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These notes assume you're already familiar with basic electrical safety and design concerns when working with high-current mains powered equipment and will not explicitly mention all safety precautions needed. If you are not confident of your knowledge and skill to safely handle this then please seek assistance.

For ratings under about 500VA you may be able to find DIY transformer kits already including the core, bobbins and a design guide, in which case ignore all of this and just follow the instructions supplied.

Most transformer design processes start with picking of the core as it's usually the part with the most restricted set of choices available

If you already have a core and primary coil available

Then your task is going to be a LOT easier!

  1. The rating of the transformer you're taking apart for this should already tell you how much current your primary coil can take, that gives you your power rating (less a few % for efficiency)
  2. Don't dismantle your transformer yet
  3. Wrap between 10 and 100 turns of quite thin wire around the existing secondary (whatever you can fit) power it up, and measure the RMS voltage on that new coil, don't try to pull any current from this coil
  4. Divide by the number of turns, now you know the volts-per-turn of the transformer you've inherited. Depending on size this could be anything from millivolts to tens of volts, smaller Xfmrs usually having lower values
  5. OK so work out the number of turns you're going to need for your secondary to get the voltage you're seeking
  6. Work out how big the wire will need to be to support the current you're trying to get. You'll need high-temperature enamel wire and rate it VERY conservatively as you'll be packing in hundreds or thousands of parallel wires carrying the current. Plausible values for current density in the wire range from around 2A/mm^2 for a winding buried deep inside the transformer up to about 4A/mm^2 for a thin winding right on the outside of the bobbin.
  7. So now you have the thickness of your wire and the number of turns, so from this you can get total cross-sectional area of your secondary coil
  8. Increase this theoretical area by between 25%-35% depending on how complex your secondary coil is and how carefully you're going to wind it
  9. Check if this is actually going to fit in the remaining space in the core window once the existing secondary is removed, allowing for suitable insulation and bobbin
  10. If yes - Good to go! If no - you're going to need a new (larger) core, see the design-from-scratch section
  11. Two options here, first is removal of secondary
    1. Cut the secondary coil away, being very careful not to damage the primary
    2. Wind the core with insulation and apply separator plates
    3. Wind a new secondary around the core, possibly including thermal sensors. This will work best if the secondary has relatively few turns (<150) otherwise see complete disassembly
    4. Pay attention to providing insulation and safe creep distances to avoid flash-overs either between layers or primary-to-secondary
    5. Apply suitable terminations to the secondary
    6. And you're done! Test thoroughly and check for any thermal issues
  12. Second option is complete disassembly and rebuild
    1. In order to fit a new secondary with very many turns it will be necessary to get the core apart
    2. Cut away the secondary being very careful not to damage core or primary in order to get more access
    3. Examine your core carefully, you're looking for a weld line running across 2 sides of it, if it is a welded core
      1. Grind or mill the weld away
      2. Tap the core gently to separate the parts being careful not to split laminations apart
    4. If it's not a welded core this is going to be harder
      1. You need to gently pry the laminations apart and slip them out from the center of the primary coil, noting the pattern in which they were laid up
      2. A little heat may help here
      3. Be very careful not to damage or kink any of the laminations, you're going to need to re-use all of them
    5. Once you have them fully separated you can 3D print or mill a new bobbin for your secondary
    6. Wind your new secondary, the metal lathe may help you on larger secondaries, or you can borrow the small coil winder machine (post on the mailing list to ask), possibly including thermal sensors
    7. Pay attention to providing insulation and safe creep distances to avoid flash-overs either between layers or primary-to-secondary
    8. If your core was welded then you need to re-clamp and re-weld your core (TIG will be required). If it was non-welded then slip all the laminations back in with the new secondary in place
    9. You must replace ALL of the core laminations that you had, if you leave any out then the leakage current and resulting heating in your transformer may increase drastically
    10. You may wish to impregnate your core with polyurethane varnish which will reduce buzzing noises
    11. Apply suitable terminations to the secondary
    12. And you're done! Test thoroughly and check for any thermal issues

If you're starting from scratch

Then this is going to get a lot more mathematical....

  1. Make a first-guess at a likely core size, this is based on experience but as a rough guide 50mm core might do 50VA, 100mm core might do 250VA, 200mm core might do 2KVA, 300mm core might do 5KVA, 500mm core might do 25KVA, 750mm core might do 100KVA. The relationship between core size and power handling is (more than) cubic, so small increases in core width give large increases in power capacity
  2. From a manufacturer's catalog find a lamination that fits this size and get the technical drawing for it
  3. This should tell you the size of the core window you're working with
  4. Consider the number of coils you are needing to fit and suitable separations to allow for safe creep distances that are needed, work out the actual cross-sectional area that you have for windings
  5. Now look at your windings, consider the voltage ratings, as all windings on one core have the same volts-per-turn you now know the relative number of turns on each winding but not the absolute numbers
  6. Work out the proper wire size that you're going to need for each winding. High-temperature enamel wire is strongly recommended and de-rate it considerably to account for the fact that a large number of turns are going to be tightly packed
  7. By multiplying wire cross-section and number of turns for each winding you can calculate the relative area that each winding will require in the core window
  8. You already have the the calculated available window, so divide it up accordingly and you now have the cross-sectional area of each winding
  9. From these reduce these numbers by between 25% and 35% to allow for winding inefficiencies and errors or carelessness in windings
  10. You can now calculate a number of turns for each winding by dividing area available by cross-sectional area of each turn
  11. Given the number of turns for the primary and the voltage it must support (remember that it must withstand the maximum permitted voltage of 230V +10% = 253V rms) you can work out the voltage per turn that must be supported
  12. If you're lucky your chosen manufacturer will list a value for volts-per-turn-per-lamination so you now know how many laminations you need and so how thick a stack you need, but if not.....
    1. Find out the maximum magnetic flux density that your chosen core material can withstand without saturation, the manufacturer should provide this information but typical values are between 1-2Tesla for most common silicon steels.
    2. You can calculate the total flux that will be needed to support your target volts-per-turn
      1. Include how here later
    3. Having got that you can work out how thick your lamination stack needs to be.
  13. Compare the thickness of your lamination stack to the width of the central pole piece of your laminations
  14. If the aspect ratio is <1:1 then you might be able to use a smaller core, if it's >3:1 you probably need to use a larger core, either way go back to step 2 with the new core size. If it's in the range 1:1 to 3:1 then you've got a suitable core size
  15. Knowing how many laminations you need, the manufacturer will list a weight per lamination and will probably sell them by weight, so you can order the right quantity
  16. You can now make bobbins, wind your coils and assemble your transformer as per the "Complete dissassembly and rebuild" section above