I have built a really simple 120VAC to 12VDC 2A power supply. Here is the schematic and the PCB.


simulate this circuit – Schematic created using CircuitLab

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As you can see, the power supply does strange things. The frequency of that bump is about 5 ms and it has a width of about 0.2 ms. I think it is generating inconsistency in my op-amp readings because it gets extremely noisy.

Anyone could help me improve that design with some explanations?

  • \$\begingroup\$ Is this with a load attached? \$\endgroup\$ – evildemonic Nov 17 '17 at 23:26

There's no decoupling capacitors. See the application notes for 78XX regulators. The capacitors are needed for stability and low noise. It's essential to connect them with as short leads as possible to the input and output of the regulator. Use only specified capacitor types.

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    \$\begingroup\$ I'm not convinced an output capacitor would be necessary in this particular circuit. And looking at the board, the filter cap between the rectifier and the LM7812 is probably adequate. I would personally have decoupling caps close to the regulator, just to avoid trouble, but I fear this is not causing his current problem. \$\endgroup\$ – evildemonic Nov 17 '17 at 23:34
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    \$\begingroup\$ decoupling capacitors solved everything, one of 0.33uf at the input of each regulator and 0.1uf at each output .Thank you, solved it right away! \$\endgroup\$ – PyThagoras Nov 17 '17 at 23:51
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    \$\begingroup\$ A 3000uF capacitor is unlikely to have low enough ESR or ESL to provide stability for those load transients. Not having bypass caps on the input or the output of the 7805 makes things even worse. Add small, low ESR caps (0.33uF on the input, 0.1uF on the output) and retest. Just solder leaded caps (leads as short as possible) directly to the bottom of the 7805 pads with your current board and retest. \$\endgroup\$ – Dean Franks Nov 17 '17 at 23:52

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