I am reverse engineering a consumer product that I have, and the circuit seems to be mains voltage (85-250VAC) -> resistor (10ohm) -> bridge rectifier -> buck regulator -> linear regulator -> load.
The current is around 0.3A, output voltage is 3.3V. I would expect the resistor would be a very inefficient way of dropping the voltage. The application requires a very small power supply circuit and so a flyback with a transformer can not be used.
Is using a resistor + buck regulator as a power supply okay, or is there a much more efficient way of doing so?