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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.

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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.

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  • \$\begingroup\$ I have searched for this in the internet, and that may be the only option for now not to burn my finger 55 \$\endgroup\$ – Chatthana Janethanakarn Sep 6 '14 at 3:44
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I won't get into details (since there are more things to consider that one might think to get it absolutely right and efficient) and just give a quick overview to get started. Lets begin with the transformer. When choosing a transformer there are three main requirements to consider

  1. Power rating - What are your load requirements?
  2. Primary input voltage - 220V in your case
  3. Secondary output voltage(s) - discussed below

So what output voltage should you choose for your example? Lets look at your requirement to generate 12V DC using an linear regulator. Since the regulator has some internal voltage drop, we should consider feeding it at least 14V DC as input voltage.

Lets take a quick look at how we would get a DC voltage from the transformers secondary:

enter image description here

So in this example, there are V1 = 12V AC (transformer secondary) going through a full wave rectifier. The rectified output is then smoothed by a capacitor (ignore R1 for this example). Why is the output voltage shown as 15V? The actual peak voltage on the transformer is higher than the given RMS average (peak V is theoretically ~1.414 times the RMS voltage). So we would get about

12V * 1.414 = 16.97V

minus the drop from the diodes (about 0.7V for each polarity). So we end up with

16.97V - 2 x 0.7V = 15.57V

This would mean that the linear regulator would have to drop about 3.5V which could result in significant power dissipation, depending on your load current (3.5W @1A). So one should consider choosing a lower output voltage on the secondary.

What about the other two voltages, 9V and 5V? Well, one way would be to just feed the 12V DC output from the first regulator to the input of the 9V regulator and use that one to supply the 5V regulator. However, this would waste plenty of energy (again, depending on your load current). A more efficient option would be to use a custom transformer with 3 secondary windings, each feeding a carefully matched voltage to each of your regulators.

Be careful to select an capacitor with an appropriate voltage rating and low ESR. The output voltage of a transformer can be much higher under no-load conditions. So a 16V rating would not be sufficient for C1.

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  • \$\begingroup\$ Thank you very much, this helps me a lot. So i have more things to consider... \$\endgroup\$ – Chatthana Janethanakarn Sep 6 '14 at 3:42

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