ESP32-Based Relay Board with Power Monitoring and PoE Integration

esphome_relay_control.md

ESP32-Based Relay Board with Power Monitoring and PoE Integration

Objective: Design and build a custom relay board using the ESP32 microcontroller with integrated Power over Ethernet (PoE) functionality. The board will control 8 relays, each capable of handling up to 10A at 24VDC, and include current monitoring for each relay using ACS712 sensors. The system will be compatible with ESPHome for easy integration with Home Assistant.

Project Requirements:

1. Functional Requirements:

• Relay Control:

  • 8 relays, each rated for 16A at 250VAC and 10A at 24VDC.
  • Relay model: G5RLU-1A-E DC3 (Omron Electronics), with a 3V coil voltage.
  • Controlled via the ESP32 microcontroller.
  • Maximum Load: 10A per circuit at 24VDC.

• Power Monitoring:

  • Each relay circuit will include an ACS712 current sensor for real-time current monitoring, calibrated for up to 10A.
  • The current sensors will be configured and calibrated for accurate power measurements in ESPHome.

• PoE Power Supply:

  • Power input through an Ethernet RJ45 connector (TRP Connector B.V. 2250506-1).
  • PoE module: Silvertel AG9903-MTB, providing 3.3V output for the ESP32 and relays.

• Microcontroller:

  • ESP32-WROOM-32 or ESP32-WROVER module with Wi-Fi and Bluetooth support.
  • Programmed using ESPHome for integration with Home Assistant.

• Relay Driver:

  • ULN2803A 8-channel Darlington array for driving the relay coils.

2. Technical Specifications:

• Input Voltage: 36V - 57V DC from the PoE module.
• Output Voltage: 3.3V DC, up to 2A (provided by the PoE module).
• Relay Coil Voltage: 3V DC, 200mA per relay.
• Load Voltage: 24VDC, maximum current per circuit: 10A.
• Current Measurement Range: 0A - 10A per relay (using ACS712 sensors).
• Communication Protocol: Wi-Fi (802.11 b/g/n) for ESP32.

3. PCB Design Considerations:

• PCB Layout:

  • Compact design with all components securely mounted.
  • Proper trace width for high-current paths (at least 2mm) to handle up to 10A per circuit.
  • Adequate isolation between high-voltage (24VDC) and low-voltage (3.3V) sections.

• Component Placement:

  • PoE module and RJ45 connector placed close together to minimize power loss.
  • Relays and current sensors placed with sufficient spacing to reduce interference.

• Ground Planes:

  • A solid ground plane for noise suppression and signal integrity.

4. Software Requirements:

• ESPHome Configuration:

  • Define GPIO pins for each relay.
  • Configure each ACS712 sensor for accurate current monitoring.
  • Implement calibration logic for the current sensors based on the 10A max load.

• Integration with Home Assistant:

  • Each relay and sensor will be configured as an entity in Home Assistant.
  • Ability to control relays and monitor current consumption remotely.

5. Deliverables:

• Schematic Design: Detailed schematic diagram of the complete system.
• PCB Layout: Optimized PCB design files in Gerber format.
• BOM (Bill of Materials): Complete list of components with part numbers and quantities.
• ESPHome Configuration Files: YAML configuration for ESPHome with all relays and sensors defined.
• Prototype Board: Fully assembled and tested prototype for evaluation.

6. Additional Notes:

• External Fusing: No internal fuses will be included on the board. It is assumed that each load will be fused externally using a separate fuse block.
• Flyback Diodes: For inductive loads (e.g., motors or solenoids), flyback diodes should be added externally to protect the relay contacts from voltage spikes.

Below are the 16 issues, each with a clear answer and datasheet / official-doc evidence.

1. Relay part is latching, not a simple “ON while energized” relay

Proof: The Omron datasheet describes set/reset pulse width requirements and explicitly discusses latching relay operation.
Newyang

Answer: If you designed this like a normal relay (coil held on while load is on), it’s the wrong part/drive model.
Engineer action: Either:

Switch to a non‑latching G5RL variant, or Keep latching and implement pulse control + proper set/reset drive (see #2/#11).

2. Single‑coil latching relay needs polarity control (can’t be “ULN sinks one side” only)

Proof: Omron notes to “check carefully the coil polarity” and provides separate set/reset behavior and pulse timing.

Answer: With a single‑coil latching relay, you typically need reverse polarity to reset. A ULN2803A low‑side sink alone cannot reverse coil polarity.
Engineer action: Use a coil H‑bridge (or equivalent polarity-reversal driver) per relay, or change to a two‑coil latching variant (set/reset coils), or change to non‑latching.

3. ULN2803A voltage drop likely makes a 3V coil marginal from a 3.3V rail

Proof: ULN2803A VCE(sat) at 200 mA is up to 1.3 V. STMicroelectronics

Relay coil is 3 V, 200 mA, 15 Ω, and the “must operate voltage” is 70% max of rated. Newyang

Answer (with numbers):
Worst case coil voltage ≈ 3.3 − 1.3 = 2.0 V
Must-operate threshold max = 0.70 × 3.0 = 2.1 V
So worst-case is below spec → risk of non‑actuation across tolerance/temp.

Engineer action: Replace ULN2803A with logic‑level MOSFETs (very low Rds(on)), or a relay driver with low drop. If staying latching, pick an H‑bridge approach.

4. ULN2803A thermal is unacceptable if coils are ever held on (fault case)

Proof: RθJA ≈ 55 °C/W.
STMicroelectronics

At 200 mA/channel, VCE(sat) ≈ 1.1–1.3 V.
STMicroelectronics

Answer (worst case): 8 relays × (0.2 A × 1.3 V) ≈ 2.08 W in the ULN → ~114 °C rise (2.08×55) above ambient. That’s a “will cook itself” failure mode.
STMicroelectronics

Engineer action: MOSFET drivers + firmware safety limiters, or distribute load across multiple packages, and design so a stuck output can’t hold a coil on indefinitely.

5. PoE module output power is tight for 8 relays + ESP32 + sensors

Proof: AG9903MTB is 3.3 V, 6 W (70°C) / 4.5 W (85°C).
Silvertel

Max continuous output current for AG9903M/MT/MTB is 1.8 A.
Silvertel

ESP32 Wi‑Fi TX can be 160–260 mA depending on RF mode.
SparkFun

Relay coil is 200 mA each.
Newyang

Answer: If coils were ever simultaneously energized continuously: 8×200 mA = 1.6 A, leaving only 0.2 A margin for ESP32 + anything else; Wi‑Fi peaks alone can exceed that margin.

Engineer action: Ensure you never have “8 coils on continuously.” If you truly need non‑latching behavior, pick a higher‑power PoE solution (or different relay/coil rail).

6. PoE module has minimum load + MPS behavior risk at low draw

Proof: Silvertel warns that operating below minimum ILOAD can create audible noise and may cause the PSE to fail its Maintain Power Signature (MPS).

Answer: If your board spends time in low-power idle (ESP32 modem sleep, relays idle), you can drop below what the PD module expects → PoE power may drop.
Engineer action: Confirm minimum-load requirements in the exact AG9903 datasheet section and design for it (bleeder resistor or periodic load), especially if aiming for low idle consumption.

7. PoE front-end needs the exact bridge/cap network and placement

Proof: Silvertel’s typical connection shows bridge rectifiers from VA/VB pairs and calls out C1=100 µF (input) and C2=10 µF (output) in the reference diagram.
Silvertel

Answer: If you omit/misplace these, you’ll get start-up instability, ripple issues, or PoE negotiation problems.
Engineer action: Implement Silvertel reference exactly (bridges + caps) with tight placement to the module pins.

8. PoE output capacitors: value/type constraints matter (stability + ripple)

Proof: Silvertel specifies capacitor requirements including C1 ≥ 100 µF and output capacitor guidance (MLCC/electrolytic options, ripple current considerations).

Answer: Random bulk capacitance selection is a common cause of brownouts when relays switch or Wi‑Fi transmits.
Engineer action: Follow the capacitor requirements and include local bulk near ESP32 as well (see #10).

9. ACS712 cannot run on 3.3V (it’s a 5V part)

Proof: ACS712 supply voltage is 4.5–5.5 V; supply current is ~10 mA typ.
College of Engineering

Answer: With AG9903 providing 3.3V, ACS712s require an additional 5V rail (or a different current-sense IC).
Engineer action: Either add a 5V rail (and re-check PoE budget), or change current sensors to a 3.3V-compatible device.

10. ACS712 output range can exceed ESP32 ADC safe range depending on variant

Proof: Zero-current output is VCC × 0.5 (so 2.5 V at 5 V).

Sensitivity examples from the datasheet:

~100 mV/A variant: 96–104 mV/A
College of Engineering

~66 mV/A variant: 63–69 mV/A
College of Engineering

Answer (10A case at 5V supply):

100 mV/A → 10A shifts +1.0 V → ~3.5 V output (over 3.3 V rail domain)

66 mV/A → +0.66 V → ~3.16 V (fits under 3.3 V, with little margin)

Engineer action: Lock the ACS712 variant selection deliberately and/or add scaling/clamping before ESP32 ADC.

11. ESP32 ADC2 is not reliable with Wi‑Fi (ESPHome uses Wi‑Fi)

Proof: ESP-IDF docs: ADC2 is used by Wi‑Fi; ADC2 reads may be blocked/fail while Wi‑Fi is running.

Answer: Don’t design your 8 analog channels assuming ADC2 will be usable in ESPHome.
Engineer action: Use ADC1 pins only (or external ADC/mux) for current channels.

12. ESP32 ADC attenuation: don’t assume you can safely measure “up to 3.9V”

Proof: ESP-IDF docs: 11 dB attenuation gives ~3.9 V full-scale, but also notes the maximum voltage is limited by VDD_A.
Espressif Systems

Answer: Designing to measure signals >3.3V into ESP32 ADC is not a robust plan; you still need proper scaling/protection.
Engineer action: Keep ADC input within ESP32 analog supply limits; use dividers/RC and/or buffering.

13. Boot/strapping pins can be accidentally driven by relay circuitry

Proof: ESP32 strapping pins include GPIO0, GPIO2, GPIO5, GPIO12 (MTDI), GPIO15 (MTDO) and have required boot-level constraints; ESP32-WROOM datasheet explicitly warns about IO12/flash voltage selection.
Mouser Electronics

Answer: If your relay driver inputs, pullups, ADC networks, etc. touch these pins incorrectly, the board can fail to boot or enter wrong boot mode.
Engineer action: Constrain pinout to avoid strap pins for anything that could change at reset, and enforce correct default levels.

14. ESP32 supply decoupling: you need local bulk, not just the PoE module caps

Proof: ESP32-WROOM datasheet notes a capacitor should be connected to VDD33.
Mouser Electronics

Answer: Relay actuation + Wi‑Fi bursts are classic brownout triggers without strong local decoupling.
Engineer action: Add bulk + HF decoupling near ESP32 module power pins (and verify brownout margin under worst-case switching).

15. Omron “-E” high-capacity contacts require using both terminals

Proof: Omron explicitly states high-capacity “-E” models connect two terminals for one contact and you should use both; using only one may not meet performance.
Newyang

Answer: Your PCB and external wiring must tie/route both pins per pole correctly, or you risk contact heating / derating.
Engineer action: Ensure the PCB nets and connector mapping use both terminals as Omron instructs.

16. “2mm traces for 10A” is not a safe blanket rule (needs IPC-based sizing)

Proof: Digi-Key provides an IPC-2221-based trace width calculator/resource.
DigiKey

Answer (IPC-2221-style math, 1 oz external copper): 10A continuous typically wants multiple mm of width for reasonable temp rise; 2mm is generally only acceptable with very high allowed ΔT, short runs, planes, or heavier copper.
Engineer action: Define copper weight (1 oz vs 2 oz), allowed ΔT, and length; then size pours/planes/vias accordingly and validate with an IPC calculator + thermal check.

Extra: Relay contact protection for inductive loads (your “external diodes” note is directionally right)

Proof: General relay contact suppression guidance exists (Omron surge suppressor circuits; DigiKey contact protection), and flyback diodes are standard for DC inductive loads.

Engineer action: Document required external suppression per load type (diode for DC coils, RC snubber/TVS where appropriate), and consider adding optional footprints if you want the board to be more foolproof.