[GreenKeys] New 60ma current loop driver board design

Sat Oct 29 21:52:47 EDT 2016

"In general, the current demand is managed by the device, not the power
source.  The USB jack is constant voltage up to its current limit,"

And this was my understanding, up to the point of this discussion about
'smart' USB power ports. While I haven't had the inclination to actually
read through the relevant bus specs, it seems the the answer is "it varies,
depending upon the version and specific implementation" as to how smart the
power mgmt. actually is.

I do know that pre-2.0 ports generally cannot supply 500mA. For instance,
if you want to use a port-powered USB 2.0 hard drive on a 1.X controller,
you have two options - either use an external PSU for the drive, or one of
those 'siamese' cables that require two USB ports for the single device.
With that setup, a first port supplies power + data, the second port only
additional power.

Between the two, they sum to 500mA or so, enough to power the 2.0 drive -
though it only talks at 1.X speeds, of course.

On Sat, Oct 29, 2016 at 6:35 PM, Scott Johnson <scottjohnson1 at cox.net>
wrote:

> In general, the current demand is managed by the device, not the power
> source.  The USB jack is constant voltage up to its current limit, which
> might be 500mA, 900mA, or up to 3A in some dedicated power ports.  It is
> incumbent upon the device design to protect the device from overcurrent,
> and
> to provide sufficiently large conductors in the charging/power cable to
> support maximum charging current.
>
> Scott V. Johnson W7SVJ
> 5111 E. Sharon Dr.
> Scottsdale, AZ 85254-3636
> H (602) 953-5779
> C (480) 550-2358
> scottjohnson1 at cox.net
> scott.johnson at ieee.org
>
> -----Original Message-----
> From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of
> Ralph Mowery
> Sent: Saturday, October 29, 2016 4:05 PM
> Cc: 'Green Keys' <greenkeys at mailman.qth.net>
> Subject: Re: [GreenKeys] New 60ma current loop driver board design
>
> This may help.  Seems the original usb 1 and 2 could deliver 500 ma.  The
> newer connectors have more pins to negotiate how much power to send out.
>
> http://www.extremetech.com/computing/115251-how-usb-
> charging-works-or-how-to
> -avoid-blowing-up-your-smartphone
>
>
>
>
> > -----Original Message-----
> > From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On Behalf
> > Of Rob Doyle
> > >
> > > So how is it then, that "dumb" USB-powered devices, such as these
> > > little USB fans & lamps, can pull a current from the USB port
> > > without sending any sort of request-to-enable signal? I was always
> > > under the impression the the +5V was available at all times, on the
> > > port -
> > 500mA
> > > for 2.0, (less) for 1.X and (more) for newer 3.X specs?
> >
> > I'm pretty sure that a computer will only deliver low power (100 mA)
> > until the powered device requests, via the enumeration process, a high
> > power (500 mA) setting. That's what the USB 2.0 spec says.
> >
> > I have a USB desk lamp that definitely does not enumerate. I've always
> > assumed that it draws less than 100 mA. Maybe I'll measure it.
> >
> > > And if the USB port doesn't supply constant power, how does the
> > > USB-powered device make that request without a power source?
> >
> > The USB controller that John used has an internal EEPROM that controls
> > the 'Max Power' parameter. He said "This is reprogrammed to talk 45
> > baud, and to ask for 250mA of power from the USB port. Once the USB
> > port has come up and the other end has agreed to supply 250mA, ENABLE
> > goes high and U2 turns on power." In other words, his USB-powered
> > device does not draw high power until after enumeration.
> >
> > > This is genuinely confusing.
> >
> >
>
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