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Very simple, possibly transformer-less AC to 5v dc power supply

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Hi everyone,

I've been looking around for a low cost, low current, likely transformerless power supply to convert mains (AC 240/127V) to a stable, relatively low noise 5v DC to power a around 1mA load microcontroller.

I have seen designs like this before, but I assume their efficiency is very low, using a somewhat high-value resistor in parallel to drop the intake current.

Since mains operates in AC, for a short period of time the tension is assumed to be close to 5 volts. Would it be possible (or smart) a design that would allow current to flow and charge an output capacitor only when AC input is close to 5v?

Advise, discouragements or other recommendations?

Many thanks!

That is a capacitor dropper, and while they have purely CRAP power factor, the efficiency is actually not horrible, most of the voltage is dropped across the cap.

HOWEVER, they are non isolated, which limits the use case to things that have no accessible user connections apart for the power input, EVERYTHING on that board (and on any boards connected to it) must be assumed to be live at full mains voltage (Which makes plugging in the debugger a pain).

Anything you come up with that is better is likely to be more complex and less reliable.

Do a web-search with this: capacitive dropper

I have used capacitive droppers to drive a carefully-insulated LED from mains without a transformer.  Note that a good (high-Q) series capacitor should have no loss or heating, but it's a good idea that the load current is sinusoidal, such as a medium-size (resistance and power) resistor in series with a LED in parallel with a reverse diode, or in series with a bridge as in your drawing.  It's probably not politically correct to have such a high power factor, and insulation safety is paramount, as dmills stated.

You can get small isolated SMPS modules/boards just for this sort of thing. Or build your own linear supply, though the dual voltage requirement complicates that.


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