lamaPLC: Multi-channel Solid State Relay Module

Microcontroller-compatible Solid-State Relays (SSRs) connect low-power microcontrollers to high-power AC devices. Unlike traditional mechanical relays, SSRs lack moving parts and instead use light and semiconductors for safe, quiet switching of electrical power.

A key feature of an SSR is safety via optical isolation. When an Arduino pin outputs a HIGH signal (usually 5V or 3.3V), it activates a small internal Infrared (IR) LED. The chip contains a physical gap that prevents high-voltage AC from returning to the microcontroller while allowing light to pass. A light-sensitive semiconductor, such as a phototriac, is positioned opposite the LED. When it detects IR light, it becomes conductive, completing the circuit and powering the load.

Crucial Limitations

• AC Loads Only: The SSR uses a TRIAC output and cannot switch DC loads. Attempting to control a DC motor or LED strip will keep the relay in the “ON” position until power is completely disconnected.
• Minimum Load Requirement: SSRs have a small leakage current of approximately 1.5 mA. If you connect a load that draws very little current, such as a tiny 0.5W LED bulb, the light may glow faintly or flicker even when the relay is off.

If you want to switch DC or increase power on AC, consider using a multi-channel (optocoupled) relay module.

A Zero-Cross function is a feature in AC Solid State Relays (SSRs) that synchronizes switching. It ensures the relay opens the circuit precisely when the AC voltage sine wave crosses zero volts, minimizing electrical noise and stress. In typical household outlets, AC voltage oscillates smoothly between positive and negative peaks, such as +325V and -325V, at 50 or 60 Hz.

If your Arduino sends an “ON” signal at the peak of the wave (+230V) without a Zero-Cross function:

• A massive, instantaneous rush of current enters the load.
• This generates Electromagnetic Interference (EMI) and radio noise.
• It creates severe electrical stress on the component, shortening its lifespan.

The Solution: How the Zero-Cross Circuit Works

1. The Trigger: Arduino outputs a 5V signal to the SSR input.
2. The Hold: The internal zero-cross detector captures this signal. If the AC waveform is at a high voltage, the detector keeps the switch open.
3. The Cross: The AC waveform drops to 0 volts.
4. The Action: The detector immediately triggers the internal TRIAC, turning on the load smoothly with zero initial voltage.

The OMRON G3MB-202P is a low-cost, subminiature, PCB-mounting Solid State Relay (SSR) designed for high-density applications. It is widely used in DIY electronics, Arduino, and Raspberry Pi projects to safely switch AC loads up to 2A using low-voltage DC signals.

Key Specifications

• Load Voltage: 100 to 240 V AC (50/60 Hz).
• Maximum Load Current: 2 A.
• Input Control Voltage: 5 V DC (standard model variant).
• Switching Method: Zero-cross function (turns on when AC voltage crosses zero to reduce noise).
• Form Factor: 4-pin compact SIP (Single In-line Package) layout.

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