GPS - Re: A portable GPS should use standard mini-USB power (but which

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Re: A portable GPS should use standard mini-USB power (but which

Jack Erbes

2007-05-29, 10:33 am

sarah bennett wrote:

<snip>
> USB is not just for computers. Think of it as a "universal power adapter".
> It happens to conform the the power supply requirements for USB, but
> there's no "data" in a power adapter.
>
> With a "universal power adapter" (ie the USB B power supply), you can
> charge your telephone, your earbud, your mp3 player, your camera, and your
> GPS ........ all from the same "universal power supply".
>
> This is so very important for portable electronics that I'm shocked that
> the GPS review web sites don't make it one of the number one criterias
> (after performance metrics, of course).
>
> Is there a web site that explains which power adapter is used by the
> various GPS units under test?

Sarah,

You're bordering on becoming a "zealot" for mini-USB as a charging
source without a complete understanding of it all. And I don't say that
with harsh intents or as an insult.

Mini-USB (and USB) are intended to provide up to a 500 mA power source
for devices. That they will charge some devices is purely incidental.
Their strong point is that, in most devices, they send a signal to the
device that the battery need not be used because 5V power is available
and that lets a smart device use that for power instead of depleting the
internal battery.

Another factor is the battery voltage of the device and the electronics
in the device. If the device has a 3.7V battery and the right
electronics, it is practical to use the USB's 5V source to charge the
battery. The device would have to regulate the charging voltage and
current down to levels that will charge the battery.

If the device has a 5V (or higher) battery, or if the size of the
battery requires a charging current of more than 500 mA, the USB port is
not a good choice for a charging source. And if you have a 5V device
that has too large a battery and demands too high a charging voltage and
current rate, you are threatening the electronics in the USB port,
especially if it is in a computer.

There is a complex interplay of battery voltage, battery capacity, and
charging voltages and rates that need to be considered before a USB or
Mini-USB port can bee considered for use as you describe.

Jack

Jack Erbes

2007-05-29, 3:33 pm

sarah bennett wrote:
> On Tue, 29 May 2007 09:07:03 -0500, Jack Erbes wrote:
>
>
> Hi there Jack,
>
> My Blackberry AC adpater outputs has a mini USB connector yet it outputs
> 750ma at 5 volts.
>
> MODEL = PSM04A-050RIM
> PART NUMBER = ASY-07040-001
> INPUT = 100 to 240 VAC, 50-60 Hz, 0.2A
> OUTPUT = 5 volts c.c. 0.75A
>
> BTW, what does "c.c." mean?
>
> sarah

I don't know what "c.c" is, maybe current capacity? That is not a
standard abbreviation as far as I know.

The USB and Mini-USB connectors have become widely used as a means to
provide power to, and charge batteries contained in, electronic devices.

I was not saying that USB cannot satisfy the power/charging needs you
describe, just that trying to charge devices from a USB port intended
for data I/O needs consideration on a case by case basis.

Just because you received an AC or DC adapter with a USB connector for
use in charging a device, does not mean that you can charge that device
from anything with the USB connector.

Your Blackberry adapter is an example. By its specs, it can provide at
least 50 per cent more power (750 mA) that a USB port on a computer (500
mA). If you assumed that the Blackberry with a nearly depleted battery
needed most of the AC adapter's capacity, a USB port on a computer would
not be able to meet the need. If your Blackberry were trying to draw
600 mA of current from a USB port, that is more than a USB port is ever
supposed to provide.

There is a good explanation of the power plan in USB ports and also some
discussion of how some devices that can misuse the USB power scheme here:

http://en.wikipedia.org/wiki/USB#Power

The USB and Mini-USB connectors have become a convenient way to supply
power to devices, but they also can be misused. Especially in the
marketing of low cost "gadget" items and in low cost replacements for
more expensive OEM items. The OEM items may have contained
sophisticated electronics and have been much more sophisticated than
that inexpensive replacement that does the same thing.

Jack

Ron Hunter

2007-05-30, 10:33 am

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