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# Switch mode power supplys Answered

I am looking into different methods of power supply's when I found out about switch mode power supply's. These are the type of supply's that you have in your phone charger (or at least i believe so). The type of supply I need, is something that can supply a voltage of 2.5 volts @ 70 amps, and plugs right into the wall. To do this, I would use whats called a buck converter (because it lowers the voltage). So I was looking on amazon for things that would work for me, and I found nothing anywhere. so I decided to lower my expectations, and just look for any buck converter that would work with a input of 120 volts dc from the wall, after it was rectified, and filtered, and I could find nothing. Then I tried looking into building my own power supply, given a switch ic, which I looked for, and still found nothing, that could handle an input of 120 volts. What I was wondering, was how come basically every single thing powered by the wall outlet is powered by these, if none of them can handle an input of 120 volts?  And how do I find one that can. Also, how would I create my power source that can step down the main wall outlet power to 2.5 volts @ over 30 amps at least? Also, why is it that I could not find any switch ic that could handle 120 volts, if every phone charger has one that can? Do they not sell them to the public? Why is it that I could not find a Built buck converter on amazon that could handle an input of 120 volts? Could I modify a 30 volt one to work with 120 volts, or put something between the filtered dc wall current and the buck converter to do this? Thanks for the help, it is really appreciated.

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## Discussions

You are not the first to ask for insame amp levels and next to no voltage at all.
For those areas a normal power supply, even if it is switchmode model won't do you any good.
To get it going you need to do serious calculations for the parts in use, including their electrical losses.
Your best option IMHO is to either rewire the secondary on a welding transformer or to modify an inverter style stick welder.
For short term use a rewired MOT will do as well, the heating coil should supply around 2.7V by default.

The buck converter is one of may different switching mode power supplies.. Each has their strong point and weak points. I would not use a clasic buck converter at 120 Volts because if the transister swtich fialed 120VAC would be supplied directly to your 2.5VDC device. There is no isolation between the input and output. At the link below you will see a 12VAC to 16VDC witching mode power supply schematic I found on the web.