CL1B power distribution

John Watlington wad at laptop.org
Sun Apr 26 00:55:08 EDT 2009


On Apr 26, 2009, at 12:40 AM, Benjamin M. Schwartz wrote:

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> John Watlington wrote:
>> Quick straw poll on how many people think it is useful enough have
>> individual
>> control over the power supplied to each connector to raise the cost
>> of the laptop
>> by $0.15 ?
>
> Turning off a single port to which nothing is connected saves no  
> power,
> right?

Correct.

> I don't see the appeal.  Maybe for deactivating power to passive
> devices (e.g. usb sticks) during suspend, but such devices are  
> cheap to
> power anyway, and may not shut down cleanly if their power supply is
> killed.  Moreover, I am persuaded by your argument that the  
> software is
> unlikely to get smart enough to use it.
>
> Also, these "switches" are actually transistors, with some leakage  
> current
> and some effective resistance, right?

The leakage current is negligible (sub uA -- especially with nothing  
connected...)

With a little design, you can get effective resistances around 22  
milliohms
for the price I mentioned (OK, maybe $0.18).   This yields a loss of  
6 mW (0.25%)
at full rated power (0.5 A), and 22 mW (0.44%) at our rated power (1A  
through any
connector).

> So it seems like we pay for the flexibility of these switches with  
> a small
> increase in power requirements.

The price is the dominant factor.

wad



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