If I use a powered USB hub with my RPi ModelB, as well as plugging in the RPi's normal mini-USB power cord, will the 2 power sources work in combination with each other as far as amperage is concerned?
For instance, the Pi board draws 700mA. But my USB hub's power supply is only 500mA. I'm assuming using just my USB hub's power supply, at 500mA, won't be enough to power both the Pi and my USB devices.
But if I also plug in the Pi's own mini-USB power cord (1A), would this give me an effective combined total of 1.5A (1A + 500mA) or is this not how the Pi works, power-wise?
I was curious to find this out before I invest in a more expensive powered USB hub.
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FTrevorGowen
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Re: USB hub power supply with RPi's adaptor?
Unless you're certain that your "powered" USB hub does not "back-power"** it's probably not a good idea to use two separate power supplies. A good powered (4-port) hub should have (be used with) a 1.5 - 2A PSU, since all four ports may require 500 mA. My personal preference is to use a 1.5A or 2A "multi-port" USB power supply to power the Pi and hub via separate leads. Another, single supply option, is to use a powered hub that has a"charging port" (which can supply more than 500mA) and power the Pi via that. Of course, if you're not concerned about bypassing the Pi's polyfuse and overvoltage protection you can use a single supply with a "back-powering" hub and power both that way. W.r.t. total current flow and the relative currents from each of two separate supplies, you also have to take account of the exact voltage of each supply and any effective series resistances (internal, cable, polyfuse etc.) so it's not a "simple sum".brgr88 wrote:If I use a powered USB hub with my RPi ModelB, as well as plugging in the RPi's normal mini-USB power cord, will the 2 power sources work in combination with each other as far as amperage is concerned?
For instance, the Pi board draws 700mA. But my USB hub's power supply is only 500mA. I'm assuming using just my USB hub's power supply, at 500mA, won't be enough to power both the Pi and my USB devices.
But if I also plug in the Pi's own mini-USB power cord (1A), would this give me an effective combined total of 1.5A (1A + 500mA) or is this not how the Pi works, power-wise?
I was curious to find this out before I invest in a more expensive powered USB hub.
Trev.
** Examples of "non-back-powering" hubs (which meet the USB specs.) and "back-powering" hubs (which don't) can be found here:
http://www.cpmspectrepi.webspace.virgin ... esChk.html
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, 2xB2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, and a A+) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm
Re: USB hub power supply with RPi's adaptor?
If your USB hub has a power supply of only 500 mA, if I were in your shoes, I would do one of two things:
1) replace the hub's power supply with one that supplies at least 2A. ~1A for the Pi, and ~1A for all of your peripherals. Use that to back-power the Pi. Note that this bypasses the polyfuses that protect the board from drawing too much current, so if there's a risk of shorting the 3.3 or 5v GPIO pins to ground, this might not be the best option for you, unless you install similar polyfuses in the USB hub.
2) Use the 1A Pi power supply, and disable the backpower on the USB hub. To do this, you can open up the hub and clip the red wire that supplies 5v to the host device (RasPi.) Alternately, you could block the lead at the end of the cable from making contact with something like electrical tape, but good luck positioning it just right so that that pad and no others are blocked, and not having it move around on you.
1) replace the hub's power supply with one that supplies at least 2A. ~1A for the Pi, and ~1A for all of your peripherals. Use that to back-power the Pi. Note that this bypasses the polyfuses that protect the board from drawing too much current, so if there's a risk of shorting the 3.3 or 5v GPIO pins to ground, this might not be the best option for you, unless you install similar polyfuses in the USB hub.
2) Use the 1A Pi power supply, and disable the backpower on the USB hub. To do this, you can open up the hub and clip the red wire that supplies 5v to the host device (RasPi.) Alternately, you could block the lead at the end of the cable from making contact with something like electrical tape, but good luck positioning it just right so that that pad and no others are blocked, and not having it move around on you.