From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 05/09/18 13:37, Robert Roland wrote:   
   > I have been thinking about the way a variac works. One thing that   
   > puzzles me, is the wiper design:   
   >   
   > If the wiper is too narrow, the output voltage will cut out when the   
   > wiper is between two turns of the coil.   
   >   
   > If the wiper is too wide, it will bridge two contact points, causing   
   > one turn of the transformer to be directly shorted. Although it is not   
   > a lot of voltage, it is also not a long piece of copper, so the short   
   > circuit current, I expect, would be considerable.   
   >   
   > Obviously, getting the wiper the perfect with, and also the wire   
   > spacing equally perfect is not a practical approach.   
   >   
   > How to they do it in practice? Is there some kind of snap action that   
   > makes the wiper click from one turn to the next?   
   >   
   I'm not a big variac guy, but in the couple I've seen the wiper is made   
   of graphite, and spans a couple of turns.  The series resistance is   
   small enough not to matter much for the main output, but large enough   
   not to cause serious loss due to the shorted turn due to the low voltage   
   drop between turns.   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC / Hobbs ElectroOptics   
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