XPost: rec.electronics, sci.electronics, sci.electronics.basic   
   XPost: sci.electronics.basics, sci.electronics.equipment   
   From: CFKinsey@military.org.jp   
      
   On Thu, 20 Feb 2020 01:22:00 -0000, Jasen Betts  wrote:   
      
   > On 2020-02-20, 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?   
   >   
   > because if there's enough voltage on the 5V no effort is made to   
   > supply the 12V   
      
   Sounds like a horrid design making way too many assumptions.   
      
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