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 18:54:56 -0000, Phil Hobbs  wrote:   
      
   > 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.   
      
   Strangely, with no 5V load, I get 5.2V and 10.5V.  A small rise and a large   
   drop.   
      
   I can't understand why the following happens: No 5V load, 12V is out.  Small   
   (2A) 5V load, 12V is ok.  Yet if I draw 30A from 5V, the 12V is still ok?  How   
   can zero load upset it, but 2A or 30A (big difference) both be ok?   
      
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