Protecting the Power Rail on ESP32 and Similar Boards

I've killed more ESP32 boards than I'd like to admit. Some died in ways I still don't fully understand — they just stopped responding one day. Some died from clearly identifiable mistakes: wrong polarity from a reversed JST connector, a 12V supply accidentally connected to a 5V rail, a brown-out during a motor startup transient that corrupted the flash.

The ESP32 itself is reasonably robust for a microcontroller. It tolerates 3.3V I/O, can be powered from 3.0–3.6V, and has some internal protection. But it has no protection against reverse polarity, limited tolerance to overvoltage, and sensitivity to significant voltage rail transients.

The good news: basic protection is cheap and adds almost no complexity to a design. The components involved cost a few rupees each and the circuit is straightforward.

Reverse polarity protection

The simplest approach: a Schottky diode in series with the positive supply rail. The diode only conducts in the correct polarity direction. Connect the supply backwards and the diode blocks current entirely.

Downside: Schottky diodes have a forward voltage drop of 0.2–0.4V. For a 5V supply, this is usually acceptable. For a 3.3V supply with tight headroom, it may not be.

Better approach for low-voltage applications: a P-channel MOSFET as an ideal diode. The MOSFET turns on when the supply is correctly polarised and turns off when it's reversed, with minimal forward voltage drop. The gate logic requires a few passive components. Many application notes cover this circuit.

For a bench/debug context where you're connecting and disconnecting frequently: at minimum use the diode. It's one component, it's always right, and it saves the board in the most common wiring mistake.

Overvoltage and transient protection

A Zener diode from the power rail to ground clamps the voltage at a defined level. A 3.6V Zener on a 3.3V rail will clamp any spike above 3.6V by sinking current to ground. Combined with a series resistor or upstream impedance, this limits the voltage seen by the regulator and microcontroller.

For the 5V input on a development board: a 5.6V Zener clamps the input and prevents accidental 12V connection from reaching the onboard regulator (the regulator itself may not handle more than 6–7V input without damage).

TVS diodes (transient voltage suppression) are a better option than Zeners for transient clamping. They have lower clamping impedance and respond faster. For ESD protection on I/O lines and power rails, TVS diodes are the professional approach.

A bulk capacitor (10–100µF) on the supply rail helps absorb fast current transients from motor startup or switching loads. This stabilises the voltage during brief transient events that might otherwise cause a brown-out reset.

Current protection

For an ESP32 running solo without attached peripherals, the supply current is modest — under 300mA peak during transmit. A polyfuse rated at 500mA–1A holds current provides reasonable protection.

For a board driving servos, motors, or other high-current peripherals: don't rely on a single polyfuse for the whole system. Use separate current protection for the high-current load (a polyfuse or circuit breaker sized for that load) and keep the microcontroller supply clean via a decoupling regulator.

The principle: the microcontroller should never see the current transients from motors or servos directly. Decouple them electrically — separate power rails or at minimum a ferrite bead and decoupling capacitors between the motor supply and the microcontroller supply.