From: CFKinsey@military.org.jp   
      
   On Thu, 20 Feb 2020 04:56:55 -0000, Michael Terrell  wrote:   
      
   > 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.   
      
   I said "They could even have it shut off if there was enough external load."   
      
   > 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.   
      
   A decent supply made by Corsair lets you take the full 850W from the 12V   
   line.  The designers of this supply are morons.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)