main PSU on Diecimila's 5V?

Forum 2005-2010 (read only) Hardware Troubleshooting
1

hi!

somehow my USB-GND is at about 40mV (at the main board's usb busses) and even about 250mV (at my USB hub) above PATA-5V-GND.

Since I read that those little black AC adapters (electiricity vampires) are very inefficient, I try to use my PSU's PATA-5V-GND and PATA-5V line. My DSL modem likes it.

How can I connect my PATA-5V to Diecimila's 5V and PATA-5V-GND to Diecimila's GND connector?
I mean: Would it be like a short circuit, when I connect USB-GND to PATA-5V-GND (or is there a big resistor between them?)?
::slight_smile: :-[

And why is there a difference?
I mean: Why isn't GND of all subsystems on the same level? i'm puzzled
:-? :stuck_out_tongue:

Thx.

Bye
Arne

2

There is a jumper next to the USB connector that selects where power comes from. Set it to EXT to disconnect the USB power.

3

does that mean, that Diecimila can have one leg in USB-power-world and the other in PATA-5V-world?

I mean: They do not share GND?

-Arne

4

I mean: Would it be like a short circuit

No because you are only connecting one leg of the power supplies together so ther is nothing to drive any current flow.

You make measurements saying what the GND is on different supplies but what are you measuring it with respect to? In fact these supplies are floating that is not connected to GND at all and the measurements you are making are not valid, they don't mean anything.

What can make GNDs go to different potentials is the amount of earth current flowing. This is normally only a problem with units separated by a large distance. When this occurs you have to resort to optical isolation for the signals but in your case just connect them together.

5

i c
But: I connect two cables that r connected to the same PSU (i. e. the main PSU of my computer).

GND----[USB cable]-----[USB controller]-----[main PSU]----[PATA 5V cable]----GND
^^^^ in this setting i had a difference of 40mV between left(+) GND and right(-) GND, which frightened me a little.
funnily that difference nearly disappeared as soon as my CO2 meter sucked on the PATA-5V with 500mA for 20msec and then it went back to 40mV.

i connected both GNDs (USB and PATA-5V-GND) via an ampere meter and measured about 40µA (0.00004A).

That might be neglectable...???
Or will i damage my PSU sooner than necessary?

Can i plug that PATA-5V cable into the 5V connector in the power connector row (after i set the jumper on EXT)?
Or would that disturb the regulator?
:o

-Arne

6

i just removed that jumper completely,
so that neither the regulator nor the USB-5V-line is disturbed...

the current between PATA-5V-GND and USB-GND seems to be no problem in my case...