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PCM2702 USB DAC Revision B - Page 10

post #136 of 670
10/22/05 at 11:02pm
Quote:
Originally Posted by DaKi][er
The 500mA limit of the usb port isnt really something you have to stick that close to, there is no current limiting on it and the only limits you have that are of any effect are the psu at the computer end and the interconnects on the way to the device under power.
Just curious, if this is true why don't manufacturers that build enclosures for HD's directly draw power from the USB port (and not just the laptop HD's) instead of requiring a separate switching PSU? If the limit is directly related with the PC's PSU, then wouldn't just a note on the necessary system requirements easily solve the HD enclosure manufacturer's problem? I think their may be other problems with this idea, but I'm not even vaguely familiar with USB specs and won't presume to know anything factual... (slowly covering my own arse
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post #137 of 670
10/22/05 at 11:11pm
Quote:
Originally Posted by grasshpr
Just curious, if this is true why don't manufacturers that build enclosures for HD's directly draw power from the USB port (and not just the laptop HD's) instead of requiring a separate switching PSU? If the limit is directly related with the PC's PSU, then wouldn't just a note on the necessary system requirements easily solve the HD enclosure manufacturer's problem? I think their may be other problems with this idea, but I'm not even vaguely familiar with USB specs and won't presume to know anything factual... (slowly covering my own arse
I am 90% sure there is a limit of current drawing from USB port directly from computer.
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post #138 of 670
10/23/05 at 12:10am
Quote:
Originally Posted by diablo9
I am 90% sure there is a limit of current drawing from USB port directly from computer.
And I'm 90% sure there isn't

I'll do some testing here now to prove/disprove it, an old P2 motherboard can be my victim, results to be posted after I'm done
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post #139 of 670
10/23/05 at 12:12am
if you draw too much, windows will error saying that the device is sucking too much juice and won't work.
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post #140 of 670
10/23/05 at 1:33am
Quote:
Originally Posted by cire
if you draw too much, windows will error saying that the device is sucking too much juice and won't work.
that's exactly what I am saying.
if you try a external hard drive enclosure on USB port without power connected, windows won't recognize it.
For some 2.5 inch hard drive enclosure, the power cord is often optional, meaning it usually works without power cord connected, but on some computer it won't recognize it, when this happens, you just connect the power and everything is OK.
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post #141 of 670
10/23/05 at 1:43am
The setup -
1 * AB-AH6 motherboard
a usb cable tapping into just the 5V and GND lines
an 80mm PC fan as the initial load and to provide some cooling for the 1/2W 56ohm resistors that became the increasing load as i plugged more into a breadboard

Some numbers -

At idle, the voltage was 5.07V coming out, and as i loaded it up more it slowly dropped
250mA - 4.62v
315mA - 4.54v
390mA - 4.48v
390mA - 4.48v
460mA - 4.42v
523mA - 4.36v
590mA - 4.30v
650mA - 4.25v
710mA - 4.20v
775mA - 4.15v
830mA - 4.10v
890mA - 4.05v

Conclusion? Seems there is some limiting but noting stopped me getting to 900mA and probably nothing stopping me further going over 1A, just that the voltage would continue to fall at the same rate
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post #142 of 670
10/23/05 at 3:58am
we are not talking about the same thing. it's probably OK on hardward level, won't blow anything or fry anything, but on software level, Windows might freeze or do some weird things on the device of this USB port.
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post #143 of 670
10/23/05 at 4:33am
obviously wasnt in windows when you did that experiment....
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post #144 of 670
10/23/05 at 5:13am
Quote:
Originally Posted by cire
obviously wasnt in windows when you did that experiment....
It was in XP, and i wrote that post on that same computer while it was running
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post #145 of 670
10/23/05 at 5:17am
Quote:
Originally Posted by DaKi][er
It was in XP, and i wrote that post on that same computer while it was running
XP could be running while you are doing the exp, but funny thing could be happening on that USB device, too. You can try to connect an external drive to that USB port and draw different current from the same drive while operating the drive.
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post #146 of 670
10/23/05 at 5:26am
Quote:
Originally Posted by DaKi][er
It was in XP, and i wrote that post on that same computer while it was running
then you must not've been sending, recieving data from that slot. try that with something that actually interacts with the computer.
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post #147 of 670
10/23/05 at 6:11am
Quote:
Originally Posted by DaKi][er
The setup -
Conclusion? Seems there is some limiting but noting stopped me getting to 900mA and probably nothing stopping me further going over 1A, just that the voltage would continue to fall at the same rate

What you have measured is pretty much a perfect output impeadance of 1.15 Ohms. Thus far you have not hit any internal current limiting, but this neither says that there is any, or where it is. But neither does it say that the USB power is totally happy with you either.
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post #148 of 670
10/23/05 at 9:54am
Quote:
Originally Posted by cire
then you must not've been sending, recieving data from that slot. try that with something that actually interacts with the computer.
No, i didn't have anything else connected to the port (or any other port) and it was just a straight cable that ZI chopped the end off and wired into the voltage lines
Next weekend i'll do the same thing with a device connected if you want and see what happens there

Francis_Vaughan, I could see the changes happening quite linearly, I just didn't bother to do any calculations on them
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post #149 of 670
10/23/05 at 10:17am
Exactly. The results are almost a perfect 1.15 Ohm output resistance. One might suspect that in fact there is a resistor in the way. The question about what you can reasonably draw is, however, not properly answered.
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post #150 of 670
10/23/05 at 6:23pm
Quote:
Originally Posted by grasshpr
Just curious, if this is true why don't manufacturers that build enclosures for HD's directly draw power from the USB port (and not just the laptop HD's) instead of requiring a separate switching PSU? If the limit is directly related with the PC's PSU, then wouldn't just a note on the necessary system requirements easily solve the HD enclosure manufacturer's problem? I think their may be other problems with this idea, but I'm not even vaguely familiar with USB specs and won't presume to know anything factual... (slowly covering my own arse
Harddrives need a 12V rail too.
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