[GreenKeys] New 60ma current loop driver board design

[COURYHOUSE at aol.com]

Thanks -   with flaming phone batteries and such I  think we  are all 
concerned about  thinks like this.... Ed#
 
 
 
 
 
In a message dated 10/29/2016 4:35:49 P.M. US Mountain Standard Time,  
scottjohnson1 at cox.net writes:

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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