Tube DIY Asylum

Or, in other words,

128.125.179.205

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In Reply to: RE: Backward Power Transformer posted by Paul Joppa on September 21, 2017 at 21:24:04

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.

Follow Ups:

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!

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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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