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Wi-Fi DISCUSSION
On my own PCB an ESP32-WROOM module is powered from 5 V through a small 3.3 V regulator. The board runs simple code perfectly, but as soon as WiFi.begin() is called it restarts, and the serial log shows "Brownout detector was triggered". On USB power from my PC it sometimes connects; from a phone charger with a long cable it usually does not.
How much current does the module really need, and what should the supply and decoupling look like?
The message means the supply dipped below the brownout threshold while the chip was running, and the timing tells you why. When the radio starts, the module performs RF calibration and then transmits, and the current jumps from a few tens of mA to pulses of several hundred mA. Espressif's hardware guidelines ask for a 3.3 V supply able to deliver at least 500 mA.
If the regulator, the source or the wiring cannot supply the pulse quickly enough, the rail sags for a fraction of a millisecond, which a multimeter will never show but the brownout detector catches.
Check three things in the supply path. The regulator: small LDOs rated for 150 to 300 mA are a frequent cause; use one rated for 600 mA or more, and mind its dropout voltage and thermal limit. The source: a long or thin USB cable drops a noticeable voltage at several hundred mA. The layout: a thin trace from the regulator to the module adds resistance and inductance exactly where the pulse flows.
Worked example: 0.5 Ω of total cable and connector resistance at a 400 mA pulse is a 0.2 V dip at the regulator input. Add a regulator with 1.1 V dropout and a 5 V source that is really 4.7 V, and only 3.4 V is left, so the margin is nearly gone.
Decoupling is what covers the fast part of the pulse. Place a bulk capacitor right at the module's 3V3 pin, not at the regulator. Reference designs show 10 µF to 22 µF ceramic there as a minimum, and adding 100 µF or more in parallel is a common and effective fix for exactly this symptom, with a 100 nF ceramic beside it for high frequencies. Keep the path from the capacitor to the module's supply and ground pins short and wide.
Check that the regulator is stable with the capacitance you add; some older LDO types need a minimum ESR, and the datasheet says which capacitor types are acceptable.
Confirm with an oscilloscope rather than by swapping parts. Probe the 3V3 pin at the module with a short ground lead, trigger on a falling edge at about 3.0 V, and call WiFi.begin(). A healthy rail stays within a few tens of millivolts; a sagging one shows dips in step with the transmit bursts.
Do not disable the brownout detector to make the message go away. That hides the symptom while the chip runs outside its supply specification, and the typical results are random crashes and corrupted flash writes later.
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