bridge rectifier. For filtering of rectified voltage there are C1 and C2. Next are positive and negative voltage regulators 78L12 and 79L12 with decoupling capacitors C3 to C6 close to regulators. Next are small filter capacitors and also signaling LEDs connected via resistors. Output voltages are connected to 3 pin connector. For signaling of presence of voltage is enough only one LED. We can also use 2 pin connectors for LED connecting.

Schematics diagram

Schematics diagram schematics in Eagle 6 format


First we check if all holes are correctly drilled. We assemble parts from a smaller to bigger. We begin with resistors, small capacitors, leds, regulators, fuses and rectifier. Next we continue with connectors, transformer and big capacitors. It doesn't matter on the assembling order. Beware of polarity of electrolytic capacitors and orientation of diodes and regulators.

Component view

Component view

Printed circuit boards

Board is designed single-sided. It allow to create it in a amateur conditions. I ordered it at small local company Kohout Spoj. I tried to design them symmetrically.

Board PDF format Eagle6 format




Almost all components are usually available. Transformer in this package create a lot of manufacturers. If voltage on the big capacitors will be under 14.5 volts, that we must use transformer with bigger secondary voltage (2x 15V) for 12 volts on the output. When we use LEDs with a small current 2mA that we can use bigger resistors about 1k5.

Part list
name value and type quantity
R1, R2 820 (1k5 for 2mA LED) 2x
C1, C2 2200uF/25V 105c electrolytic RM5 2x
C3-C6 100nF foil RM5 2.5x7.5mm 4x
C7, C8 100uF/25V 105st electrolytic RM2 2x
IC1 78L12 TO92 1x
IC2 79L12 TO92 1x
B1 Bridge rectifier 1.5A/250V RB1A 1x
LED1, LED2 LED green (2mA) 2x
TR1 Transformer 230V/2x 12V 3VA EI30-2 1x
F1, F2 100mA fuse to PCB 2x
F3 50mA fuse to PCB 1x
J1 Molex 3pin PSH02-03P 1x
X1 Frame terminal AK300/2 1x



Supply works without troubles on the first power-on. When we require another output voltage for example +/-15V or +/-5V that we need another transformer and regulators. We must also check allowed voltage of capacitors.