Power supply circuit capacitor selection
Why C1 and C2 selected 2200 uF but C3 and C4 selected 1000uF ? (C3 is 1000 uF picture is wrong)


C1 and C2 are in series with the negative supply. The impedance is given as \$ Z_C = \frac {1}{2 \pi fC} \$ and for 2200 &mu;F at 50 Hz that's just under 1.5 &Omega; each. (See Capacitor impedance calculator.) If you tried to pull 1 A from the negative supply you would drop 3 V across the capacitors so your supply would not be symmetrical.

C3 and C4 are holding the voltage up between half-cycles of the supply. For modest levels of ripple the ripple will be given by \$ V_r = \frac {I_{load}}{2 f C} \$. See Electronics notes for more.


  I tried to use 220uF and 100uF and voltage levels were same I think C3 and C4 for stabilizing the output but what's the purpose of C1 and C2 ?


C1 and C2 create an isolated supply for the negative rail since you haven't got a centre-tapped transformer.


  Also if capacitor selection only effects on output current , is there equation for it ?


Given.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A much simpler supply.

You can make a much simpler supply as shown in Figure 1. This won't suffer the voltage drop in the negative rail that your circuit does and will give symmetrical voltages if the currents drawn are symmetrical.