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I'm kinda stuck in a situation where i have a 18650 Li-ion battery which is enough to power my atomizer throughout the day, but yet I want to be able to use some low-resistance (sub-Ohm) atomizers from time to time just to have some fun. I don't want to buy any 100/150/200W box mods, because they cost too much and, when being fired at 200 Watts they'd drain any battery in a matter of minutes, and i don't feel like sitting the whole day near the power inlet waiting for my batteries to charge. So what i need is a mains powered high-current power supply for indoor use only.

Voltage of the fully charged Li-ion battery is 4.2V and sub-Ohm atomizers may have really low resistance, so we're looking at at least 30-40 Amps of current.

The only idea i come up with is using 220/12V toroidal power transformer. I could buy one, and then insulate or even remove 12V winding. Then I'd add my own winding with the number of turns just enough to get 4.2 from it. I was thinking about a ~300W transformer, so no matter if we gonna drain even 50 Amps from the secondary winding, our first winding is gonna be able to handle it, since it was factory designed for even higher amounts of current.

I'm not afraid of connecting myself to the mains through my mouth, because primary and secondary windings are gonna be isolated from each other, but I do have some concerns about self-induction. When you close/interrupt an electrical circuit which has an inductive element in it, circuit's voltage spikes just for a little moment. If you will try to quickly connect and disconnect even a 4V battery to the primary winding of such transformer with your bare hands, you gonna feel like Zeus shook both your hands. But our secondary winding in this particular case is not gonna have too much turns, so self-induction in it isn't gonna be too high, isn't it? So, is it gonna be any dangerous or not?

I'm sorry, I wish I knew more about the physics, so I wouldn't have to ask such weird questions. If you have any better ideas how to build a safe power supply which can handle that amount of current, I'd appreciate your answers too. Thanks in advance!

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    \$\begingroup\$ If you have no experience, stay away from the mains. It will sneak up behind you and kill you dead. Buy any existing commercial 4V SMPS. \$\endgroup\$ – Tom Carpenter Apr 30 '16 at 20:34
  • \$\begingroup\$ @TomCarpenter I totally agree with you, but to be honest... How can two wires separated from each other by multiple layers of thick isolation be any dangerous? And using SMPS for that kind of "project" isn't really a good idea. They aren't designed for such use, and the most interesting thing is: they have half of the mains voltage between any of their output lines and the earth, so it's not a good idea to touch it's output and any mains-powered device at the same time. \$\endgroup\$ – John Doe Apr 30 '16 at 20:46
  • \$\begingroup\$ why stop at mains when you can go for the high tension wires? \$\endgroup\$ – Tim Spriggs Apr 30 '16 at 21:03
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    \$\begingroup\$ @TimSpriggs didn't find any near my house. \$\endgroup\$ – John Doe Apr 30 '16 at 21:26
  • \$\begingroup\$ @DoxyLover I think you are forgetting a somewhat important word in your comment. \$\endgroup\$ – efox29 May 1 '16 at 0:03
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I would not mess with what you are asking. But, you do dont need 100w to subohm. 40 is plenty and you should be able to buy a safe one for under $30.

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You could use the filament winding on a microwave oven transformer - they're designed to deliver single-digit volts at several tens of amps. Or just buy a small arc welder...

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  • \$\begingroup\$ Aren't you referring to microwave oven transformers which hobbyists have hacked, replacing the high voltage magnetron winding with a low voltage winding of their own construction? \$\endgroup\$ – Harper - Reinstate Monica May 1 '16 at 3:00
  • \$\begingroup\$ @Harper Unmodified MOTs come with a low voltage, but well-insulated winding, just a dozen or so turns of heavy wire, to power the filament of the magnetron. This is separate from the HV winding. \$\endgroup\$ – pericynthion May 1 '16 at 7:49

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