3v3 Power

All Raspberry Pi models since the B+ can provide up to 500mA on the 3v3 pins, thanks to a switching regulator. In some cases it may be possible to draw more but, due to lack of documentation and testing on the actual limits, 500mA is given as a rule of thumb.

The 3v3 supply pin on the early Raspberry Pi had a maximum available current of only 50mA.

The 5v supply coupled with a 3v3 regulator is recommended for powering 3.3v projects.

The Piversify blog has an exploration of the 3v3 supply rail on the Raspberry Pi B+