I want to build my own power supply section for my project. The condition is that I want to connect the 220V AC power source to the board and the power section is about to reduce this 220V to 5V, 9V, and 12V using the bridge rectifier.
I am now planning to use 3 linear regulator which are LM7805, LM7809, and LM7812 but I have been wondering for a while about what kind of transformer I have to use to reduce the voltage before supplying it to the bridge rectifier section.
I found that there are DC-DC step down converter but those are for DC not AC.
Best Answer
You can use an appropriately rated bridge rectifier for converting the incoming AC to DC (you'll need a smoothing capacitor too also appropriately rated). Then you can use a DC-to-DC converter PROVIDING it's capable of taking a few hundred volts at its inputs. Many are capable of doing this but more often than not a DC converter might only be rated to use up to 48V DC or less.
If your AC supply is 240 volts then the peak DC voltage after rectification is going to be about sqrt(2) higher or around 340 volts and this doesn't consider over-generation or load drop-outs or lightning etc. Usually the smoothing cap is rated at 450V DC.
Also, the DC converter, if rated for an input appropriate for rectifed AC, will almost certainly provide isolation so it's relatively safe to touch the output terminals.
If you are a beginner though, I'd consider using a step-down transformer - less chance of burning your fingers.