[sdiy] usb power?

[Barry Klein]

USB hard drives draw about an amp on spinup and 500-700mA during operational 
modes.
None are USB compliant as they cannot meet the 500mA limit.
I have found that some Apple and Sony laptops will fail to provide more than 
750mA or so, but
most things will supply over an amp without port shutdown.  Critical thing 
is not to present
a large discharged cap to the port.  The high current spike will cause a 
detected current surge and
the port may be shut off or at least you will get a fault message.
Another thing you could maybe do is to multiplex the LED's (strobe them) to 
reduce power or go
all the way and have a lipoly power source that is charged/backed up by the 
laptop power when necessary.
Also, you can get very bright leds cheap off ebay - way brighter than in the 
usual cheap multiLED arrays.
You can use less of these for the same amount of light... Check Maxim and 
other driver chips too.

Barry

----- Original Message ----- 
From: "Loscha" <loscha at gmail.com>
To: "db" <dbarton at pacbell.net>; <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, March 22, 2009 1:12 AM
Subject: Re: [sdiy] usb power?


> http://en.wikipedia.org/wiki/USB#Power
> "A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA
> in USB 3.0"
>
> Stick to around 100 mA and you should be sweet.
>
> -ErJ
>
> On 3/22/09, 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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>
>
> -- 
> www.loscha.com
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