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USB device durrent draw

How can I find out how much current a USB device draws?  I fired up an
XP VM and checked the current of each thing listed as a "USB Hub" in
"Device Manager", but they were all 0.  So either that's not where you
find it, that is not the data I'm looking for, or VMware doesn't emulate
that properly.

Long story: You know those hand boilers, thin glasswork helices and
other curves filled with colored liquid?  Well, I got one powered by
USB.  It uses an incandescent bulb to make the heat and once warmed up
it sends about 1 bubble/sec up the helix.  The manual says the bulb is
non-replacable; we shall see how "non-replacable" it really is when it
dies.

But to forestall that, I aim to make either a rheostat or a set of
series resistors with a selectable tap point (probably a rheostat, it's
easier) and trade off bubbling speed for lifetime.  To select the
right resistance range I need to know how much current it draws right
now.  No more than 500 mA, so that's an upper bound.

-- 
-eben      QebWenE01R@vTerYizUonI.nOetP      http://royalty.mine.nu:81
AQUARIUS:  There's travel in your future when your tongue freezes to the
back of a speeding bus.  Fill the void in your pathetic life by playing
Whack-a-Mole 17 hours a day.  -- Weird Al, _Your Horoscope for Today_
0
ebenZEROONE
2/25/2011 6:25:21 AM
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Hactar wrote:
> How can I find out how much current a USB device draws?

USB devices must tell the controller how much current they draw with
a field in their device descriptor structure, and in theory, the
controller driver could disable some devices to prevent them from using
more power than the controller can supply.  However, devices like hard
disks that draw more power than the USB specification allows just lie
about their maximum current, and many other devices don't bother to
report anything accurate, and there are devices that just take power
from the USB power lines without being an actual USB device.

If you trust your device's firmware, have a look at the MaxPower field
in the output of "lsusb -v" for your device.  Otherwise, you have to
splice a multimeter into the USB power line.


Regards,
Clemens
0
Clemens
2/25/2011 8:46:48 AM
In article <8sp8haFfvU1@mid.individual.net>,
Clemens Ladisch  <clemens@ladisch.de> wrote:
> Hactar wrote:
> > How can I find out how much current a USB device draws?
> 
> USB devices must tell the controller how much current they draw with
> a field in their device descriptor structure, and in theory, the
> controller driver could disable some devices to prevent them from using
> more power than the controller can supply.  However, devices like hard
> disks that draw more power than the USB specification allows just lie
> about their maximum current, and many other devices don't bother to
> report anything accurate, and there are devices that just take power
> from the USB power lines without being an actual USB device.
> 
> If you trust your device's firmware, have a look at the MaxPower field
> in the output of "lsusb -v" for your device.  Otherwise, you have to
> splice a multimeter into the USB power line.

I just checked "lsusb -v"'s output without and with it connected.  Same.
**sigh**  I guess I can assume it's about the same power draw as another
device with a bulb of similar brightness, which is 1W.

So when it's running it passes 200 mA (1W/5V), so it acts as a 25 ohm
resistor (5V/0.2A).  So this:
http://www.radioshack.com/product/index.jsp?productId=2062299
would probably work.

-- 
-eben    QebWenE01R@vTerYizUonI.nOetP    royalty.mine.nu:81

          Logic is a systematic method of coming to
            the wrong conclusion with confidence.
0
ebenZEROONE
2/25/2011 4:54:55 PM
On Fri, 25 Feb 2011 11:54:55 -0500, Hactar rearranged some electrons to
say:

> In article <8sp8haFfvU1@mid.individual.net>, Clemens Ladisch 
> <clemens@ladisch.de> wrote:
>> Hactar wrote:
>> > How can I find out how much current a USB device draws?
>> 
>> USB devices must tell the controller how much current they draw with a
>> field in their device descriptor structure, and in theory, the
>> controller driver could disable some devices to prevent them from using
>> more power than the controller can supply.  However, devices like hard
>> disks that draw more power than the USB specification allows just lie
>> about their maximum current, and many other devices don't bother to
>> report anything accurate, and there are devices that just take power
>> from the USB power lines without being an actual USB device.
>> 
>> If you trust your device's firmware, have a look at the MaxPower field
>> in the output of "lsusb -v" for your device.  Otherwise, you have to
>> splice a multimeter into the USB power line.
> 
> I just checked "lsusb -v"'s output without and with it connected.  Same.
> **sigh**  I guess I can assume it's about the same power draw as another
> device with a bulb of similar brightness, which is 1W.
> 
> So when it's running it passes 200 mA (1W/5V), so it acts as a 25 ohm
> resistor (5V/0.2A).  So this:
> http://www.radioshack.com/product/index.jsp?productId=2062299 would
> probably work.

Using this would cut the power going to the bulb in half at most, if that 
is what you want to do.  
0
david
2/26/2011 12:16:51 PM
In article <ikaqvj$6cp$1@news.eternal-september.org>,
david  <none@nospam.com> wrote:
> On Fri, 25 Feb 2011 11:54:55 -0500, Hactar rearranged some electrons to
> say:
> 
> > In article <8sp8haFfvU1@mid.individual.net>, Clemens Ladisch 
> > <clemens@ladisch.de> wrote:
> >> Hactar wrote:
> >> > How can I find out how much current a USB device draws?
> >> 
> >> USB devices must tell the controller how much current they draw with a
> >> field in their device descriptor structure, and in theory, the
> >> controller driver could disable some devices to prevent them from using
> >> more power than the controller can supply.  However, devices like hard
> >> disks that draw more power than the USB specification allows just lie
> >> about their maximum current, and many other devices don't bother to
> >> report anything accurate, and there are devices that just take power
> >> from the USB power lines without being an actual USB device.
> >> 
> >> If you trust your device's firmware, have a look at the MaxPower field
> >> in the output of "lsusb -v" for your device.  Otherwise, you have to
> >> splice a multimeter into the USB power line.
> > 
> > I just checked "lsusb -v"'s output without and with it connected.  Same.
> > **sigh**  I guess I can assume it's about the same power draw as another
> > device with a bulb of similar brightness, which is 1W.
> > 
> > So when it's running it passes 200 mA (1W/5V), so it acts as a 25 ohm
> > resistor (5V/0.2A).  So this:
> > http://www.radioshack.com/product/index.jsp?productId=2062299 would
> > probably work.
> 
> Using this would cut the power going to the bulb in half at most, if that 
> is what you want to do.  

That should be fine.  Incandescent bulbs are non-ohmic devices; IOW you
give them half the voltage, they draw way less than half the current.
Since that's a rheostat, I could dial in anything between "normal
brightness" and "no brightness".

But what I'll probably do is this:  I found cords for a HSF with
embedded 12, 25, and 50 ohm resistors, two of each.  So if I wire three
switches appropriately, I can get 0-7 * 6 ohms.  That might be enough
precision.  Stuff I have that might work wins out over stuff I need to
buy that'll probably work.

-- 
A well-lovd and corrctly traind domstc cnine is gnrlly slobbry, excitbl,
noisy, scatologically obsessed, xenophobic, pathetically unjudgmental,
embrrssngly uninhbtd, unreasnngly dvtd, hrtbrkngly dpndnt and wretchedly
craven.  All othr knds of dog cmpre unfvrbly wth ths picture. - PB, AFCA
0
ebenZEROONE
3/1/2011 5:24:39 AM
In article <nhcu38-ram.ln1@pc.home>, Hactar <ebenZEROONE@verizon.net> wrote:
> In article <ikaqvj$6cp$1@news.eternal-september.org>,
> david  <none@nospam.com> wrote:
> > On Fri, 25 Feb 2011 11:54:55 -0500, Hactar rearranged some electrons to
> > say:
> > 
> > > In article <8sp8haFfvU1@mid.individual.net>, Clemens Ladisch 
> > > <clemens@ladisch.de> wrote:
> > >> Hactar wrote:
> > >> > How can I find out how much current a USB device draws?
> > >> 
> > >> USB devices must tell the controller how much current they draw with a
> > >> field in their device descriptor structure, and in theory, the
> > >> controller driver could disable some devices to prevent them from using
> > >> more power than the controller can supply.  However, devices like hard
> > >> disks that draw more power than the USB specification allows just lie
> > >> about their maximum current, and many other devices don't bother to
> > >> report anything accurate, and there are devices that just take power
> > >> from the USB power lines without being an actual USB device.
> > >> 
> > >> If you trust your device's firmware, have a look at the MaxPower field
> > >> in the output of "lsusb -v" for your device.  Otherwise, you have to
> > >> splice a multimeter into the USB power line.
> > > 
> > > I just checked "lsusb -v"'s output without and with it connected.  Same.
> > > **sigh**  I guess I can assume it's about the same power draw as another
> > > device with a bulb of similar brightness, which is 1W.
> > > 
> > > So when it's running it passes 200 mA (1W/5V), so it acts as a 25 ohm
> > > resistor (5V/0.2A).  So this:
> > > http://www.radioshack.com/product/index.jsp?productId=2062299 would
> > > probably work.
> > 
> > Using this would cut the power going to the bulb in half at most, if that 
> > is what you want to do.  
> 
> But what I'll probably do is this:  I found cords for a HSF with
> embedded 12, 25, and 50 ohm resistors, two of each.  So if I wire three
> switches appropriately, I can get 0-7 * 6 ohms.  That might be enough
> precision.  Stuff I have that might work wins out over stuff I need to
> buy that'll probably work.

I couldn't find my box of switches (etc), so I wired up two 12-ohm
resistors in parallel and soldered them into the + line of a USB
extension cord.  The light appears maybe half the brightness it used to
be, but humans in general (and I in particular) are pretty bad judges of
such things.  The heat from the bulb runs a boiler (I think it was in my
original post), and the bubbling is a bit slower than it was, maybe 2.x
per second.  It's fast enough.

-- 
-eben      QebWenE01R@vTerYizUonI.nOetP      royalty.mine.nu:81
     If you need someone to blame
     Throw a rock in the air
     You'll hit someone guilty -- U2, _Zooropa_, "Dirty Day"
0
ebenZEROONE
3/2/2011 8:31:11 AM
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