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 23:46:45 -0000, Jasen Betts  wrote:   
      
   > On 2020-02-19, Commander Kinsey  wrote:   
   >> 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?   
   >   
   > The difference between nothing and 2A is a factor of infinity   
   > The difference between 2A and 20A is only a factor of 10   
   >   
   > Actually the fisrt is more like the difference betweem 2mA and 2A   
   > because the internal feedback takes avout 2mA to run the LM431 and the   
   > optocoupler. So going to 2A loads the 5V output by 1000 times more, better   
   > than infinity. but not by much.   
      
   If it's going to be so shit, they could have added a dummy load inside the PSU   
   to make it work properly.  They could even have it shut off if there was   
   enough external load.   
      
   > Diode voltage drop is logarythmic vs current so the voltage on the   
   transformer   
   > needed to make 5V on the output is less with a 2mA load than it is with   
   > a 2A load.   
      
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