In message <telecom25.429.7@telecom-digest.org> Emily
<emilymoberg@hotmail.com> wrote:
> On Fri, 29 Dec 2006 01:46:56 GMT, DevilsPGD wrote:
>> Depending on the device design, the chargers may or may not be
>> interchangeable. I'm currently charging my girlfriend's Blackberry on
>> a Motorola USB charger I purchased for my Razr.
> Hi DevilsPGD,
> The part of your statement that gets me is the part that says U S B.
> If it's USB, then it should be USB. Right?
Well -- There are a few factors. First off, I'm going to assume we're
only talking the same USB connector, there are both 4-pin and 5-pin
versions of the typical mini-B connector.
USB male-A
http://www.computercablesource.com/images/usb-am-am-cable-white.jpg
USB male-B
http://www.computercablesource.com/images/usb-am-bm-cable-black.jpg
USB mini-B, 4pin
http://www.computercablesource.com/images/usb2-am-mb4-06.jpg
USB mini-B, 5pin
http://www.computercablesource.com/images/usb2-am-mb5-06.jpg
Most small portable devices these days use a mini-B 5pin, most
removable drives seem to use a male-B. I have a few devices that
accept a male-A as well, typically cheap (non-manufacturer) cradles.
> For example, like you, I also have a Blackberry and a Motorola Razr.
> Here is a pairing of USB DEVICE requirements with the USB POWER
> SUPPLY capabilities:
> DEVICE = Blackberry 8700 SUPPLY = TCPRIM2ULSSN 5.0vdc 750mA
> DEVICE = Motorola V195 SUPPLY = PSM5037B 5.9vdc 375mA
> DEVICE = Motorola RAZR SUPPLY = DCH3-05US-0300 5.0vdc 550mA
> DEVICE = Motorola Earbud SUPPLY = FMP5185B 5.2vdc 450mA
> Can I "assume" if I hook the Motorola V195's USB power supply (5.9vdc
> 375mA) to the Blackberry 8700 device, that the Blackberry will be getting
> more voltage than it 'expected' and that the current it will try to suck
> out of the charger will be more than expected (due to the higher voltage),
> yet the current the power supply can supply will be half of what it
> expected (due to the 375 mA limitation of the power supply).
The USB spec requires 5.0vdc +/-0.25v (or +/-5%) -- So your Motorola
V195 charger isn't within spec.
The USB spec puts out a maximum of 500mA power. However, there is no
harm in providing more mA then required as long as the voltage is
regulated to stay within spec under higher load.
In other words, the other power supplies are within spec.
Also note that many devices can accept a wide range of voltages. I
definitely wouldn't recommend experimenting, you may fry equipment if
you go too high, or batteries if you charge a LiON too low, but it's
not uncommon for equipment to be able to accept a much wider range
then is printed on the device (largely to enable them to charge from a
PC)
> If this is true, this implies that USB chargers are NOT interchangable!
> (The T-Mobile store clerks just might have been right.)
> But, what irks me is they all have the SAME CONNECTIONS!
> They all "LOOK" the same to me!
Sure ... Read on though, I'll explain why :)
> Does EVERYONE label all their USB chargers so they don't mix them up?
> Or am I missing something fundamental here. If it says it's a USB
> charger, but that we can't use them interchangably, then are they
> REALLY USB power supplies?
Yes -- They're supplying power via USB, therefore they are power
supplies with a USB end.
> I'm still confused on the fact that the charger advertises it is USB but
> it's not USB if it doesn't fit all USB devices.
I think the problem, ultimately, is that there isn't a clearly defined
spec for USB as a power supply independent of USB as a data cable that
happens to provide a bit of power.
> Can someone clear up the USB part of the confusion here?
Besides a data link, USB provides 5 +/- 0.25 volts to power an
otherwise unpowered device or charge a battery in a self powered
device. A USB controller or hub is required to power one unit load
(100 mA, a low power load).
It can optionally power up to 5 unit loads (500 mA, a high power
load). The spec is silent about intermediate loads, so I assume that
the sources deal with just the two cases. Either a load is low power
(like a keyboard or a mouse) or high power (like an iPod or PDA).
A device cannot assume that 500 mA is available until it gets
permission, when you're talking to a computer this is done over the
data connection. In general, the way devices will handle this is to
only draw 100 mA (which is typically enough to run, but not to charge
a battery), link up, and once the driver is installed, ask the driver
for permission, then switch up to the 500 mA (or higher in many cases)
mode.
In practice, with modern USB ports in desktops, and powered hubs, you
can pretty safely draw well over 100 mA without asking, and over 500 mA
with permission. However, this will actually overload some computers
(especially older laptops that were built to spec and not a penny over,
meaning that there was no current overload protection)
So what do you do when you hit a USB charger? Well, per the spec, only
100mA can be drawn safely, which means no charging batteries.
One option would be to provide a second charger port, using the USB
port only for data and for charging when a PC was attached and using
the separate power connector as a safe charging port. However, this
would incur additional costs, and is an additional component to get
damaged.
Another option, the one most well designed hardware uses, is to have
some way for the charger to identify itself to the device, giving the
device permission to safely draw more current.
The third option, the one many cheap "use your USB ports to charge
your device" works is to simply assume higher power capabilities are
present and charge.
So, in the real world, what does this mean? Well, take my Treo 650.
It went with solution #1, it has separate data and charge connectors
(well, it's all one custom adapter, but you can connect a data and
power cable at the same time) -- As a result, it cannot USB charge
directly. So now some aftermarket company comes out with a cable that
uses technique #3, it just hot-wires the Palm's 5V in to my USB power
and chargers. Works great, although it potentially draws too much
current.
The Motorola Razr is a case of #2, it will only charge if it has
permission (drivers installed, or it detects a dedicated charger)
Another example is my MP3 player, a Zen Micro ... This is the ultimate
choice, as far as I'm concerned -- It uses solution #2, which means it
won't overload a PC with an underpowered USB port. However, if you
switch the controls to "Lock", it will charge if there is power but no
driver connection -- This means I can charge off of any USB power
adapter if I want to force the issue, but when communicating with a
PC, it will only charge if authorized.
Now, with all that being said, I've yet to see a device capable of
charging off of USB that my Motorola Razr chargers won't power, so as
a result I've bought several of these on eBay (You can get genuine
Motorola ones for ~$10 on eBay on a good day), and haven't ever had an
issue.
Whew, hope that post wasn't too long :)
Going to war over religion is fighting to see who's got the
better imaginary friend.
DevilsPGD (spam_narf_spam@crazyhat.net)
