XPost: rec.electronics, sci.electronics, sci.electronics.basic   
   XPost: sci.electronics.basics, sci.electronics.equipment   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 2020-02-19 10:43, 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?   
   >   
   > Sorry for the crosspost, I'm not sure which of these groups are active.   
      
   A lot of cheap supplies regulate only one output, and rely on   
   cross-regulation via the transformer to control the others.  If the   
   regulated output isn't loaded, it rises out of spec and so do the others.   
      
   Cheers   
      
   Phil Hobbs   
      
   --- SoupGate-Win32 v1.05   
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