300V Flyback converter schematic

I'm intending on building this circuit, but re-specified for an output of about 150V @ 5mA. Enough to light two neon bulbs from a 12V supply.

Why do flyback transformers such as the Wuther's seem to be either specially made for this purpose, or hand wound in the DIY world? There are numerous web examples of people hand winding these things with spooled wire onto various formers.

This circuit comes with an LTSpice model. LTSpice treats all transformers and inductors similarly, and the model does not mention things like "magnetic saturation, eddy current, flux density". The simulation works very well (it's what LTSpice was written for) so there seems to be no modelled difference to using a standard mains step down transformer. Indeed, this item available on a well known auction site clearly shows a 12-0-12 mains transformer-

SMPS driver board with mains transformer

There is a 1:10 transformer in my original above example. That's very similar to a standard 230v – 24v transformer backwards. Why hand wind or look for the elusive Wurth?

  • 1
    \$\begingroup\$ Transformers ... and inductors ... are pretty much the only components you CAN manufacture to meet your own requirements. \$\endgroup\$
    – user16324
    Commented Mar 30, 2016 at 22:07
  • \$\begingroup\$ You're not going to be able to use a normal 230V transformer in this application, they're designed for completely different use cases and have very different properties than the transformer you need in the converter shown. \$\endgroup\$
    – pipe
    Commented Mar 30, 2016 at 22:16
  • \$\begingroup\$ @pipe -why please? \$\endgroup\$
    – Paul Uszak
    Commented Mar 30, 2016 at 22:20
  • \$\begingroup\$ @PaulUszak I'm not experienced enough to answer that question, but it has to do with magic words like magnetic saturation, eddy current, flux density, all of which I know little about. Another stack exchange question would be better suited for this. \$\endgroup\$
    – pipe
    Commented Mar 30, 2016 at 22:37
  • 2
    \$\begingroup\$ A mains (50-60Hz) transformer is typically wound on a laminated silicon steel core. These work great for that application, but are prone to eddy losses which become oppressive when the frequency rises much higher than say a few hundred Hz. The steel is a conductor, and a current is induced in a conductor subjected to a varying magnetic field. That current circulates in the steel (thickness-wise) and generates heat. The reason the steel is laminated (and each layer insulated) is to keep those eddy current loops small (limited to one layer thickness). Switching cores are typically ferrite. \$\endgroup\$
    – scanny
    Commented Mar 31, 2016 at 2:42

2 Answers 2


Flyback transformers require a very small air (or other dielectric) gap within the magnetic core structure. This helps prevent core saturation but lowers inductance. Most transformers that you're familiar with aren't the flyback type so they would saturate early and fail to function well as a flyback so you can't just swap in a random transformer from some old, no longer used electronics usually. Between difficulty finding the exact right type of transformer for your application, cost, availability, and ease of finding magnet wire, it's not terribly surprising that many choose to make their own for one-offs.

Lastly, although using old transformers from other electronics may work, it likely won't work well because there's engineering tradeoffs that are made that probably aren't anywhere near ideal for your application. In total, not only are you looking for the correct ratio of turns, you want wire thickness that will keep your resistive losses to a minimum, inductance that's suited to your circuit, and in this case, an airgap in the magnetic core.

See here for more info on why flyback transformers need the special air-gap.

  • \$\begingroup\$ When you say "won't work well," do you mean anything other than the efficiency will be compromised? \$\endgroup\$
    – Paul Uszak
    Commented Apr 2, 2016 at 19:31
  • \$\begingroup\$ @PaulUszak Efficiency will be greatly compromised, your output voltage likely won't be as high, you may immediately cause catastrophic failure because you're creating a voltage higher than the insulation breakdown voltage was spec'd to, or with all the extra heat wasted, you may melt the insulation causing premature failure. As an experiment you can try it, but don't expect it to last unless you know all of the design parameters of the old transformer actually match your application. \$\endgroup\$
    – horta
    Commented Apr 3, 2016 at 14:40
  • \$\begingroup\$ Even more so with cheap mains transformers that often do not only not have a gap, but have their laminations welded with reckless abandon (giving better mechanical stability but disgusting idle losses).... \$\endgroup\$ Commented Sep 26, 2017 at 10:33

You rather answered your own question. People hand-wind transformers because commercial models don't exist, or they are too expensive, or they are not available in small quantities, or an odd ratio or wire gauge is needed, etc. etc.

  • \$\begingroup\$ I'm not sure I can accept this (other than the fact that people mysteriously do). My Chinese switch mode supply just arrived. In the middle, is a big yellow through the hole transformer that looks as if it was wrapped by a machine rather than a small lady sat in the back of a factory. Thought: could it be that people wrap their own because it's fun /can't be bothered to design a circuit that uses an off the shelf one? Isn't it easier to find a commercial transformer /inductor than a bobbin to hand wind? RS Components has shed loads that you can buy individually. Cheapest £2.29 for 1. \$\endgroup\$
    – Paul Uszak
    Commented Apr 11, 2016 at 13:04
  • \$\begingroup\$ Your Chinese switch mode supply was made in "production quantities" (at least 100s if not 1000s). When you are talking about that volume, sure you can set up an automatic winding machine to make as many as you need for the production run. \$\endgroup\$ Commented May 12, 2016 at 21:35
  • \$\begingroup\$ RS Components has shed loads that you can buy individually (118 today). uk.rs-online.com/web/c/power-supplies-transformers/transformers/… Cheapest £0.82 for 1. \$\endgroup\$
    – Paul Uszak
    Commented May 13, 2016 at 22:25

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