Quote: Originally Posted by
gthing 
Hey I just have a couple of questions about USB device considerations. I am using a Carnetix P1900 Power supply which provides a +5V/3A output for power a USB hub/etc.
My question is how do I take advantage of this output - I mean, specifically.
Right now I have a 3 inch USB extender that I have spliced into. I found the pinout on pinouts.ru for USB, cut the 5V (red) wire, and wired in the 5V from my power supply. I left the computer side of the red wire disconnected so that 5V from the power supply does not feed back into the computer (I don't know if this is necessary but it seemed like a good idea).
VERY good idea. Most power supplies will get mad if they are trying to power the same power rail. very unexpected results can occur.
Quote: Originally Posted by
gthing I have kept the USB ground connected to the computer, even though my power supply provides an extra ground wire as well. Is that okay?
You need to make a ground connection from the power supply into the USB hub, but maintain the ground between the hub and computer. the computer needs a reference to compare the data signal voltages.
Quote: Originally Posted by
gthing As it stands, my USB hub seems to work for mouse and external sound card (cheapo 5.1 thing), but it will not spin up an external hard drive which can normally run off of USB - this I have to connect to the computer.
So is there a better way to power this? On 3 amps, how many USB devices can that little hub be expected to handle?
Like others have said each USB port is good for 500mA, including the one comming from your computer. usually each port has a current sensor to make sure it is within spec.
Without a powered hub it wont work to daisy chain a HD off the hub. HD's usually max's out the USB port. If the HD takes 500mA, the mouse takes 50mA, the 5.1 takes 30mA and the hub it self takes 20mA the power draw from the computer USB is well over the 500mA available. A powered hub is deffinately the way to go.
Quote: Originally Posted by
gthing What are the problems/issues with simply leaving a bunch of USB devices plugged into the back of the computer. I seem to have problems waking from sleep and am guessing that they are due to USB device issues.
Thanks in advance for any help.
I'm designing my system so that 2 powered usb hubs connect to
all USB devices. make removing the computer easier and the power draw is signifigantly lower to the computer PSU (extending lifespan and upping stability).
Quote: Originally Posted by
gthing I see. My USB hub does have a power connector port, so basically I just need to splice into that.
Yes. be sure and observe polarity closely. connecting the power backwards will fry the hub and all thats connected.
Quote: Originally Posted by
gthing So if I wire in a 3A connection from my power supply, and hook it up to USB, the hub will have a total of 3.5A available to it, meaning I am pushing ~500mA more than it is designed for.
Current works different from voltage. current draw is scaled based on demand. a 10A PSU with a 2A draw is just fine. Currrent is kinda like horse power of an engine; it may be capable of 150HP but if it only takes 15HP to travel at highway speed on flat ground your not doing the car any damage by not using the other 135HP. the rest of the HP are just waiting in reserve.
So... providing 2.5A will work just fine. If you look at your devices and research them you'll probably find your way under 2.5A.
Hope that helps.