Worked with an Intel server system that did the same thing but kept up
about 3V.  Became a problem when the keyboards would lock up.  Intel's
solution? Beat up the keyboard vendor to change their controller reset
threshold.  Never mind that some customer is later going to change
keyboards and have problems.

Gary Crowell 

> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Howard Winter
> Sent: Monday, November 13, 2006 9:12 AM
> To: PICLIST@MIT.EDU
> Subject: [EE] PC Power Supplies - stray voltage on +5V
> 
> I tried building a device into a PC which was powered from 
> the +5V line on one of the disk drive power connectors, and 
> found that it was very 
> unreliable in starting up - more often than not it didn't!
> 
> So I put a meter on the connector and found that when the PSU 
> has mains connected, but is turned off, there was 1.8V on the 
> 5V line, so powering 
> the PC down and up does not power the device I mentioned down 
> properly.  From cold it seems to work if you connect the 
> power lead and then press 
> the power-button within a couple of seconds - more than that 
> and it locks up the device.
> 
> I tried another PSU and that was showing 0.8V and had the 
> same problem.  I know there is a separate 5V Standby supply, 
> but I would have thought 
> that the main 5V should drop to zero when power is off, shouldn't it?
> 
> Has anyone else encountered this?  Am I likely to find a PSU 
> that does behave "properly"?
> 
> If not I'll have to use the +12V and put my own 5V regulator 
> in-line, which will be annoying but will solve it (the +12V 
> line does drop to zero).
> 
> Cheers,
> 
> 
> Howard Winter
> St.Albans, England
> 
> 
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