XPost: rec.electronics, sci.electronics, sci.electronics.basic   
   XPost: sci.electronics.basics, sci.electronics.equipment   
   From: CFKinsey@military.org.jp   
      
   On Thu, 20 Feb 2020 21:06:22 -0000, default  wrote:   
      
   > On Thu, 20 Feb 2020 20:07:47 -0000, "Commander Kinsey"   
   >  wrote:   
   >   
   >> On Thu, 20 Feb 2020 03:43:58 -0000, default  wrote:   
   >>   
   >>> On Thu, 20 Feb 2020 00:32:05 -0000, "Commander Kinsey"   
   >>>  wrote:   
   >>>   
   >>>> On Wed, 19 Feb 2020 23:31:01 -0000, Jasen Betts  wrote:   
   >>>>   
   >>>>> On 2020-02-19, 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?   
   >>>>>   
   >>>>> because all the output voltages come from taps on the same transformer   
   >>>>> and the voltage regulation is applied to the input to that transformer   
   >>>>> and the voltage regulation only watches the 5V line.   
   >>>>   
   >>>> Ok, but why does current need to be taken from 5V to make the voltage   
   monitor work?   
   >>>   
   >>> It is designed to be in a computer, and there's always some load on   
   >>> the 5 V line.  It probably didn't seem terribly important to worry   
   >>> about a high voltage condition where none should ever exist.   
   >>   
   >> What load would that be?  In my newest computer for example (which has a   
   decent supply which doesn't need the 5V load), it uses an SSD which draws fuck   
   all power.  The CPU and graphics card and memory all operate from 12V with   
   their own regulator    
   modules.  I can't think of anything drawing much 5V, and the dodgy supplies I   
   have need approaching 2 amps!  I tried 1 amp and that didn't completely   
   stabilize the 12V line.   
   >>   
   >>> Some power supplies sit and oscillate if they don't have a load on the   
   >>> 5 volt line...   
   >>>   
   >>> I notice my desktop has 5v present on the USB connector even when it   
   >>> is turned off, turned on, or just in standby.  I suspect it may have a   
   >>> small independent supply to run the USB connectors for power, and   
   >>> perhaps that also supplies the CMOS memory so the clock and settings   
   >>> don't drain the battery.   
      
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