I am a certified computer programmer, but not an electrician, after researching on google, youtube, reading few books and referring to some local technicians, I made a transformer on toroidal core with 100 turns/ 18 SWG wire for both primary and secondary, when I tried testing it, my home fuse burnt out. I guess my calculations were not right.

I want to know formulae and calculations one should follow in order to make a transformer of a particular type (step-up, step-down or isolation) for a specific load of VA.

My requirement is that, I'll connect it directly to my mains of 230 volts AC instead of a variac (to save variac cost) and I want to run a 100 watts incandescent light bulb that consumes 0.4347 amps at 230 volts. I want to know what core size for EI type cores, should I choose, what size SWG wire should I choose for primary & secondary, how many turns should I put on primary and secondary so that transformer consumes 230V & .6A at primary and delivers 230V & .4A (after losses) at secondary. Should I also be aware of the type of tape, paper I choose to laminate core before winding? And is there any other thing I should be aware of in order to build a successfull transformer?

Please englighten me. Thanks!

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    This might be a better fit on electronics.stackexchange.com Commented Jul 11, 2022 at 20:59
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    To avoid an over-current situation with no load on the transformer, the primary coil and core need to produce sufficient inductance. The size of the wire is not a major factor.
    – Hot Licks
    Commented Jul 11, 2022 at 21:19
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    What are you trying to accomplish? Way too many questions. Not a good idea to hook up unlisted self made devices to your mains. If you can't figure this out, you shouldn't try to make it.
    – JACK
    Commented Jul 11, 2022 at 21:25
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    What the H.E. double hockey sticks are you trying to accomplish? Any transformer with the same number of windings on the primary and secondary will put out the same voltage on the secondary side as the primary. Why not just connect the darn bulb to your normal (mains) circuit, since, according your post is also 240? Sorry, but I'm going to vote to close because this is clearly not a home improvement question. Commented Jul 11, 2022 at 22:00
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    I’m voting to close this question because it's clearly not a DIY home improvement question. Commented Jul 11, 2022 at 22:01

1 Answer 1


This is madness. 230V power is dangerous!

You don't build things like that. You buy them commercial off the shelf, in enclosures, with the necessary government certifications (BSI, CSA etc.) from competent suppliers.

There are many electronics skills worth building, but transformer design is not one of them unless you plan to do a lot of them or something really weird.

A lot of electrical injuries are not deaths, they are heinous bodily damage.

Remember that even if your circuits are RCD protected, that protection is void on the secondary side of the transformer.

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