holowey
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"Two-way" power dangerous?

Thu Oct 18, 2012 9:34 pm

I got my RPi and used genuine Samsung charger originally meant for Galaxy S2 (provides 5V 700mA). Just connected power, keyboard, mouse and UTP cable. It booted up just fine. It's not as fast as I expected but that's another story.
However, I wanted to use my hard drive so I bought powered USB hub. Just as a precaution, I shut down system, unplugged my charger, connected the hub into one of the two RPi's USB ports and then first plugged USB hub's power adapter before I would plug in my SGS2 charger.
I was quite surprized when my RPi started booting without a need to plug in my SGS2 charger. It seems that my powered USB hub back-feeds power to RPi through USB port. Is it dangerous? Should I discard this USB hub? Or should I plug in SGS2 charger as well?
Specifications of the USB hub read that each of the USB ports provides 450mA of power. Don't have a Voltmeter to check it, but I'm puzzled how come that RPi boots if it requires at least 700mA?
I do have couple more questions regarding power issue but I'll get back to it when someone clears above mentioned problem. Thank you in advance.

Wheel_nut
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Re: "Two-way" power dangerous?

Thu Oct 18, 2012 10:18 pm

Without knowing the detailed design of your hub, it is difficult to answer why the "Specifications of the USB hub read that each of the USB ports provides 450mA of power." and yet it powers the RPi by back feeding the USB Port.

However, I can tell you that most cheap Hubs have NO current limit to each port and simply distribute the +5V bus to all ports. Some Hubs have a 500mA Polyfuse in series with each Port +5V line but even that is a pretty coarse limiter.

Also, The RPi will boot quite comfortably from a 500mA Power source. It is specified at 700mA on the basis that each USB Port can draw 100mA wiithout problem. Note also that on the Rev 2 Board and latter Rev 1 Boards, the 100mA Polyfuses at the USB Ports have been omitted and bridged.

holowey
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Re: "Two-way" power dangerous?

Thu Oct 18, 2012 10:37 pm

So, what is the verdict? Should I keep it as it is? Or should I plug in SGS2 charger as well? I'm thinking 450mA from the hub plus 700mA from the SGS2 charger would be within 1200mA max power, it would just come from different port. Is this redundancy possible at all?

Wheel_nut
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Re: "Two-way" power dangerous?

Thu Oct 18, 2012 11:02 pm

If you Hub is powered by a 2Amp Power Supply AND your Hub does NOT have Polyfuses or other Current limiting devices on its ports then power the RPi solely from the Hub.

Having said that, there is not a lot wrong with also plugging in your MicroUSB PSU as long as you power the Hub first; the hazard being that you have power hungry devices on your Hub that would draw current from the MicoUSB PSU through the RPi USB port until the Hub PSU was plugged in. This would likely blow the 1 Amp Polyfuse just inboard of the MicroUSB Socket.

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mahjongg
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Re: "Two-way" power dangerous?

Thu Oct 18, 2012 11:10 pm

This is only okay when you have a revision two board, with mounting holes, the older revision 1 board had polyfuses in between the PI and USB port which would prohibit backfeeding.

But with the new revision 2 Board this way of powering the PI is allowed, as the PI only uses 500mA because it doesn't have to power USB devices. As such its quite safe, as the PI can be considered just as any other 500mA consuming device. Cheap hubs in general do not have overcurrent protection on their ports anyway. And if you try powering a Hard-disk with it, the hard-disk will quite possibly consume more than 500mA. Just don't power the hub with a PSU that can deliver more than say 3A and it should be reasonably safe.

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redhawk
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Re: "Two-way" power dangerous?

Thu Oct 18, 2012 11:16 pm

You should never drive the PI from a USB powered hub because there's no polyfuse protection and may contain ripples in the power supply.
To prevent the hub from powering the PI you need to open it up and cut the "red" wire (the +Ve / +Vcc connection).

Richard S.

JustPat
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Re: "Two-way" power dangerous?

Fri Oct 19, 2012 1:11 am

mahjongg wrote:This is only okay when you have a revision two board, with mounting holes, the older revision 1 board had polyfuses in between the PI and USB port which would prohibit backfeeding.

But with the new revision 2 Board this way of powering the PI is allowed, as the PI only uses 500mA because it doesn't have to power USB devices. As such its quite safe, as the PI can be considered just as any other 500mA consuming device. Cheap hubs in general do not have overcurrent protection on their ports anyway. And if you try powering a Hard-disk with it, the hard-disk will quite possibly consume more than 500mA. Just don't power the hub with a PSU that can deliver more than say 3A and it should be reasonably safe.
Hmm, so knowing (assuming) that the polyfuses you mention are the only thing stopping you from backfeeding power to the pi, would it be possible to solder a connection around the fuses? Also, what are the side effects of backfeeding power without the micro USB plugged in on a Rev 1 board?

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Lob0426
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Re: "Two-way" power dangerous?

Fri Oct 19, 2012 2:52 am

JustPat wrote:
mahjongg wrote:This is only okay when you have a revision two board, with mounting holes, the older revision 1 board had polyfuses in between the PI and USB port which would prohibit backfeeding.

But with the new revision 2 Board this way of powering the PI is allowed, as the PI only uses 500mA because it doesn't have to power USB devices. As such its quite safe, as the PI can be considered just as any other 500mA consuming device. Cheap hubs in general do not have overcurrent protection on their ports anyway. And if you try powering a Hard-disk with it, the hard-disk will quite possibly consume more than 500mA. Just don't power the hub with a PSU that can deliver more than say 3A and it should be reasonably safe.
Hmm, so knowing (assuming) that the polyfuses you mention are the only thing stopping you from backfeeding power to the pi, would it be possible to solder a connection around the fuses? Also, what are the side effects of backfeeding power without the micro USB plugged in on a Rev 1 board?
Yes you can jumper the polyfuses and then safely back-power your RasPi. There are some problems with it, but they all can be easily overcome. Most hubs are designed to not provide power back through the uplink port. They can usually accept power through the uplink so they can be used as an unpowered hub.

If you would like more information on back-powering:
http://www.raspberrypi.org/phpBB3/viewt ... 41&t=16473
I have been experimenting with it for a while now!
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holowey
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Re: "Two-way" power dangerous?

Fri Oct 19, 2012 4:32 pm

redhawk wrote:You should never drive the PI from a USB powered hub because there's no polyfuse protection and may contain ripples in the power supply.
To prevent the hub from powering the PI you need to open it up and cut the "red" wire (the +Ve / +Vcc connection).

Richard S.
I can't open the hub, I tried but failed, so this option of cutting the red wire is out.
The hub's charger output provides 5V 2100mA, so if it gives 450mA for each of the four ports, plus backfeeding additional 450mA there got to be some power shortage if I plug in two external drives. Or not?
The hub, if not powered, works just as any other hub: I plugged in mouse and USB memory stick and it's working fine.
So, from one of the previous posts I understood that if I plug in powered USB hub first and then plug in SGS2 charger, as long as hub's power adapter is only 2100mA, it would work. I'm just cautious not to fry the RPi.

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Burngate
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Re: "Two-way" power dangerous?

Fri Oct 19, 2012 4:57 pm

Using both PSUs is unlikely to fry your Pi

Also, If you are using the hub for your keyboard and mouse as well as other items, it's unlikely that the total load on the hub, including thePi, is going to exceed the maximum 2100mA.
If the Pi takes <700mA and the keyboard and mouse each takes <100mA then you've got (2100-700-100-100=)1200mA for everything else on the hub.
What your hard drive takes is an unknown, but even if it needs two ports to power it, it's unlikely to exceed the 900mA you say the hub should supply through two ports

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