From: phil_a@tpg.com.au   
      
   "Big Bad Bob is off with the Fairies"   
      
      
   >> Transformer VA is rated with resistive load. I.E. voltage and current   
   >> sine waves. Rectified and capacitor filtered DC, however, doesn't draw   
   >> a current sine, the entire DC current draw is compressed into a short   
   >> duration high current capacitor charge pulse.   
   >   
   > Now I think I see where this is going.  Transformers apparently don't   
   > handle 'duty cycle power averaging' very well if there are 'hot spots' in   
   > the windings due to high current pulses.   
      
   ** Nonsense.   
      
   Pulses of current cause the heat build in wires to be greater than if the   
   same *average* amount of current were delivered smoothly. Do the math.   
      
   Eg:   
      
   Imagine a wire with 1 ohm of resistance and you pass 1 amp of DC through   
   it - the heat loss is 1 watt, OK ?   
      
   Now instead, you decide to pass 10 amps switched so that it is on for 1   
   second and off for 9 -  ie the same, 1 amp average current.  The heat loss   
   in the wire is now 10 times more since that loss is proportional to current   
   squared while the duty cycle is only 10%.   
      
   100 x 0.1  = 10.   
      
      
      
   > You can get away with average power based on duty cycle for MOST devices   
   > but there are instantaneous maximum ratings that you don't want to exceed   
   > for just about anything.   
      
   ** More nonsense.   
      
      
   > FYI I've noticed that rectified and filtered output from transformers   
   > tends to approach the RMS voltage when you get close to the maximum rated   
   > current, even if you use gigantic filter capacitors.   
      
      
   ** That is mostly due to resistive * voltage drop *  in the copper windings   
   which have to pass the current pulses.   
      
      
   > I think it has less to do with resistance than it does with flux density   
   > and inductive action within the transformer itself.   
      
   ** Nope.   
      
   It is almost entirely due to simple resistance.   
      
   Few transformers have enough primary to secondary " leakage"  inductance to   
   matter at 50 or 60 Hz.   
      
   And never imagine the "saturation" is the culprit  - cos the MORE  you load   
   the secondary of a transformer,  the LESS it is likely to become   
   magnetically saturated.   
      
      
      
   ....  Phil   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)