power supply
Hi kunwart.
First you need a transformer to step down the 220 volts AC to say about 28 to 30 volts AC and a current of 1amp or so would do, also stated as ( VA ) volt amps, 25 to 30 would do fine, next you need to build a bridge rectifier to covert the AC to DC which will be say 28 volts DC, from there it needs smoothing or filtering with an electrolytic capacitor, say 40 volts 2200uf.
After that comes the voltage regulator a 78L24 this is a 24 volts + fixed voltage regulator at a current maximum of 1 amp, so ok as you only want 300 milliamps.
Then you need a couple of ceramic disk or other decoupling capacitors, and then a 40 volt 100uf capacitor or a bit less capacitance, but same voltage 35 to 40 volts is fine.
Finally a load resistor on the output say 1k ohm 0.5 watts.
You must fuse the input on the live rail, and its best to fuse the out put on the live rail.
All the above would keep you busy for a wile, if your set on building a power supply.
One of the fixed voltage regulator circuits using the components above will be fine.
Or if its just to get a quick power supply, then there are plug in power adapters, but for 24 volts there not all that cheap, which way to go is up to you.
Power supply link below.
Dave.
The circuit below is an example, the 35 to 40 volts 100uf capacitor can be left out, or drop the capacitance.
You really need a transformer, or opt for a plug in adapter.