Electronics > Projects, Designs, and Technical Stuff
how to specify a transformer for manufacture
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james_s:
I've bought cores and supplies for winding my own from here: http://www.bridgeportmagnetics.com/index.html

At the time at least they also sold toroid cores with pre-wound primaries over which you wind your own secondaries. They may do prototyping as well, I don't recall.
Jeroen3:

--- Quote from: mikeselectricstuff on December 22, 2018, 08:28:41 am ---No manufacturer will be interested in 1 or 10 - they are probably just fobbing you off as they think you are a timewaster.

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Just ask the right place. We ask one or two transformers varying from several kVA to several VA all the time.


--- Quote from: jaunty on December 23, 2018, 10:02:58 pm ---PS - does anyone have a good technique for removing the epoxied cloth wrapping on one of these?

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You need strong acids to remove polyurethane based epoxy resins. This requires chemical ppe! In short, it's destructive to all non-metals.
Although it does get brittle when hot, so you can try that.
If it isn't potted, regular thinner with toluene (poisonous) should work (slowly).

For larger potted transformers, to know whats inside, use pyrolysis.
Globe Collector:
Rewinding toroids of 2-300VA or bigger  with voltages less than 25 or so is not too difficult.

As the VA rating, (and core size) increases the number of turns per volt decreases, so the winding gets easier.

One point of note....in professionally would transformers with multiple secondaries the total cross-sectional-area of copper in the primary will be equal to the sum of total-cross-sectional-areas of all the secondaries and each secondary will possess a fraction of the total VA rating which is proportional to its cross-sectional-area divided by the cross-sectional-area of the primary.

For example, imagine you have a 180VA transformer with two 18v secondaries...if the wire gauges of those secondaries are the same, then each should have half the cross-sectional-area (or ampere-turns) of the primary and each will deliver 90VA or 5A RMS. However, imagine that (although the number of turns are the same) that one winding is wound with thick wire and the other with thin wire....then the thick one will deliver most of the power...say 9A (162 VA's worth) of the total, whereas the other winding will only have 18VA's worth available.

 If you "lash" an extra secondary onto an existing design, you have to bear in mind that you are increasing the total cross-sectional-area of the secondaries over and above that of the primary, so you may need to de-rate the loads on the existing secondaries, particularly if your new secondary draws a significant proportion of the available VAs (I'd ball park 10% of the total VA rating or more on your overwind) .

If you simply need a few VA for some 5v DC for some 4000 CMOS or -25v to bias some valve grids or something like that, then don't worry, but if you have a decent solid, low impedance load on your overwind, like a motor or class AB output stage of an amplifier then you will need to do something about de-rating the loads on the existing secondaries otherwise you might "let the magic smoke" out of the Primary liker Dave Jones did with that soldering station.
soldar:

--- Quote from: jaunty on December 23, 2018, 09:42:48 pm ---soldar - i'm in southern california and yes - what you say could VERY WELL be the case ... i just asked them the info they wanted and a pic of the faulty one i'm trying to replace and there seemed to be far too much 'back and forth' going on ... i told them i would send it in when i am ready to go on it ... so they can get all the info necessary when necessary but they wanted it just for an estimate - which seems silly to me .. the quote doesn't need to be so accurate - just 'in the ballpark' .. i.e. $10? $100? 10 000??? i can't be sending it out to 10 different manufacturers just for quotes ... ugh! that will take the better part of a year probably.
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That is not how things work and I am not surprised they just brushed you off. Rather than answer your vague question he would rather spend his time answering the questions of others who definitely know what they want and are much more likely to result in orders which will bring in dollars which he can invest in feeding and clothing his family. Your question seems to him like it might probably result in more confused questions and more wasted time. He has no interest in doing "approximate" studies when it costs him less effort to do an exact pricing on something clearly defined.   You might as well ask how much it would cost by weight and it would give you a rough idea. 


--- Quote from: jaunty on December 23, 2018, 09:42:48 pm --- though reading through responses to this thread makes me think MAYBE i should get an 'off the shelf' unit with the right secondaries (need 2 18V IIRC) and just wind my own extra low voltage secondaries .. an 8v secondary might not take so many turns ...
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If you can use off the shelf commercially available units that would be the easiest and cheapest route. Depending on the voltages and currents you can use two or more transformers. You can also combine windings of a transformer with multiple windings. There are many ways of going about this. If your requirements are somewhat flexible, you have a lot of space, etc. then you should be able to find an off the shelf solution. If your requirements are very tight, space is very limited, etc. then the first thing you need to do is determine and write down all those specs very clearly. But don't expect the guy at the transformer place to hold your hand through all this. He has other fish to fry.

We're mostly whistling in the dark here. Why don't you tell us what you want exactly? Pretend we're the transformer guy and tell us what you are looking for exactly.
jaunty:

--- Quote from: james_s on December 23, 2018, 10:21:33 pm ---I've bought cores and supplies for winding my own from here: http://www.bridgeportmagnetics.com/index.html

At the time at least they also sold toroid cores with pre-wound primaries over which you wind your own secondaries. They may do prototyping as well, I don't recall.

--- End quote ---

MIGHT be just the ticket! thanks
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