ESP32 MiniPLC

Relay Outputs

The MiniPLC provides 6 SPDT mechanical relays (HF115F/005-1ZS3) for switching AC or DC loads. Each relay exposes NO / NC / COM contacts and is driven via optocoupler-isolated control circuitry.

System/board rating: 3 A MAX per output (system limit). The MiniPLC board/system can safely carry ONLY 3 A per relay output. Using more than 3 A directly can overheat and permanently damage the board.

Relay contact (component) rating: 12 A @ 250 V AC (resistive). This value belongs ONLY to the relay component itself and is NOT usable as a system output rating. The MiniPLC must NOT be used above 3 A.

Loads above 3 A (or inductive/inrush loads) MUST be switched using an external contactor or power relay. The MiniPLC relay may be used as a control signal for the external contactor. Do not attempt to switch loads above 3 A directly through the MiniPLC relay outputs.

⚠️ External protection required: Every relay output MUST be protected by an external fuse or circuit breaker (max 3 A per channel). Relay output circuits are not internally fused. External overcurrent protection is mandatory for safe operation.

Relay Specifications

Relay Wiring Examples

Example wiring using Normally Open (NO) and Normally Closed (NC) relay contacts.

• Each relay has COM / NO / NC terminals
• Use NO for default OFF loads
• Use NC for default ON loads
• ⚡ Loads may carry hazardous voltage
• Supports AC and DC switching (DC and inductive loads require derating)

⚠️ External protection rule: Every relay output MUST be protected by an external fuse or circuit breaker. The rated current of the protective device shall not exceed 3 A MAX per output (board/system limit).

⚠️ Common protection warning: If a single common fuse or circuit breaker is used to protect multiple relay outputs (as shown in the wiring example), the protective device shall NOT be sized by summing relay outputs. The rating of the common protective device shall not exceed 3 A MAX per output (board/system limit).

For inductive or DC loads (contactors, solenoids, motors), use appropriate suppression (RC snubbers, MOVs, or flyback diodes). Loads exceeding the recommended PCB current limit shall be switched using external contactors or wiring methods that bypass PCB copper paths.

Analog I/O (0–10V)

MiniPLC provides 4 analog inputs and 1 analog output with a standard 0–10V signal range. Use these channels for common sensors, transmitters, and control devices that accept/produce 0–10V signals.

Analog Inputs (AI) — 4 Channels

Analog Input Specifications

Analog Input Wiring

Example: multiple 0–10V sensors powered from a shared supply (PS) with common 0V/GND.

Wiring Checklist (AI)

• Signal: 0–10V
• Reference: connect sensor 0V to AI GND
• Use a shared 0V/common ground between sensors and MiniPLC
• For long runs, use a twisted pair (Signal + GND)
• Keep analog wiring away from relay outputs and mains/AC wiring

Analog Output (AO) — 1 Channel

Analog Output Specifications

Analog Output Wiring

Example: 0–10V analog output connection to an external device.

Wiring Checklist (AO)

• Signal: 0–10V
• Reference ground: AO GND
• Use a twisted pair (AO + AO GND) for long runs
• Keep AO wiring away from power switching and motor/VFD cables
• Typical use cases: analog setpoints (VFD speed), valve/actuator positioning, controller reference signals.

Temperature Inputs

MiniPLC supports RTD sensors (PT100/PT1000) and 1-Wire temperature sensors (DS18B20 or compatible) for reliable cabinet and process temperature monitoring.

Temperature Input Specifications

Wiring Examples

1-Wire wiring example.

RTD wiring example (4-wire).

RTD wiring example (2-wire).

RTD wiring example (3-wire).

Communication & Protocols

• Modbus RTU (RS-485) – UART-based communication for expansion modules and field devices
• Wi-Fi – ESP32 integrated (Improv onboarding supported)
• Ethernet – Optional via LAN8720 PHY

RS-485 Communication (Modbus RTU)

The MiniPLC provides a half-duplex RS-485 interface with integrated protection and fail-safe biasing. The interface is available on the A / B / COM terminals.

Hardware Features:

• RS-485 transceiver: MAX485 (half-duplex)
• Fail-safe biasing network on A/B (idle state defined)
• Common-mode choke for EMI suppression
• TVS surge and ESD protection on A/B/COM
• Resettable PTC fuses on A and B lines
• Bidirectional logic level shifting between MCU and transceiver

RS-485 Wiring

Connect A, B, and COM to the bus.

Add 120Ω termination only at the two physical ends of the bus.

Termination & Biasing

• Terminate with 120 Ω only at the two ends of the line.
• Do not terminate intermediate devices.
• Fail-safe biasing is already provided inside the MiniPLC.

COM / Reference Ground

• Connect COM between all RS-485 nodes.
• This limits common-mode voltage and prevents communication faults.

Power Supply for Extension Modules

• Recommended: Use the same power supply for MiniPLC and all RS-485 extension modules.
• Distribute power in a star topology from the PSU.
• If separate PSUs are used, connect 0V references together at one point (unless the extension module is galvanically isolated).

A common 0V reference prevents RS-485 common-mode voltage errors and communication faults.

Tip: Cable recommendations, shield bonding, and routing rules are defined in the Cable Recommendations & Shield Grounding section below.

Cable Recommendations & Shield Grounding

This section applies to Analog (0–10V), Temperature (RTD / 1-Wire), and RS-485. Use shielded, twisted constructions and bond shields correctly to reduce EMI and ground-loop issues.

General Routing Rules

• Route low-level signal cables (Analog/RTD/1-Wire/RS-485) separately from mains, relay outputs, contactors, VFD motor cables, and power wiring.
• If crossing power cables is unavoidable, cross at 90°.
• Keep cable runs as short as practical and avoid parallel runs with high-current conductors.

Analog (0–10V) Cable

• Construction: twisted pair (Signal + GND) per channel
• Shielding: overall shield (standard) or individually shielded pairs (high-EMI)
• Examples: J-Y(ST)Y (overall shield) or LI2YCY PIMF (shielded twisted pairs; one pair per channel)

Temperature Cable

RTD (PT100/PT1000)

• Recommended: shielded multi-core for 2/3-wire; shielded pairs for best accuracy (4-wire)
• Examples: J-Y(ST)Y (overall shield) or LI2YCY PIMF (pairs; e.g. 2×2×0.50 for 4-wire)

1-Wire (DS18B20)

• Recommended: shielded 3-core ( +5V / DATA / GND ) for typical installations
• High-EMI / long runs: shielded pairs + overall shield (e.g. LI2YCY PIMF 2×2×0.50)
• Topology: daisy-chain (bus). Avoid star wiring.
• Stubs: keep sensor stubs ≤ 0.5 m.
• Pull-up (DATA): 4.7 kΩ typical; 2.2–3.3 kΩ for long/heavy loads.

RS-485 Cable

• Construction: twisted pair for A/B
• Characteristic impedance: 120 Ω (recommended)
• Shielding: overall shield is recommended in cabinets; use individually shielded pairs + overall shield in high-EMI
• Examples: J-Y(ST)Y 2×2×0.5 mm² or LI2YCY PiMF 2×2×0.50
• Use one twisted pair for A/B. Use the second pair for COM (0V reference) or spare.

Shield Grounding

• Default recommendation: bond cable shield(s) to cabinet PE/EMC ground at the PLC end only.
• Do not connect shields to signal terminals (AI/AO/RTD/1-Wire/RS-485 A/B/COM).
• If both ends are in equipotential bonded cabinets, shields may be bonded at both ends using proper 360° clamps.