How much current can I put into my RPi?

The Pi will only draw the current it requires. However there's a polyfuse on the input that limits the possible current. If the Pi plus any usb devices exceeds the threshold then the fuse will 'blow' and the current will be limited.

Supplies of this sort are constant voltage, variable current up to their rated values. So your 5v 2.1A adapter will (in theory) supply a constant 5v at any current between 0 and 2.1A. You're not forceing 2.1A into the Pi, so nothing to worry about. You could happily use a 5v, 100A supply and the Pi would still only draw the 500mA or so that it requires.

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.

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.

True but a high current capable power supply is very unforgiving of any mistakes like shorts. Then you will get in to the track melting situation rather than the over current trip on the power supply being activated.

What is the fuse rated at (or until the track melts)?

I know the USB ports are rated at 150mAh

... but how do I know that zero power is being drawn from the RPi and all the power is coming from my powered hub (when devices are plugged in)?

For this you will need to research the powered hub that you want to use as they do vary from almost no current use to trying to run all devices attached if the power supply to the hub is removed.  If the latter condition is true the system will fail as will the PI, but since the usb power and the PI itself are current limited it should not cause a major problem other than a system failure that can be corrected.

Is there anyway that I can make sure that the RPi's power is not being drawn by the hub (ie. a data only USB cable)?

I attemped to make a data only cable by cutting the Green and White wires in the USB cable, but then the RPi would not recognise it.

I think it would be more inexpensive on you to try a generic powered hub if your app works you are fine if not then try a defferent hub. Of course you have a usb cable with cut power leads, so put a current meter inline and check the current draw. you will have a means to test any hub you try.

Thanks Mike for this suggestion.

I might try this when I get a chance (to go out and buy an ammeter).

Fergus Byrne wrote:

 

Is there anyway that I can make sure that the RPi's power is not being drawn by the hub (ie. a data only USB cable)?

 

I attemped to make a data only cable by cutting the Green and White wires in the USB cable, but then the RPi would not recognise it.

According to Wikipedia's USB article (http://en.wikipedia.org/wiki/USB#Physical_appearance), Green and White are the data pair so you don't want to cut them to make a data-only cable.  You want to cut the Red (VBUS) wire, or whatever color is connected to the +5V connector pins.  Leave the Black (GND) line intact.

The best way to make sure RasPi is not driving a powered hub is to power RasPi directly from the hub using a Standard A to Micro B cable.  This does not always work, because the hub may have current limiting circuitry.  Cheap hubs usually connect a common +5V to all downstream ports, so they'll work.  An expensive hub may limit a port's current to 500 mA to protect the other ports from a bad USB device, and that may not provide enough current for RasPi.  A "Y" cable that combines the current from two downstream hub ports should work in this case.

If RasPi and the hub have independent power supplies, there's no way to ensure that the hub +5V is at least equal to RasPi.  If RasPi is higher, it will drive the hub until F1 or F2 fails.  If the hub is higher, it may drive RasPi if it's a really cheap hub and omits an ORing diode in its upstream port.

You usually cannot tell from a hub's published information what it's going to do -- you have to take it apart and play with an Ohmmeter to see how it's connected up, and whether there's an upstream ORing diode.

John Beetem wrote:

 

Fergus Byrne wrote:

 

Is there anyway that I can make sure that the RPi's power is not being drawn by the hub (ie. a data only USB cable)?

 

I attemped to make a data only cable by cutting the Green and White wires in the USB cable, but then the RPi would not recognise it.

According to Wikipedia's USB article (http://en.wikipedia.org/wiki/USB#Physical_appearance), Green and White are the data pair so you don't want to cut them to make a data-only cable.  You want to cut the Red (VBUS) wire, or whatever color is connected to the +5V connector pins.  Leave the Black (GND) line intact.

 

The best way to make sure RasPi is not driving a powered hub is to power RasPi directly from the hub using a Standard A to Micro B cable.  This does not always work, because the hub may have current limiting circuitry.  Cheap hubs usually connect a common +5V to all downstream ports, so they'll work.  An expensive hub may limit a port's current to 500 mA to protect the other ports from a bad USB device, and that may not provide enough current for RasPi.  A "Y" cable that combines the current from two downstream hub ports should work in this case.

 

If RasPi and the hub have independent power supplies, there's no way to ensure that the hub +5V is at least equal to RasPi.  If RasPi is higher, it will drive the hub until F1 or F2 fails.  If the hub is higher, it may drive RasPi if it's a really cheap hub and omits an ORing diode in its upstream port.

 

You usually cannot tell from a hub's published information what it's going to do -- you have to take it apart and play with an Ohmmeter to see how it's connected up, and whether there's an upstream ORing diode.

Now I know why my data-only cable wasn't working.  I cut the ground and the 5v.  I can check this later now I know what was wrong with my data only cable.

Thank you.