When to "wind-your-own" vs "scavenge" a transformer?

Question

I often see answers for one-off designs including the advice, "Just wind a transformer...." Like it's no big deal. Did I miss a class in which they handed out a sheet listing the formulas for core size, composition, geometry; wire gauge and insulation; and winding style as a function of voltage, power, frequency, duty cycle, and load application?

But I also frequently see people scavenging and reverse-engineering the pins on flyback transformers from CRTs.

So I'm deducing that there's an envelope inside which one realistically hand-winds a custom transformer, and outside of which it would be difficult/wasteful/dangerous to do so. What are the parameters of that envelope, and why?

Answer 1

Cost (money and time), and availability.

There are several "standard" transformers that can be bought ready, for example it is likely that you can buy 1:1 with different cores and wire gauges. If you need an uncommon ratio, multiple secondaries, off-center taps or a combination of these, then you find yourself in the "long tail" of the market, where specialist firms will gladly build transformers for you...for a price.

At this point, it usually becomes cheaper and faster to build your own, especially for one-off projects. The formulas can be found in the relevant literature or on the Internet, and I'm sure these are mentioned more than casually in EE courses as well.

Answer 2

Yes you did! NAB=Vt for any core saturated topology including all 50/60 Hz transformers, and Vrms=4.44fNAB for sinusodial waves. You can assume 1 for B for laminated iron core or perhaps 0.8 for a conservateive design (you are trading no-load losses for copper losses here).

Wire gauge is normal thermal heating for all low frequency transformers or rule of thumb 4 A/mm^2 for you most conservative and very long life time even at high ambient temperatures up to about 10 A/mm^2 for very small with good cooling and only ambient temperature.

If you have pulse applications, core saturation equations are the same but wire gauge can be heavily compromised.