I have started graduating in electrical engineering in the beginning of 2020. However, due to the pandemic, I had only a few classes in university before it went all online.

I have been trying to practice on my own and learn a few things about electronics.

One of the things I have been trying to make is a transformer using copped wire. I learned about Lenz’s Law and how the winding turn ratio works.

I used a steel screw 10cm tall and 18 gauge wire.

I wrapped the wire around the screw in order to make the primary winding, around 80 turns, then I added some electric tape on top of that and continued wrapping. I did this process 6 times, then added more electric tape to it. After that, I started to wrap the secondary winding and I did the same process mentioned above twice. So the turn ratio would be around 1:3. When connecting to the power outlet (220V ~ 60Hz), my transformer started getting very hot and my house lights went all weak, so I immediately disconnected it. I am not very sure what is happening and after doing some research, could not yet figure out the problem. Is there anything I am missing?

Please, forgive me if I made any nonsense stupid mistakes. It is, as I said, my humble attempt to learn electronics on my own.

  • 1
    \$\begingroup\$ Think about the inductance of your primary and what the magnetization current at 60 Hz would be. Very likely the inductance of your steel screw core construction is way to low. I would recommend experimenting with lower voltages for safety reasons. \$\endgroup\$ Commented Aug 27, 2021 at 18:09
  • \$\begingroup\$ Can you confirm that the wire insulation has a sufficient voltage rating? \$\endgroup\$ Commented Aug 27, 2021 at 18:11
  • \$\begingroup\$ @LarsHankeln I have experimented with lower voltages and it worked quite well. However, when trying to step down the outlet power by a third, it heats up a lot and very fast. \$\endgroup\$
    – Octonions
    Commented Aug 27, 2021 at 18:18
  • \$\begingroup\$ @AndrewMorton I have no way to check that beside trusting in a voltage per gauge rating table found on the internet. I have no power supplies to work with changeable AC, which made me go after making a transformer using stuff I had nearby. \$\endgroup\$
    – Octonions
    Commented Aug 27, 2021 at 18:20
  • \$\begingroup\$ @Octonions Where you wrote "18 gauge wire", it is not clear if that is bare copper wire or insulated. If you really want to experiment with deadly mains power, at least put a suitable mains-rated fuse in series with the experment, perhaps 350 mA or 1 A slow-blow. Wearing safety goggles could save your eyesight from exploding things. \$\endgroup\$ Commented Aug 27, 2021 at 18:32

2 Answers 2


As I understand it, you are using a screw as a core. The problem is that the screw does not form a complete path with high permeability. In the end, the magnetic field has a long way to go 'in the air', this leads to a high reluctance which, in turn, leads to a low inductance.

Screw Core

What happens then is that your transformer's 'excitation current' is too high. I suggest completing the magnetic path.

  • \$\begingroup\$ So that explains it. I need six more screws then. Thanks a lot! \$\endgroup\$
    – Octonions
    Commented Aug 27, 2021 at 19:16

Many things can be the reason for heating up very fast the equipment.

  1. Your wire have a failure on its insulation, causing a little short circuits, or a bad connection (high resistance, high Joule losses);
  2. Transformer excitation current is to high that the wire can not support (cant dissipe all the heat);
  3. Make sure about what is your "charge" on your secondary winding, or if its terminals are really oppened. If its with terminals connected, you have a real short circuit on it.

You can try to use more turns on it, because your core can be saturated.

Use the formula bellow to calculate your magnetic induction and make sure about the maxium of its core material can support.

enter image description here

Where Vf is your voltage, N is your turns, f is the frequency and S is the cross section of the core.

Hope I helped you.


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