0

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?

schematic

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

pcb

JRE
  • 67,678
  • 8
  • 104
  • 179
Ed Lewis
  • 1
  • 1
  • Check that you have used the correct pin out for the 7912 regulator; it's not the same as the 7812. – Andy aka Apr 02 '21 at 18:27
  • 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? –  Apr 02 '21 at 18:27
  • Please use the site's built-in schematic editor to include a schematic of your circuit. – jwh20 Apr 02 '21 at 18:33
  • @GregoryKornblum I have a 50Hz supply, do you recommend reducing the output load? – Ed Lewis Apr 02 '21 at 18:47
  • @Andyaka I double checked the output pins and they are correct – Ed Lewis Apr 02 '21 at 18:48
  • Well you didn't do such a good job with your schematic because you got that wrong so, if you correct that and apply pin numbers on both regulators, you'll make everyone happy. Other than that your schematic is correct. – Andy aka Apr 02 '21 at 18:52
  • @Andyaka appologies, pin out corrected – Ed Lewis Apr 02 '21 at 18:58
  • "the negative side produces -26V and the positive produces +10V." Where exactly are you measuring these values? – devnull Apr 02 '21 at 19:06
  • @vangelo directly after the rectifier and before the first set of caps – Ed Lewis Apr 02 '21 at 19:08
  • The total voltage (36V), considering almost no load, is ok, but obviously not the asymmetry, considering 12-0-12. If you are using a solder-less breadboard for the test fell free to move this to a chat so we can try to trouble shoot this. – devnull Apr 02 '21 at 19:20
  • @vengelo I have actually made a pcb and have soldered it together. I have checked all joints and potentially shorts and it all looks good. – Ed Lewis Apr 02 '21 at 19:22
  • Verify all gnds. are actually 0V. Then the Vac of each output and expect 10% above 12Vrms with no load. – Tony Stewart EE75 Apr 02 '21 at 19:51
  • Consider adding some detailed pictures of this PCB you've built to your question. – Unimportant Apr 02 '21 at 20:47
  • @EdLewis It's possible that you need to re-check your 7812. If it is an [L7812](https://www.hobbytronics.co.uk/datasheets/L7812.pdf) then your pinout for the TO-220 package may be different than other makers show. I'd like to see a photo of your 7812 and 7912, if possible. – jonk Apr 02 '21 at 23:22
  • @Unimportant pcb added – Ed Lewis Apr 03 '21 at 09:02
  • @jonk They are indeed L7812 and L7912, however I used their specific pin outs in Eagle – Ed Lewis Apr 03 '21 at 09:03
  • Is your transformer ground wired to your PCB ground ? If not it would explain the uneven voltage splitting. – tobalt Apr 03 '21 at 09:19
  • @tobalt The centre tap of the transformer is wire into the middle screw terminal to act as ground – Ed Lewis Apr 03 '21 at 09:25

0 Answers0