XPost: rec.electronics, sci.electronics, sci.electronics.basic   
   XPost: sci.electronics.basics, sci.electronics.equipment   
   From: CFKinsey@military.org.jp   
      
   On Wed, 19 Feb 2020 16:11:07 -0000, RobH  wrote:   
      
   > On 19/02/2020 15:43, 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.   
   >   
   > Why don't you buy decent, not cheap, PC power supplies.   
      
   I could do, but I have several of these lying around and hate wasting stuff.    
   I like to put old equipment to good use.  They work fine with the 5V under   
   load.  Just wondering why I have to do that.  I've currently got the two   
   offending supplies connected    
   to the dip and full beam of a car headlamp on their 5V lines, which works fine.   
      
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