2 Replies Latest reply on Aug 22, 2012 8:42 PM by John Beetem

    Is maximum current from 3.3V pin really 50mA?

    Drew Fustini

      Hi - I've seen on the eLinux wiki that:

       

      http://elinux.org/RPi_Low-level_peripherals#Power_pins

      The maximum permitted current draw from the 3.3 V pin is 50 mA.

       

      I saw this reiterated in a recent post:

       

      http://www.element14.com/community/message/58840#58828

      50 mA is for the 3.3V power pin (P1-01) which is the output of RasPi's 3.3V regulator RG2.  You can use it for +3.3V ICs and pull-ups on an external board provided that the total current is under 50 mA

       

      On the Raspberry Pi schematic, I can see that RG2 is a NCP1117 along with a note that the max dissipation is 800mW.  I think this would indicate the maximum current that one should draw is ~240 mA.

       

      Anyone know how the 50 mA value is derived?

       

      Thanks,

      Drew

       

      (the reason for this question is that I like to power 3.3V I2C devices from the Pi's 3.3V pin.  Simon Monk pointed out to me he used the 5V pin and a logic level converter for his I2C 7-segment display tutorial becuase he was worried about the 50 mA current limit on 3.3V pin.  The I2C 7-segment display can run off 3.3V, so it seems ashame to have use a logic level converter just because the 5V pin provides more current).

       

      UPDATE: I just realized that additional regulators seem to be fed from the NCP1117, so I guess that would make sense the 3.3V rail can't use the whole 800mW dissipation.  But I'm still wondering how that 50mA for 3.3V was calculated.  I'm only using serial port when using these 3.3V I2C devices so I hope this would lessen the demand on the other rails (no ethernet connected, no usb connected, no hdmi connected).