From: terrell.michael.a@gmail.com   
      
   On Wednesday, February 19, 2020 at 7:33:30 PM UTC-5, Commander Kinsey wrote:   
   > 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.   
      
   That is absolutely stupid. A dummy load wastes power, it generates extra heat   
   and the heat shortens the equipment's life. Those are dedicated power   
   supplies, not bench or lab supplies. They were designed to operate with a   
   minimum load. If you don't like    
   it, go to a four or five output supply with trimmers to calibrate each output.   
   Be prepared to pay $250 to $400 for one.   
      
   You bitch about only getting 650 Watts output. Read the specifications. that   
   850W maximum is fully loading all outputs at the same time. Do yourself a   
   favor and take some basic Electronics classes.   
      
   --- SoupGate-Win32 v1.05   
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