From: terrell.michael.a@gmail.com   
      
   On Wednesday, February 19, 2020 at 10:44:04 PM UTC-5, default wrote:   
   > On Thu, 20 Feb 2020 00:32:05 -0000, "Commander Kinsey"   
   >  wrote:   
   >    
   > >On Wed, 19 Feb 2020 23:31:01 -0000, Jasen Betts  wrote:   
   > >   
   > >> On 2020-02-19, Commander Kinsey  wrote:   
   > >>> Why do (cheap? expensive ones may be better) PC ATX power supplies need   
   current drawn from the 5V line to make the 12V line work correctly?   
   > >>>   
   > >>> I have a PC with 3 graphics cards running scientific applications.  I   
   acquired three old graphics cards that take about 300W each, and have loads of   
   cheap (CIT) PSUs that are rated at 650W on the 12V line, which is what those   
   cards use.  So I run    
   each card off its own supply.  But the 12V line at no load, or even at 300W,   
   is only giving out 10 to 10.5V.  If I attach a small dummy load of an amp or   
   so to the 5V line, the 12V line suddenly becomes 12V.   
   > >>>   
   > >>> Why are the two lines related in any way?   
   > >>   
   > >> because all the output voltages come from taps on the same transformer   
   > >> and the voltage regulation is applied to the input to that transformer   
   > >> and the voltage regulation only watches the 5V line.   
   > >   
   > >Ok, but why does current need to be taken from 5V to make the voltage   
   monitor work?   
   >    
   > It is designed to be in a computer, and there's always some load on   
   > the 5 V line.  It probably didn't seem terribly important to worry   
   > about a high voltage condition where none should ever exist.   
   >    
   > Some power supplies sit and oscillate if they don't have a load on the   
   > 5 volt line...    
   >    
   > I notice my desktop has 5v present on the USB connector even when it   
   > is turned off, turned on, or just in standby.  I suspect it may have a   
   > small independent supply to run the USB connectors for power, and   
   > perhaps that also supplies the CMOS memory so the clock and settings   
   > don't drain the battery.   
      
      
      Look at the ATX Power Supply specifications. It should be on the Intel   
   website for free.   
      
      There is a +5VSB output, (5 volt standby output) which is a low power   
   switcher to provide power for the electronic power switch. Some computers   
   power the USB port at all times, while others like this Dell Optiplex 780   
   shuts it down if the computer isn'   
   t active.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)