ESP32-S3 resetting when relay turns ON (suspected EMI / power dip)

[kajarekar.sujay]

**Title:** ESP32-S3 resetting when relay turns ON (suspected EMI / power dip)

Hi everyone,

I am facing an issue with my ESP32-S3 where the controller resets exactly at the moment a relay is energized.

### System Overview

* MCU: ESP32-S3 (Arduino IDE)
* Supply:

* 230 VAC → SMPS → 12 V DC
* 12 V → 3.3 V regulator → ESP32
* Relay:

* 12 V coil driven via transistor
* Flyback diode already placed across coil
* Application: EVSE (AC switching to EV gun)

---

### Problem Description

* ESP32 works normally
* The moment the relay turns ON → ESP32 resets
* Happens consistently

---

### Observations

* MOV + EMI filter already present **inside SMPS input**
* Issue still persists
* Suspected causes:

* EMI from relay switching (AC side)
* Voltage dip on 12 V / 3.3 V rail
* Ground noise coupling

---

### Tests Done

* Flyback diode across relay coil → no improvement
* Planning to check reset reason logs (brownout vs other)
* Considering adding:

* Bulk capacitor (470 µF) on 12 V rail
* Additional decoupling on 3.3 V
* MOV near relay contacts

---

### Questions

1. Does this look more like a **power dip (brownout)** or **EMI issue**?
2. Should I:

* Improve power supply decoupling first?
* Or focus on suppressing relay switching noise?
3. Is adding a **MOV across relay contacts (AC side)** recommended in this case?
4. Any best practices for **grounding/layout** to avoid such resets?

---

### Additional Notes

* I am avoiding RC snubber across AC output due to EVSE safety (leakage to gun)
* Layout may have shared ground paths between relay and MCU

---

Any guidance or similar experience would be very helpful.

Thanks in advance!

Regards,
Sujay Kajarekar

[Sprite]

A power dip should be easy to measure using an oscilloscope, I'd think.

Can you post your PCB design? It's usually pretty easy to see if that is EMC-sensitive.