[GreenKeys] New 60ma current loop driver board design

[Rob Doyle]

On 10/28/2016 9:09 PM, drlegendre . wrote:
> Gil said:
>
> "I suppose we need to add some brass and tung-oiled cherry to the bot
> :)"
>
> Just put some gears on it.. ;-)
>
> Ed said:
>
> "ok so what I experience is the port at it its limit then... it will
> not get better.. Ok ! "
>
> @John
>
> 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.

Yes. It's complicated because it is fairly easy to damage the computer
by drawing too much power.

There are currently a lot of stories about new USB-C devices being
damaged by cheap Chinese cables that don't follow the protocols. It's a
very expensive mistake.

<http://www.pcworld.com/article/3002064/computers-accessories/beware-bad-usb-c-cables-google-engineer-warnswhile-naming-names.html>

> On Fri, Oct 28, 2016 at 10:53 PM, Ed Sharpe Archivist for SMECC via
> GreenKeys <greenkeys at mailman.qth.net
> <mailto:greenkeys at mailman.qth.net>> wrote:
>
> __ ok so what I experience is the port at it its limit then... it
> will not get better.. Ok ! Thanks Ed#
>
>
>
> In a message dated 10/28/2016 5:41:21 P.M. US Mountain Standard Time,
> radioengr at gmail.com <mailto:radioengr at gmail.com> writes:
>
> On 10/28/2016 10:54 AM, Ed Sharpe Archivist for SMECC via GreenKeys
> wrote:
>> OT  sortta  USB power related... .. how can I make my USB charge my
>> cell phone... faster.... seems computer is slowest form of
>> charging... thx...ed#
>
> That's because it is the slowest form of charging. USB ports are
> current limited to 500mA (or 900mA for USB 3.0) to protect your
> computer.  The Apple 'USB' charger that I use provides 2000 mA.
>
> USB type C (USB-C) provides up to 100 watts on the connector...
>
> There is all kinds of USB.
>
> Rob.
>