2-port-to-1 USB adapter for power-hungry peripherals?


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by PiON » Wed Jun 13, 2012 9:41 pm
Hi everyone,

I've read a lot of posts about USB power and the limit of 100mA that the Pi can provide through each of its two ports. It seems some devices are hungry for more than that though, and I was wondering if there might be an adapter of some sort that's available (or could be built easily) that would let a single device plug into both USB sockets - only one for the actual data, of course, but drawing power from both of them.

For example, I've got a K400 wireless keyboard/touchpad device (that I'm still trying to get working, btw) which uses a single USB port, but it has sticky keys that might be because it's underpowered. I'd like to make my Pi truly portable, and having to carry around a powered hub for my keyboard makes that a little harder. Even if I can plug it into the same portable power source as my pi, that's still at least 2 more annoying little wires I need to have floating around. I'd love to avoid that.

Does anyone know of such a device? Does anyone have the knowledge to tell me if this is even possible/practical/worth it?

Cheers!
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by Gert van Loo » Wed Jun 13, 2012 9:48 pm
Just short the fuses out. Take them off an put a piece of wire over them.
Yes, you can do it and it is safe.
The only drawback is: Make sure your HUB is not back-powering your Pi into the USB ports
It should not but we have seen plenty of posts from user who has (cheap) hubs do it anyway.
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by PiON » Wed Jun 13, 2012 10:03 pm
Thanks for the quick reply, Gert!

This method you're suggesting doesn't sound easily reversible. Something a novice like myself is uncomfortable with... This would involve dismantling the socket first, right?

The appeal of the idea of a plug-in adapter for the job is that you just unplug it and it's back to normal (still, I don't know if that's possible).
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by PiON » Wed Jun 13, 2012 10:15 pm
Aha! I knew someone must have already thought of this, and I can't believe I've never come across one of these before:

USB Y cable - 2 male to 1 female
http://www.newegg.ca/Product/Product.aspx?Item=N82E16812161003
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by mahjongg » Wed Jun 13, 2012 11:00 pm
Y-cable, in theory this could work, but most of these only loop through the power, you need one that loops the power through from two ports, but the data-lines from just one. From the description its unclear if this happens with this cable, but it might, a well designed cable should do this.

For bridging the fuses you do NOT have to take off the connector, and there are two ways to do it:

Paralleling the fuses, so both USB ports share two fuses that are paralleled, is done by simply connecting pins 5 of both USB ports together. see the picture below

connectedUsbVcc.jpeg
connectedUsbVcc.jpeg (24.91 KiB) Viewed 3086 times


The other method is simply soldering a wire across the fuse, simple to do an can be removed in a minute. F1 is the fuse for the upper port, so you can simply only bridge the "high power port", and let the other fuse alone, so you can use it with a low power device.

In both cases obviously you need a Power Supply that can deliver the extra current, something like 1.1 A.
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by jbeale » Wed Jun 13, 2012 11:07 pm
mahjongg wrote: you need one that loops the power through from two ports, but the data-lines from just one.


?? That is the way all USB "Y" cables work? AFAIK there is no other way they could work.

If you can solder, bridging one or both USB output fuses with a short piece of wire should be easy. I would not consider this truly "reversible" because the heat from soldering will raise the polyfuse resistance and it never completely recovers. On the other hand, I don't know why you would want to reverse it, the R-Pi is still protected by the input fuse, plus whatever safety circuit your +5V supply includes (you ARE using a UL-listed or CE-marked supply... right?)
Last edited by jbeale on Wed Jun 13, 2012 11:16 pm, edited 1 time in total.
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by mahjongg » Wed Jun 13, 2012 11:15 pm
I was thinking that some "power share cables" might only connect the power, not the data, but now I think about it I do think these might be rare as their use is very limited.

True about soldering on the polyfuses this way, but If you solder for a short time, and wait a bit between both points there probably wont be too much harm, remember that the polyfuses were soldered to the board in the first place, but in principle you are right.
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by jbeale » Wed Jun 13, 2012 11:22 pm
I see- ok, there might be, but I've never seen such a cable. The linked cable had two different colors on the USB inputs to show which was "just power" and which was power + data. There would be no reason to have two different colors with a power-only duplex cable.

Re: polyfuse thermal history- it was already soldered, but apparently it is somewhat cumulative (?) - also if you are inexperienced, it is very likely you'll end up with a longer high-temperature dwell than the first, presumably optimized reflow process at the factory.
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by PiON » Thu Jun 14, 2012 5:31 pm
Thanks for all the great info everyone!

mahjongg wrote:For bridging the fuses you do NOT have to take off the connector, and there are two ways to do it:

Paralleling the fuses, so both USB ports share two fuses that are paralleled, is done by simply connecting pins 5 of both USB ports together. see the picture below
connectedUsbVcc.jpeg
connectedUsbVcc.jpeg (24.91 KiB) Viewed 3053 times

The other method is simply soldering a wire across the fuse, simple to do an can be removed in a minute. F1 is the fuse for the upper port, so you can simply only bridge the "high power port", and let the other fuse alone, so you can use it with a low power device.


I've not done any soldering before - is there a way to parallel the fuses without soldering? If they both use one fuse, how much power would each port receive?
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by jbeale » Fri Jun 15, 2012 4:50 am
The way to parallel the fuses without soldering is to use the USB "Y" cable as described previously. This connects the power output of the two ports together at the cable, instead of on the board, but it is the same effect.

You don't get any more total power, it's just that this approach allows you to combine the separate 100 mA + 100 mA USB ports to get 200 mA going to a single USB device. You are still using both fuses, but they are now in parallel, giving you "in effect" one 200 mA fuse instead of two separate 100 mA fuses.
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by b455x » Wed Jun 20, 2012 10:53 pm
I'm also pretty shure that a Y-cable should work...

lots of external 2,5" harddrives are connected via such a cable, one for the data, one for additional power...
of course, most of the harddrive-cables have a mini-usb, but there are cheap adapters...
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by mahjongg » Wed Jun 20, 2012 11:32 pm
There is one problem with using an Y cable (or any other way of paralleling the two fuses) to power a device that needs more than 100mA, and that is that a side effect of any device that is an official "more than 100mA device" also won't tolerate the kind of voltage drop that "less than 100mA USB devices" officially have to tolerate.

<100mA devices must tolerate voltages as low as 4.4 Volt according to USB specs.
>100mA devices do not have to, and only tolerate the normal 5%, that is only as low as 4.75V

putting two fuses of (typically 6 Ohm) in parallel still results in a combined resistance of 3 Ohm, and that is far too much for devices that want 4.75 Volt!

The PI was really designed [url]only[/url] for USB devices that can tolerate 0.6 of a volt, (that is <100mA devices) not for devices that can only tolerate 0.25V below their nominal supply.
That means it really ever was designed only for the most simple (wired) keyboards, and mice, both being <100mA devices.

If you use any non <100mA device you must therefore power it from another source than the RPI. That is from a powered hub, or directly by cutting the 5V wire from the RPI, and directly powering it with a reliable 5V source giving more than 4.75V.

If you use a powered hub that directly connects its 5V to the RPI's using the USB cable between the hub and the RPI then you either need to cut its 5V wire (or mask off pin 1 of the connector), OR you need to power the PI from the same hub (if the hub can power the PI reliably from a single one of its ports) so the PI and hub are powered from the same single 5V source.

The only way to solve this scenario is to replace the fuses with one having only a small fraction of their current resistance, a fuse like the one used for F3 (the power input fuse), with a resistance in the neighbourhood of 0.1 Ohm.

Or you can simply solder a wire across the fuse(s), on both F1 & F2.
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by b455x » Thu Jun 21, 2012 7:45 pm
another possibility would be such a device:
http://www.amazon.de/Belkin-USB-Netzada ... 640&sr=8-5

(sorry, german amazon homepage...)

it's a PSU with two USB ports - you could use one to power your pi and another one to power one part of an Y-cable :-)
the other part of the y-cable would of course go into the usb-port of the pi...and the single end would connect with the wifi-stick or whatever device that needs more power...

that's just a solution if you do not want to use a powered hub...it's still one psu and the stick is connected to the pi, just with one cable more...
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by Montala » Thu Jun 21, 2012 9:16 pm
mahjongg wrote:..... OR you need to power the PI from the same hub (if the hub can power the PI reliably from a single one of its ports) so the PI and hub are powered from the same single 5V source.
Personally I am not too keen on taking a soldering iron to my new Pi, or on cutting leads etc. unless really necessary, so am thinking of using a Masterplug SRGDU42PB USB Extension Lead Power Block with 4 Sockets, (http://www.amazon.co.uk/gp/product/B006HFME4C) which incorporates two USB power sockets, said to provide 1000ma output (which I assume is per socket?). One of these would be used to power the Pi, and the other a powered USB hub which would be a Newlink NLUSB2-224P. (http://www.kenable.co.uk/product_info.p ... ts_id=5614)

Of the Pi's two USB outputs, one would have the 'dongle' for my Logitech K400 wireless keyboard and the other would go to the Newlink hub, into which the tiny Edimax EW-7811UN Wireless USB Adaptor (http://www.amazon.co.uk/gp/product/B003MTTJOY) would be plugged.

I had considered just using the Newlink hub with a 'Y' lead from two of the USB outlets to power the Pi, but its PSU only has a 1amp output, and also I would end up with a 'power loop' between the Hub and the Pi, which is probably not ideal, so assuming that the voltage across TP1 and TP2 is OK, I prefer my first suggestion.

Does this proposal sound OK?
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by Montala » Thu Jun 21, 2012 9:40 pm
b455x wrote:another possibility would be such a device:
http://www.amazon.de/Belkin-USB-Netzada ... 640&sr=8-5

(sorry, german amazon homepage...)

it's a PSU with two USB ports - you could use one to power your pi and another one to power one part of an Y-cable :-)
the other part of the y-cable would of course go into the usb-port of the pi...and the single end would connect with the wifi-stick or whatever device that needs more power...

that's just a solution if you do not want to use a powered hub...it's still one psu and the stick is connected to the pi, just with one cable more...

For those in the UK, a very similar option, which has just been mentioned in another thread at viewtopic.php?f=46&t=8556 is available from Currys/Dixons for just £3.97 One of the 3 USB outputs is 1 amp, so a 'Y' cable may not be necessary, assuming of course that the voltage is OK.

They might not be available in their stores but delivery is free anyway, so what the heck!
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by mahjongg » Sat Jun 23, 2012 11:49 am
Montala wrote:Personally I am not too keen on taking a soldering iron to my new Pi, or on cutting leads etc. unless really necessary,


No need for any soldering, nor cutting, just stick a bit of cello-tape on pin 1 of the USB connector, connecting the hub to the RPI.
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