[GreenKeys] New 60ma current loop driver board design

[Scott Johnson]

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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