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I've been trying to make a dual power supply from AC signal to power some op-amps, but the value I'm getting is very far from even. I'm getting +15.5V from the positive pin of the rectifier and -7.7V from the negative pin of the rectifier.

I'm using the schematic in the first picture as a reference. I thought it could be the voltage regulator I was using (I tried with TA79021S and other combinations of these same voltage regulators I had, but the difference got worse). Also tried it on another breadboard and with other capacitors. I do not have another AC power at hand. Also tried with another rectifier.

By now I'm wondering what I'm doing wrong its either my interpretation of the schematics or the building of the circuit itself. And I can go further in my project if I do not power the op-amps.

Datasheet for UA7812C and for LM7912. Also for the rectifier.

Schematics (first picture) AC power supply (second picture) Circuit from above: right side is the +, left side is the - (third picture) Circuit from the + supply perspective (fourth picture) Circuit from the - supply perspective (fifth picture)

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  • \$\begingroup\$ Please simulate your circuit. \$\endgroup\$
    – winny
    Commented Apr 7, 2023 at 15:09
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    \$\begingroup\$ How many wires come from your AC adaptor? How many power wires does your regulator circuit need? \$\endgroup\$
    – Andy aka
    Commented Apr 7, 2023 at 15:11
  • \$\begingroup\$ @Andyaka two wires come from the adaptor, I'm linking them to both legs of the rectifier (since I do not have a transformer) \$\endgroup\$
    – ludicrous
    Commented Apr 7, 2023 at 15:19
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    \$\begingroup\$ How many power wires does your regulator circuit need? \$\endgroup\$
    – Andy aka
    Commented Apr 7, 2023 at 15:21
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    \$\begingroup\$ That's the wrong answer. You need three power wires to make this circuit work. See @Justme's answer. \$\endgroup\$
    – Andy aka
    Commented Apr 7, 2023 at 15:28

2 Answers 2

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The first problem is that the schematic is for a center tapped transformer output which connects with three wires to the circuit.

Your AC adapter has two output wires so the schematics won't work with it.

Also there is no load on the regulators and at least the 7912 will not output stable 12V with no load.

If you want to use an AC adapter with two output wires, you need to modify the circuit to work with it, basically having two half-wave rectifers.

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In addition, in this photo:

Bad Electrolytic cap

The electrolytic capacitor in the bottom-left appears to have been damaged and the top has split open. Whenever one of these "aluminum can" capacitors is opened, it fails (or soon will) and needs to be recycled. This is because they have a special liquid inside (called an electrolyte) which evaporates.

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