From: CFKinsey@military.org.jp   
      
   On Thu, 20 Feb 2020 14:26:54 -0000, George Herold    
   wrote:   
      
   > On Wednesday, February 19, 2020 at 1:55:31 PM UTC-5, Commander Kinsey wrote:   
   >> On Wed, 19 Feb 2020 18:45:55 -0000, default  wrote:   
   >>   
   >> > On Wed, 19 Feb 2020 15:43:37 -0000, "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.   
   >> > \   
   >> > There's often a good reason for it.  The 5 volt supply is regulated   
   >> > and the others are not (generally speaking).  The feedback path is   
   >> > from the 5VDC output back to the mains side of the controller.   
   >> >   
   >> > The others get line (but not load) regulation via the 5V supply   
   >> > because they share a common transformer.  They are also switching   
   >> > supplies that work at a high frequency so the transformers have fewer   
   >> > turns of wire and more volts per turn which results in excellent   
   >> > transformer "regulation."   
   >>   
   >> So on a cheap shit supply, the 5V is guaranteed to be very close to 5V, but   
   the 12V will drop under heavy load?   
   >> And on a decent supply like Corsair, they must regulate both seperately?   
   >> I still don't understand why the regulation goes to pot when under 1.5A is   
   taken from 5V.  It still regulates that 5V perfectly with no load, but the 12V   
   goes wildly wrong.  Why does the regulation need  current to be flowing   
   through 5V?   
   >   
   > I didn't read all the answers, so maybe someone said this,   
   > but some SMPS (switch mode PS) topologies have a minimum current.   
   > (Mostly related to keeping current in the inductor.. you could   
   > look up the difference between continuous and discontinuous   
   > conduction modes.   
      
   In a supply which powers a computer, where currents change all the time on   
   different voltages, this is a very bad design.  I guess I just buy the better   
   ones in future.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)