# What's the different between transformer types?



## proid

I'm plannning on building a CKIII, my previous project is the starving student so now i have to learn to work with transformer. My main curious is i saw many type of transformer but the most popular one are these two:


  I don't know the first one name but the second is a toroid transformer and it's used in mostly every amps nowaday. So what's the different between these two transformers? And what will i get when i pay more money for high grade transformer?


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## jcx

the top is commonly called a "R-core", semi-toroidal - the windings are in balanced sections on the two bobbins - the core has a round cross section is actually 2 "C" cut cores clamped together
   
  the bottom is a fully toroidal transformer with the windings in layers evenly spread over the toroidal core - it has less external magnetic field leakage but much higher pri-sec parasitic C
   
  the fully toroidal construction used to be more expensive but with automated winding machines its higher material efficiency makes them cheaper, widely available
   
  a undeserved reputation for "quality" is a holdover from the early days of expensive fully toroidal transformers
   
  the low radiated magnetic field is a real advantage if you want a compact build with sensitive circuits crowded up against the transformer
   
  but the high pri-sec C couples more line noise, the core saturates hard and in driving material cost down most are working near the edge of saturation in normal operation
   
  EI laminated core, split bobbin transformers have the least pri-sec C, better saturation characteristics - but do have larger external magnetic field leakage - need more room, or shielding - but isolate better from line noise
   
  the R core is somewhat in between, its dual split bobbins double the pri-sec C  but partially cancel the magnetic field leakage


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## qusp

i've started to see some 'R-toroidal' transformers getting around too, first saw them on selectronic, who are also having a rather excellent introduction special on new models of their Rcore, tiny little PCB mount Rcores. the rarest seems to be double C core.
   
  I thought Rcore had no gap? does clamping them together close the magnetic gap entirely? I had assumed by looking at them that they were made from one piece.
   
  one question I had regarding the Rcore if you dont mind jcx, does their completely symmetrical construction mean that they really dont have an beginning or end? they would seem to be identical connected either way. I wouldnt expect any more leakage connected one way or the other unlike other types. unless i'm missing something


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## jcx

round cross section core saves a little copper - which cost more
   
  my understanding of the twin bobbin designs was that they are wound separately and the cut core is slid inside - so there should be a gap - but as small as tolerances can make it maybe < 1 mil
   
  for power transformer cores the operating point often where the relative permeability has rolled off to few 1000x anyway
   
  I'm not a expert transformer designer but have designed a couple for odd applications, selected lots more commercial designs for power supplies
   
  it looks like "wound core" R-core and "step core" designs are available, and some rectangular R-core may be "uncut" single wound strip - would have to be wound with toroidal winding machine anyway


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## fzman

r-core seem really hard to get in the USA, except for ebay.  I do mostly external supplies, and favor split bobbins from Hammond, Triad, or Signal if they are sized right for the application.  They produce quiet supplies and can be gotten from Digikey and Mouser.
   
  Toroids are easy to find, i usually would get them direct from Avel Lindberg.  I also have a soft spot in my heart for custom-wound Electra-print EI transformers (love the smell of shellac in the morning, smells like ......   DIY!   )


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