This circuit doubles the input voltage, and the maximum current depends on the transistors used. For BD135 we have something like 100 mA. The maximum input voltage is of the order of 15 V, the diodes can be 1N4004 and capacitors from 100uF to 470uF, depending on the output current. R1 is 47k, R2 and R3 is 4k7.
