Raspberry Pi Power Limitations

Question

There are very many, often contradictory, claims about the power requirements and limitations of the Pi.

What are the exact requirements?

Answer 1

Power requirements of the Pi

Raspberry Pi Ltd. (Raspberry Pi Ltd.) has recommendations for various models which range from 700 mA to 5.0 A.

These are quite generous, and all models (prior to Pi5) will operate on a decent 1A supply - I can run all my Pi3/Pi3+/Pi4 with WiFi/keyboard/mouse/HDMI from an Apple 5W supply. Extra current may be required by USB peripherals and the recommended supplies make allowance for these.

Power sources SHOULD provide 5±0.25 V and often list a current rating. This is the MAXIMUM current that can be safely drawn without causing the output to drop below its rated voltage. (Of course, very many supplies do not actually meet their published ratings, including many sold by Pi retailers. I have tested a number of PSU (with a dummy load) and have yet to find one which actually delivers the rated voltage at the rated current, except for the Apple 5W iPad Power Adapter.)

Many users worry that that they may "supply too much power" by using a higher rated supply. The Pi will only draw as much current as it requires and can not use more then 2.5 A (Pi3) or 2 A (Pi2/B+) as this is limited by a fuse, so there is no benefit in a higher rated supply. (Earlier models had a smaller polyfuse - probably 1.1 A.)

The newer Pi(3/2/B+) have a voltage monitor chip (APX803) which triggers at 4.63±0.07 V. The Pi3B+, Pi3A+, Pi4B uses a MxL7704 PMIC chip to manage power, which has the same nominal trigger point. (MxL7704 Data Sheet says Voltage rising >4.59 4.63 <4.7 Voltage falling >4.52 4.57 <4.65 .) This controls the Red Power LED.

The MxL7704 PMIC Data states Input voltage range: 4.0 V to 5.5 V.

If the Red Power LED is not illuminated this means the supply voltage is inadequate. (The newer Pi have a well engineered power circuit, and may continue to function even if the input voltage is below spec; the same may not be true of peripherals). The GUI had an rainbow indicator (replaced by a lightning bolt) which comes up in the top right if the voltage is inadequate. This has a 3 second timer, and may display even if the LED appears to be lit.

NOTE the Red Power LED on the Pi3B+, Pi3A+, Pi4B only functions if the SD Card/USB key has up to date firmware because it is controlled by software - it is meaningless otherwise.

You should be wary of cheap USB supplies. Many of these have very poor voltage regulation.

Many modern smartphones are designed to draw more current than the normal USB 500 mA max. Phone manufacturers often supply higher current chargers, either by non-standard means or by adopting the new USB Charger spec, which permits higher currents, but permits voltage to drop to 3.6 V. These are OK for charging smartphones, but NOT for voltage sensitive devices like the Pi. They may appear to work OK for a lightly loaded Pi, but may not if many peripherals are connected.

NOTE If you are having problems (low voltage indicator, or peripherals which are unreliable) This does NOT mean you need a higher current/amp(sic) rating. It is extremely unlikely your supply cannot supply the current - it just cannot supply the required current while maintaining the required voltage.

No matter how good your Power Supply if you use poor quality cables to connect to the Pi you will have problems. Many (the majority?) of μUSB cables are designed to carry data, and have very thin wiring. This makes the cables thin, light and inexpensive but they are unsuitable for power. To remain in spec there should be less than 0.25V drop which corresponds to loop resistance of 0.25Ω @ 1A. Cables designed for charging smartphones are probably the best bet, and always use the shortest possible cable.

Unfortunately there appears to be no source of quality cables with guaranteed specifications (I have been forced to make my own). I have been unable to source Micro USB plugs in small quantities, but have found many sellers on eBay offering Micro USB 5 Pin Male Plug T Port Socket. You need to supply your own strain relief, but coupled with 23/.011mm speaker cable (suitable up to 1.5m) these provide a good connection. I couple with one of the 5V Switch Power Supply Driver Adapter For LED Strip to power several Pi.

How much current can be drawn from the USB ports?

The Pi(3/2/B+) USB Current are supplied through a Current-Limited Power Switch (AP2553?) (U13), although this is not shown on the published schematics.

Maximum total USB peripheral current draw states the max USB current for Pi(2/B+ or later) is 1200 mA. Earlier models claim 500 mA.

The default for 2/B+ is 600 mA which can be doubled by setting max_usb_current=1 in /boot/config.txt. On recent Raspberry Pi OS this is set to 1200 mA.

The USB hub on the B models does not appear to be compliant to the USB specification and does not limit current. Individual ports can supply in excess of 500 mA independent of negotiation, subject to the overall maximum limit and adequate power supply.

How much current can the 3.3V pin on the expansion header supply?

The Pi 3.3 V rail is widely assumed to provide 50 mA, but this is not officially documented for recent Pi models. The original Pi has an on-board linear regulator which was limited, but the B+ and later have a switch mode regulator which can supply more. The regulator chip (which supplies both 3.3 V and 1.8 V) is rated at 1 A. The MxL7704 PMIC used in the Pi3B+, Pi3A+ and Pi4 is rated at 1.5 A.

Tests by a member indicate up to 800 mA can be used - subject to an adequate power supply.

Electrical Specifications of GPIO for best estimates of GPIO limits.

How much current can the 5 V pins supply?

There is no simple answer to this. You can roughly calculate;
min of polyfuse rating (2.5 A for PI3) and power supply rating,
less the current required by the Pi itself (~750 mA for Pi3, although this will increase for heavy use),
less USB peripheral current,
less Camera Module (~250 mA if fitted),
less HDMI port (~50 mA),
less Display (if fitted),
less 3.3 V current supplied to external devices (including GPIO).

Can the Pi be powered through the expansion header?

There are many good reasons to power through the expansion connector e.g using battery supply or powering multiple Pi from a single supply. There is no risk if you apply proper engineering practices. Indeed the Foundation Hats Master has recommendations and minimum requirements for such connection.

"It is possible to power the Pi by supplying 5 V through the GPIO (sic) header pins 2,4 and GND. The acceptable input voltage range is 5 V ±5%. ⋯ Implement a duplicate power safety diode ⋯ supply 5 V at a minimum of 1.3 A ⋯ Under no circumstances should a power source be connected to the 3.3 V pins."

NOTE the Hats Master has been updated for newer models, and includes suggested circuitry.

If you are using a Pi Zero the use of a power safety diode is probably superfluous, as the Zero does not have one, or indeed any protection circuitry.

Technical details

The release of the Pi3B+ has been followed by a description of its Power Circuitry which includes comments on earlier models. The PEN pin (next to RUN) on the J2 header is connected to the Global Enable on the power module. Pulling this low should reduce Pi current to a couple of mA.

For those interested in understanding the Pi power configuration see the Raspberry Pi3 Schematic. Schematics for other models are available. This can be a little intimidating, even for those used to these kind of things. There is a rather more readable description of the Raspberry Pi B+ which is similar. The H5V supplies the HDMI power.

A brief verbal description:-

Power In from the μUSB connector goes through a polyfuse and an ideal diode to provide 5 V which is the 5V rail on the expansion header and provides all power to the Pi including the following

• a dual step-down converter (PAM2306) which provide 3V3 and 1V8
• via a MOSFET to 5V_CORE and a Step-Down Converter (RT8088A) which generates the VDD_CORE (nominal 1.2 V). This was performed by the SOC in the original Pi.
• voltage monitor chip (APX803)
• Power Switch (RT9741) which produces H5V for HDMI

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NOTE On the B+ and Pi2 the "PWR" LED was connected to a GPIO pin as was the APX803 voltage monitor chip. This let the the Pi detect under-voltage OR control the LED.

The MOSFET controlling the Pi3 "PWR" LED is directly connected to the APX803 (which is open drain) so will ALWAYS be off if the voltage is low, but if not it should be possible to pull it low (and turn the LED off) with a program. The latest Raspbian (using kernel 4.9) seems to have restored access to /sys/class/leds/led1 which can be used to control the PWR LED on the Pi3.
Recent kernels support vcgencmd get_throttled bit0 indicates under-voltage

Answer 2

The Raspberry Pi Foundation recommendation has always been 5V plus or minus 0.25V.

In practice the Pi works from just over 3.3V to just under 6V. Personally I would limit the voltage to less than 5.8V.

Of course things you connect to the Pi may have a more limited voltage range.

The maximum current draw from the 5V and 3V3 rails depends on the Pi version and how it's powered (if powered via the microUSB all but the Pi Zero have a polyfuse fitted).

If you power via the expansion header use a 5V and ground pin.