How can I make an easy power supply?

I want to make a really easy power supply that uses direct line voltage (120) using just a capacitor(WITHOUT a transformer).  I've seen the instructable where Tool Using Animal (https://www.instructables.com/id/LED-Lightbulb/) made one for LEDS. I measured it and got around 2 volts.  That was with a .47uf cap.  I want to get around 12 volts but I don't know how to get it without buying 20 different caps to see which one works.  Is there a formula? Thanks in advance for your help..

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lemonie6 years ago

Notice that also uses a "1/4 watt 1kilo ohm resistor" - like others I don't think you should try this sort of thing.

NachoMahma6 years ago
.  I have to agree with the others on this one: you obviously don't know enough about electrical safety to be messing with mains voltage and/or an unisolated power supply.
.  Your best bet is to get a mains-to-~14V transformer, a rectifier bridge, and a 12V regulator.
Re-design6 years ago
Making a power supply like this to use in other than a totally shielded way is incredibly dangerous.

Any hobby power supply should include a transformer with separate primary and secondary windings.  Using wall power like this would become 120 volts a/c should the capacitor short out and they often short out.

A 14 volt transformer can be had for less than $10 or free if you scavenge a bit and is infinitely safer than what you propose.

With all due respect, I hope no one has the lack of common sense to give you any details.
A non-isolated power supply is not something suitable for hobbyist use: it is essentially at mains voltage and can easily be lethal.