I am making a bi-polar 12V power supply with a centre tapped (12-0-12) AC transformer. On inspecting the voltages after the bridge rectifier (KBU-4B), the negative side produces -26V and the positive produces +10V.

Both sides have a 470uF cap before and after their voltage regulators (L7812-CV and L7912-CV) for smoothing and a 4.7k resitor at their outputs to simulate a load. Their outputs after regulating are +9V and -12V.

As both sides of the circuit are identical I would assume their outputs should also be identical. Is there something I am overlooking here?


simulate this circuit – Schematic created using CircuitLab

The PCB below contains the + and - 12V outputs and a +5V output running off the +12V. I have disconnected the trace running to the 5V output whilst trouble shooting the asymmetry from the bridge rectifier. The output LEDs have also been replaced with 4.7k resistors


  • \$\begingroup\$ Check that you have used the correct pin out for the 7912 regulator; it's not the same as the 7812. \$\endgroup\$ – Andy aka Apr 2 at 18:27
  • \$\begingroup\$ Seems like a very low load. Perhaps less ohms will give more even outputs. Also it's not very clear, do you have 50 or 60Hz AC input or is it a flyback switching supply? \$\endgroup\$ – Gregory Kornblum Apr 2 at 18:27
  • \$\begingroup\$ Please use the site's built-in schematic editor to include a schematic of your circuit. \$\endgroup\$ – jwh20 Apr 2 at 18:33
  • \$\begingroup\$ @GregoryKornblum I have a 50Hz supply, do you recommend reducing the output load? \$\endgroup\$ – Ed Lewis Apr 2 at 18:47
  • \$\begingroup\$ @Andyaka I double checked the output pins and they are correct \$\endgroup\$ – Ed Lewis Apr 2 at 18:48

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