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

# Is maximum current from 3.3V pin really 50mA?

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).

• ###### Re: Is maximum current from 3.3V pin really 50mA?

Hi

i have just done some testing and from what i can see there is used around 200-250mw from the RG2

See my post here http://www.element14.com/community/message/58876#58876

Tooms

• ###### Re: Is maximum current from 3.3V pin really 50mA?

Drew Fustini wrote:

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).

I would think the 50 mA limit assumes everything else on RasPi is running at max current, so if you're not using Ethernet, not using HDMI (therefore not using GPU), and running a smallish program so you're running mostly out of cache, you probably have plenty of extra current -- at least 100 mA, more likely 200 mA.  RG2 can actually deliver 1.0A, which is enough to blow polyfuse F3.  My guess is that you'll be limited by how hot RG2 gets, which may in turn heat up F3, increasing its resistance, which then lowers 5V which then cools RG2, so you'll probably settle at some equilibrium.