Simple buck circuit reducing 240V AC to around 5V at 300mA

Question

I work with a lot of ESP8266 microcontrollers, and often need to power them from 240V AC mains. This generally involves either a wall-wart or a dedicated isolated module such as the HLK-PM03.

However, it occurred to me that - in situations where isolation from the mains supply is not a requirement - I could maybe use a simple buck circuit.¹

I came up with this circuit:

The resulting 6V-or-so could then be passed through an LDO to produce a stable 3.3V supply.

Assuming I use a 250V-rated triac to chop the mains supply into pieces, is there any good reason this circuit wouldn't work and/or be unsafe?²

In addition, would I need to use an inductor and capacitor rated for 250V? Under normal circumstances, they would spend most of their time seeing low voltage.

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¹ I toyed with the idea of capacitive droppers, but the current requirements of the ESP8266 are too high for that to be feasible.

² I am well aware of the importance of isolation, thank you. This is for use in situations where mains-referenced power is not a concern.

Answer 1

is there any good reason this circuit wouldn't work and/or be unsafe?

There are a few designs that can do this but I wouldn't recommend a triac type because of peak currents into the 120 mH inductor and the sheer physical size needed for the 120 mH inductor: -

The above regulates to 5 volts DC from an AC voltage range of 85 volts to 265 volts. It can probably be coerced into producing 3.3 volts directly too. The source is here.

Then there is this from Power Integrations: -

Source information here.

These are the preferred methods for non-isolated buck converters and note how much smaller the inductor needs to be in these designs.

$$\color{red}{\boxed{\text{These designs are dangerous in that they don't isolate high voltage mains}}}$$

The preferred method is to use a flyback design not only because the circuit provides inherent isolation properties but, because a step down transformer can deliver much more current to the output for a given (small) transformer size: -

Circuit from here and note the construction size: -

Pretty much all the space above would be taken up by a suitably dimensioned 120 mH in the OP design.

$$\color{blue}{\boxed{\text{There is very little sense in not using an isolating flyback design}}}$$