Hardware Reference
In-Depth Information
Peripheral Power
Each additional circuit that draws power, especially USB peripherals, must be considered
in a power budget. Depending on its type, a given USB peripheral plugged into a USB 2
port can expect up to 500 mA of current, assuming it can obtain it. (Pre Rev 2.0 USB ports
were limited to 140 mA by polyfuses.)
Wireless adapters are known to be power hungry. Don't forget about the keyboard
and mouse when used, since they also add to the power consumption. If you've attached
an RS-232 level shifter circuit (perhaps using MAX232CPE), you should budget for that
small amount also in the 3 V supply budget. This will indirectly add to your +5 V budget,
since the 3 V regulator is powered from it. (The USB ports use the +5 V supply.) Anything
that draws power from your Raspberry Pi should be tallied.
Model B Input Power
The Raspberry Pi's input voltage is fixed at exactly 5 V (±0.25 V). Looking at the schematic
in Figure
2-2
, you can see how the power enters the micro-USB port on the pin marked
VBUS. Notice that the power flows through fuse F3, which is rated at 6 V, 1.1 A. If after
an accidental short, you find that you can't get the unit to power up, check that fuse with
an ohmmeter.
Figure 2-2.
Model B Rev 2.0 input power
If you bring the input +5 V power into the Pi through header P1, P5, or TP1, for
example, you will lose the safety of the fuse F3. So if you bypass the micro-USB port to
bring in power, you may want to include a safety fuse in the supplying circuit.
Figure
2-3
shows the 3.3 V regulator for the Pi. Everything at the 3.3 V level is
supplied by this regulator, and the current is limited by it.
