[sdiy] usb power?

[Tom Farrand]

For the USB ports that are out there now (USB 2.0) the current limit
is 100mA until enumeration has completed.  "Enumeration" requires some
smarts in whatever is connected to the USB port.  The "smarts" ( a
processor of some sort) tells the USB interface how much current is
wanted.  This, in theory, is granted in 100mA chunks up to a limit of
500mA.  Without the "smarts" the current is supposed to be limited to
100mA.

Realistically, I have found that some computers are happy to supply up
to 500mA without enumeration of any description.  Some Dell machines
do this, though my sample is way too limited to make any claim of
universality in this regard.

>From your numbers of 25 LEDs and 4.5V, I can only guess you want some
white LEDs and intend to suck the maximum of 500mA.  Is that correct?
You can probably get away with that ... mostly.  If you are sucking
this current from a laptop computer, you might get away with it.  If
you are trying to get this from an unpowered hub .... I would suggest
that you not try this.  Use a powered hub.  In fact, I would suggest
that you use a powered hub in all cases where current of this
magnitude is consumed.  USB ports are "supposed" to be protected from
excessive current consumption.  Not all computers are compliant and it
might be quite difficult to find out which and which are not.  While
designing a medical product where I needed to suck the 500mA maximum,
I found that my development computer was quite happy to provide in
excess of 500mA without enumeration.  Yikes!  That surprised me but I
built the smarts into my product to politely ask for 500mA and error
out if for some reason this power level was not "approved" by the host
computer

How important is this to you?  Why can't you use a wall wart power
supply (or batteries for that matter) to light up the LEDs?  Get an
LED flashlight!  In short, without knowing more detail it is difficult
to guess at your limitations and why other possibilities are not
viable.  I would look for other solutions.  Then again, I would be
upset if I smoked a laptop ... maybe you are not worried about that
possible outcome.

Peace.
Tom Farrand


If one is just wanting to suck power from the

On Sun, Mar 22, 2009 at 2:59 AM, db <dbarton at pacbell.net> wrote:
>
> I want to power some lights from my laptop. These lights are a bank of 25 leds, designed to run at 4.5 volts
>
> How can I tell how many of these are ok to power from usb?
>
> I'm guessing a usb hard drive pulls more current than even 100 LEDs, no?
>
> dennis.barton:skylab2000[socal]
>
>
>
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