How much current can I put into my RPi?

Millard Watson II wrote:

 

Using a larger source curent capable supply will not cause any problem, as the PI will draw only the current that it needs.

 

The usb ports are current limited, so the problem there is that you WILL need a powered hub if you need to use power hungry devices.

 

The power supply you use MUST be regulated as the voltage must be 5.0V +/- 0.2V.

If this is the case, how come the documentation that came with the Raspberry Pi says specifically a "maximum" of 700mA for model A and 1200mA for model B? Please don't take this as an attack, I am a newb.

The Max. current is the design max draw of the PI not the max capabilaty of the supply. In fact this could be read as a minimum spec. for the supply

So what does this mean in terms of maximum current draw?

If I use a 2.1A power supply and then using the 5v GPIO pin want to draw 500mA for a motor, what will happen?  Will it be safe?

That is going off the wiki which states:

     Power consumption of the Raspberry Pi device is

• • Board A: 5V, 500 mA (2.5W) without any devices connected (e.g. USB, Ethernet, HDMI)
  • Board B: 5V, 700 mA (3.5W) without any devices connected (e.g. USB, Ethernet, HDMI)

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

     Maximum permitted current draw from the 5 V pin is the USB input current (usually 1 A) minus any current draw from the rest of the board.^[13]

• • Model A: 1000 mA - 500 mA -> max current draw: 500 mA
  • Model B: 1000 mA - 700 mA -> max current draw: 300 mA

Change the 1A above to 2.1A and you end-up with max current draw = 1400mA

Can someone please clarify this.

Fergus Byrne wrote:

 

So what does this mean in terms of maximum current draw?

 

If I use a 2.1A power supply and then using the 5v GPIO pin want to draw 500mA for a motor, what will happen?  Will it be safe?

 

That is going off the wiki which states:

 

     Power consumption of the Raspberry Pi device is

• • Board A: 5V, 500 mA (2.5W) without any devices connected (e.g. USB, Ethernet, HDMI)
  • Board B: 5V, 700 mA (3.5W) without any devices connected (e.g. USB, Ethernet, HDMI)

    

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

     Maximum permitted current draw from the 5 V pin is the USB input current (usually 1 A) minus any current draw from the rest of the board.^[13]

• • Model A: 1000 mA - 500 mA -> max current draw: 500 mA
  • Model B: 1000 mA - 700 mA -> max current draw: 300 mA

 

Change the 1A above to 2.1A and you end-up with max current draw = 1400mA

 

 

Can someone please clarify this.

If you are powering an unmodified RasPi through its Micro USB connector S1 you are limited to 1A by polyfuse F3.  If you try to draw more than 1 A (e.g., 700 mA for RasPi + 500 mA for your motor = 1.2A total), the fuse may trip.  However, before it does so the voltage drop across your Micro USB cable and through F3 may cause 5V0 to fall below 4.75V, causing other problems such as your monitor and/or attached USB devices malfunctioning.  F3's trip current and time delay depend on temperature and vary from one polyfuse to the next.  The voltage drops across the Micro USB cable and F3 are also variable.

There are various ways to solve this.  Some people have powered their RasPis through the GPIO connector P1, bypassing F3.  However, by doing so they have lost all protection from F3 so if they accidentally hook up the wrong voltage or reverse their voltage leads they can easily destroy their RasPis and any attached peripherals.  Another solution is to provide separate 5V cables for RasPi and for your motor, perhaps using an externally-powered hub as the common power source.  Make sure that 5V to the motor doesn't have a sneak path back to the RasPi through P1 or you've lost F3's protection.

John Beetem wrote:

 

Fergus Byrne wrote:

 

So what does this mean in terms of maximum current draw?

 

If I use a 2.1A power supply and then using the 5v GPIO pin want to draw 500mA for a motor, what will happen?  Will it be safe?

 

That is going off the wiki which states:

 

     Power consumption of the Raspberry Pi device is

• • Board A: 5V, 500 mA (2.5W) without any devices connected (e.g. USB, Ethernet, HDMI)
  • Board B: 5V, 700 mA (3.5W) without any devices connected (e.g. USB, Ethernet, HDMI)

    

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

     Maximum permitted current draw from the 5 V pin is the USB input current (usually 1 A) minus any current draw from the rest of the board.^[13]

• • Model A: 1000 mA - 500 mA -> max current draw: 500 mA
  • Model B: 1000 mA - 700 mA -> max current draw: 300 mA

 

Change the 1A above to 2.1A and you end-up with max current draw = 1400mA

 

 

Can someone please clarify this.

If you are powering an unmodified RasPi through its Micro USB connector S1 you are limited to 1A by polyfuse F3.  If you try to draw more than 1 A (e.g., 700 mA for RasPi + 500 mA for your motor = 1.2A total), the fuse may trip.  However, before it does so the voltage drop across your Micro USB cable and through F3 may cause 5V0 to fall below 4.75V, causing other problems such as your monitor and/or attached USB devices malfunctioning.  F3's trip current and time delay depend on temperature and vary from one polyfuse to the next.  The voltage drops across the Micro USB cable and F3 are also variable.

 

There are various ways to solve this.  Some people have powered their RasPis through the GPIO connector P1, bypassing F3.  However, by doing so they have lost all protection from F3 so if they accidentally hook up the wrong voltage or reverse their voltage leads they can easily destroy their RasPis and any attached peripherals.  Another solution is to provide separate 5V cables for RasPi and for your motor, perhaps using an externally-powered hub as the common power source.  Make sure that 5V to the motor doesn't have a sneak path back to the RasPi through P1 or you've lost F3's protection.

Thank you for explaining this.

What is the maximum current pull without PolyFuses allowed on a single USB port? Is it 500mA (the maximum allowed due to bus wieth)? or a limit of the power supply?

Fergus Byrne wrote:

 

What is the maximum current pull without PolyFuses allowed on a single USB port? Is it 500mA (the maximum allowed due to bus wieth)? or a limit of the power supply?

I assume you're asking how much current a USB port can draw when you remove polyfuses F1 and F2.  This ends up being a complicated answer.  As in my last answer, if you're providing power to RasPi through Micro USB power jack S1 then polyfuse F3 limits you to 1A for all RasPi chips (approx 500 mA) plus whatever GPIO connector P1 is drawing plus whatever your USB devices need.

If you're powering RasPi through P1, then you're limited by the current-carrying capacity of PCB traces.  For example, if the weakest PCB trace can only carry 1.5A then that trace will become a non-resettable fuse if your current exceeds that limit.  We don't know what the limits are because we don't have Gerber plots of the PCB layers and don't know how thick they are.  It's just as well, because each PCB revision would be different.

With fuses F1 and F2 gone or made Ohmless, you also have the possibility of powering RasPi through a downstream Type A USB port (S7).  While this is a violation of the standard use of a downstream USB port, it is done in practice sometimes by oversight and sometimes deliberately.  Drew Fustini describes a wonderful RasPi laptop using a Motorola Atrix Lapdock.  The Lapdock acts as a USB hub but also acts as a cell phone charger.  So it provides current the wrong way from the hub (inside the Lapdock) to RasPi.  In this case, the current from the Lapdock powers P1 and RasPi's other USB port.  You are limited by the Lapdock's current capacity and by PCB traces.

Note that if you power RasPi through S7 with Ohmless F1+F2, you've lost all F3 protection so you must trust the Lapdock or other non-compliant hub to behave properly.  However, with a rev 1.0 RasPi PCB you can replace F1+F2 with larger capacity fuses (say 1.0A) and get your protection back.

With fuses F1 and F2 gone or made Ohmless, you also have the possibility of powering RasPi through a downstream Type A USB port (S7).

Is this really possible?  I had a USB 4 port Hub (powered) and when I plugged it into the Raspberry Pi USB port, some of the power went back into raspberry Pi and started the boot process but it failed after 1 second.

From what your saying, This would succeed if I remove the the F1 and F3 fuses.

Fergus Byrne wrote:

 

 

With fuses F1 and F2 gone or made Ohmless, you also have the possibility of powering RasPi through a downstream Type A USB port (S7).

 

Is this really possible?  I had a USB 4 port Hub (powered) and when I plugged it into the Raspberry Pi USB port, some of the power went back into raspberry Pi and started the boot process but it failed after 1 second.

 

From what your saying, This would succeed if I remove the the F1 and F3 [I think you mean F2] fuses.

By "fuses F1 and F2 gone", I'm actually referring to RasPi 2.0 which -- as far as we can tell without schematics -- directly connects RasPi 5V0 to the downstream USB VBUS pins.  You can get the same effect on RasPi 1.0 by shorting out F1 and F2.  One way to do this is with 0 Ohm resistors that are the same size as F1 and F2, as RasPi has done with some recent boards.  You can also short them out with pieces of wire, leaving F1 and F2 in place.  You can also short them out using a higher-current fuse in parallel, which has the advantage of leaving in some protection.

If you remove F1 and F2 from a rev 1.0 RasPi, you won't get any connection between RasPi 5.0V and your hub's 5.0V, so you will need to provide separate external power for RasPi and your hub.

Your "reboot in 1 second" behavior is well known.  What happens is that the SoC can get enough current to start the boot process, but when it gets under way and needs more than 140 mA, the voltage drop across F1 or F2 (depending on which USB port your hub is plugged into) gets too high and 5V0 drops too low to keep the SoC alive, so it reboots.

So if you're adventurous and don't mind voiding your RasPi's warranty, shorting out F1 and/or F2 and powering RasPi through your hub's upstream connection will probably work.  After all, that's what's RasPi is allowing officially with the 0 Ohm resistor change.

Also, this will only work with some hubs.  A hub is not supposed to send current upstream.  My hub has a diode to prevent this from happening.  However, there seem to be quite a few hubs out there that do source current upstream, so RasPi decided to fuggeddabouddit and short out F1+F2 rather than de-list a bunch of hubs and listen to complaints.

This is what I mean:

Is this what you are also talking about?