On Saturday, 20 July 2019 18:35:38 UTC+1, Peter Percival  wrote:   
   > Phil Allison wrote:   
   > > Ralph Mowery wrote:   
   > >    
   > >    
   > >>   
   > >> All of that is well and good.   
   > >>   
   > >> However, is that going to be peak, average, rms or just what that the   
   > >> origional question wanted.  Is there going to be a capacitor to smooth   
   > >> out the voltage, or is the full wave rectified DC going to be used ?   
   > >>   
   > >> Lots of unasked questions in the origional question.   
   > >>   
   > >    
   > >    
   > > ** The original Q is a pile of utterly ambiguous drivel.   
   > >    
   > > If Q posters would simply reveal the PURPOSE of their question, ambiguity   
   would disappear and useful answers become possible.   
   > >    
   > > But no, they want to play at being smart and be in control the answers by   
   NOT revealing any such damn thing.   
   > >    
   > >    
   > >    
   > > ....  Phil   
   >    
   > Oh dear.   
   >    
   > I am looking at Fig 3.6 on p 42 of Marston's /110 operational amplifier    
   > projects for the home constructor/.  The circuit depicted is that of a    
   > power supply delivering 3-30V at 0-1A.  It is to be supplied with "+40    
   > to 45V (unregulated)".  The text has nothing to say about where that    
   > comes from.  I have decided to use a bridge rectifier attached to the    
   > secondary of a transformer with 240(ish)V primary; and I wish to know    
   > what secondary I need.   
   >    
   > The "2amps" in my OP was a guess of mine that if the PS delivers 1amp,    
   > then 2amps in would be more than enough.   
      
   A bit of an old post, but one that never got a fair answer.   
   To get 40v out, you need 40x 0.707 ac input = 28.3v rms plus the diode drop of   
   the bridge which under load will be 2x 1-2v. If we guess 1.5v diode drops   
   under load then you'd need 31.3v in. That much has mostly been covered   
   already, though a lot of EEs    
   mistakenly think diodes drop 0.65v all the time.   
      
   However there are 2 other factors not so far addressed.   
   First, the reservoir cap will charge to its peak v but then decay until   
   recharged. You need to supply a bit more V to counter the decay for 100th of a   
   second.   
   Second, there is mains voltage variation to consider. If you want the psu to   
   be reliable & meet mains voltage specs it needs to get enough V to the   
   regulator when V_mains is at the lowest it is permitted to go, which is a fair   
   bit under nominal voltage.   
      
   Finally you should only need a 1A supply, parasitic drain is trivial in any   
   normal 1A V reg.   
      
   And really finally, check the reg can dissipate the power at full load when   
   mains V is max, I_out is max & V_out is min. And ensure you have sound   
   short/overload protection.   
      
      
   NT   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)