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RE: Backward Power Transformer

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Paul Joppa

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In Reply to: RE: Backward Power Transformer posted by Triode_Kingdom on September 21, 2017 at 21:05:07

It only works that way if no current is drawn. If current is drawn, then the resistance of the windings consumes some of the voltage. This is why PSUD wants to know both the open-circuit (no load) voltages AND the winding resistances. Just another one of those things that are not obvious until they are. :^)

Incidentally, it works the same with audio output transformers.

Follow Ups:

Or, in other words, - sser2 22:22:53 09/21/17 (3)
the secondary's rated voltage is specified under full rated secondary current, with voltage drop in secondary's resistance and core loss factored in. As a result, turns ratio is not 1+1:1.04, but rather something like 1+1:1.35. That's why when turned backwards, the transformer puts out 91+91V.
- RE: Or, in other words, - Triode_Kingdom 07:26:18 09/22/17 (0)
  "... turns ratio is not 1+1:1.04, but rather something like 1+1:1.35. That's why when turned backwards, the transformer puts out 91+91V."
  
  Thanks! I spend a lot of time designing high-Q RF transformers, and their behavior is always bidirectional. The fact that a power transformer might be different in this respect just didn't occur to me.
  
  As it turns out, 182V will probably fit my project better than the voltage I was expecting. I need ~250V DC at 40mA, and I think this will just get there. Now I can continue with the layout. Yay!
  
  --------------------------
  Buy Chinese. Bury freedom.
- RE: Or, in other words, - Eli Duttman 02:22:27 09/22/17 (1)
  Yes, the exact same behavior is observed if you reverse connect either the Triad N-77U or Triad N-68X .
  
  The desired result can be obtained by adding a boost trafo on the AC mains side of things.
  
  Eli D.
  - RE: Or, in other words, - rongon 20:04:49 12/25/19 (0)
    Ah, good to know!
    
    So yes, figure the backwards-wired 115V+115V:115V transformer isn't 1+1:1.04 at all. Losses involved. 1+1:1.35?
    
    So it's probably best to use the N-68X wired as normal, with voltage doubling DC rectification. So you get half the current output. With an N-68X I should get 120*2 = 240VAC, rectified to 240*1.3(ish) = 312VDC under load. Correct?
    
    312V*0.1A = 31.2VA, which is within the 50VA rating.
    
    But 312V*0.2A = 62.4VA, and that's too far over the 50VA rating. Gotta bump up to the N-77U for a small stereo power amp. Oh well...
    --

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