Hi Mahmood-One reason you see circuits start with 12 volts and then step down to 5 volts is that 12 volts is needed elsewhere in the circuit. If it is not needed then you can start with a lower voltage.You still need a regulator though. A 5 volt transformer will not always put out exactly 5 volts. Transformers usually put out slightly higher than their rated voltage when there is no load or a light load, and the voltage will drop as the load increases. Microcontrollers require a closely regulated voltage. There are still other things to consider too. Let's say you started with a transformer that is rated 5 volts but it actually puts out 5.5 volts under a light load. Remember that the transformer's voltage is the RMS (root-mean-square) value of the AC voltage. When you rectify the output voltage you will end up with the peak value of the AC wave minus the voltage drop of the rectifier. Assuming a bridge rectifier, the current will be flowing through two silicon junctions at a time. If each one drops 600 millivolts, then the voltage on the filter capacitor will be 5.5 (volts) x 1.414 (the peak value) minus 1.2 volts (rectifier drop), which equals 6.577 volts. But wait- there's still another problem- the voltage on the filter capacitor will not be a smooth DC voltage. At the peaks of the AC wave the capacitor will be charging, but in between the peaks the voltage will drop a bit. This is called "ripple". Making the capacitor bigger will reduce the ripple, bu you can never completely eliminate it. That's where the regulator comes in. You must always supply the regulator with a voltage higher than the desired output voltage plus a bit more No regulator is perfect, so you must allow a bit of headroom, often referred to a "dropout voltage" (you'll find this value on the regulator spec sheet).I hope this answers your question. If not, please feel free to ask for clarification.-Rick
