Use two USB connectors for extra .5A to an mbed protoboard?

28 Aug 2010

Each USB device can only use .5A at 5VDC. For some mbed projects you could use another  .5A.  

I seem to recall that some notebook external hard drives use a special USB Y cable. Two USB cables plug into the PC and the second one is used only for more current.

Has anyone tried this?

Anyone know of a source for a cable like that which would plug directly into the mbed module?

If someone like sold it, it would be cheaper and easier than adding an AC wall wart with 5VDC out!

28 Aug 2010

As far as I know, USB ports these days supply up to 100mA on the supply pins. If you draw more than that, the operating system will (should) tell you that there is something wrong with the port and turns off the supply. In order to get the full 500 mA, the driver for the device needs to handshake for the appropriate power. This is why usb devices turn on and off and back on when you plug them in.

28 Aug 2010 . Edited: 28 Aug 2010

Interesting - I see the 100ma info in the power FAQs at for hubs along with 500ma max. The limited info I found there is confusing on that point. I wonder if 100MA is how much a device "should" draw until it gets enumerated- so that several devices can still work on a non-powered USB hub. Did it change from USB 1.0 to 2.0? I did a USB 1.0 design years ago and only recall having a chemical fuse gizmo on the 5V line.

I found Y cables several places and they all still claim "extra" power in the blurbs (but perhaps its only 100ma?)

This $7.95 one looks like it might be the Y cable needed to plug into the mbed

Heres the photo from the ad:

28 Aug 2010

Yeah, I've seen these around. I wouldn't trust it though. I don't think its a good idea to put the USB supplies in parallel, you could end up with circulating current through between the supplies and you can't really guarantee how much current is coming out of each port. It probably won't be equal. If a device requires more than 500mA, it must have a separate power supply.

You're welcome to try it, but I wouldn't risk the USB ports on my computer for an extra 500mA.

29 Aug 2010

I was at Frys today and noticed that they had a notebook hard drive external enclosure kit for $9 that has the USB Y cable that fits. They had several brands. Many of them have a different USB connector so you need to see it to be sure that it fits the mbed board. It seems to work OK and provide more power, but still not enough power for an R/C servo with both USB cables plugged directly into the PC. In fact when you plug in the power only cable, the mbed board runs, but does not enumerate so it is getting some extra current. I know you should not tie two different regulated power supplies together at Vcc (connect GNDs only), but isn't it all just one 5VDC supply inside the PC with two fused connections and wires in parallel to provide more current.

In any case, my plan B works. I dug around in the basement for an old 5VDC AC wall wart from something I was no longer using. Clipped off the strange small power connector plug and put color coded banana plugs on the wires after checking the polarity of the wires with a voltmeter. My protoboard has the banana jack power terminals on it, so this works well and is not a big mess. Thinking ahead I got the black and white banana plugs while at Frys.

I am still a bit puzzled as to why some other companies like Phidgets have RC servo control boards that run off USB power only and will run one servo, but I have always thought that was pushing the current limit. I am using the same servo that came with that board.

So I think the lesson is plan ahead and get a protoboard that has the external power connections on it, banana plugs, and a 5V DC out  1-2 Amp AC wall adapter, if you have a complex mbed project with servos, motors, or lots of external devices that draw current. We use the Global Specialties PB 103 ones here for students, they tend to last a bit longer than the inexpensive ones we have tried.


29 Aug 2010

Yeah, I've done the same thing.. Old cell phone chargers work well. And for the USB supply, I believe that each port has it's own regulator. That way you can turn off devices to save power through software. Each port is also (usually) protected from short circuits.

29 Aug 2010

I also saw some new USB 3.0 boards already available at Fry's. I just noticed that USB 3.0 devices can suck 900MA of current (up from 500MA) - so your next PC will likely have more power for mbed projects!

29 Aug 2010

There are all manner of newish USB sub-standards and de facto standards for drawing more power from USB (1A for high-power, 1A for Apple de facto iPhone/iPad standard, 1.5A for mobile phone charging standard). Maybe a future version of mbed might implement one/some of these standards for more power.

Plugging an mbed into a USB 3 port won't automatically give you permission to use 900 mA - the mbed firmware would have to request that amount from the host.

30 Aug 2010

From a recent thread post about power supply shorts:

"Hi All,

We designed the mbed to be as nice to the PC as we could to protect it from abuse it might take, so we've got protection for the USB power in the form of a FPF2123 Current Limiter; see the schematic on the mbed NXP LPC1768 page. This will kick in at around 460mA.

If you do manage to do something silly, this should stop you drawing too much current from the PC, and in some cases that means it gets very hot; once it has cooled back down, usually everything is back to normal (I think we may have seen one or two sacrifice themselves for the benefit of the PC! But failures have been very rare). In addition, most PC USB ports have protection, and many will cut themselves off for a while or until a reboot as part of their protection. So there is protection built in to the mbed, and most PC USB interfaces.

As you guessed, the mbed interface negotiates with the PC to get 500mA.

However, despite this protection, the reality is you just have to be careful!


So this explains why a device like a servo or DC motor will shut down on mbed and the Y cable will not help it any at all. The startup in rush current on motors trips the fast built-in 5V over current protection on the mbed board. My other older Phidgets USB single RC servo board did not have that. So forget the USB Y cable idea and use an AC wall wart or other power supply for some more DC power for motors or servos.  Also even if you could hack the mbed USB firmware to ask for 900MA on a new USB 3.0 port, the built-in protection circuit would still trip at 460MA.