Relay vs MOSFET vs SSR: Choosing the Right Switching Device

Choosing between a relay, MOSFET, and solid state relay (SSR) for switching loads with Arduino or ESP32 is a common decision that impacts circuit reliability, efficiency, noise, and safety. Each switching device has distinct strengths: relays offer galvanic isolation and handle AC/DC; MOSFETs provide fast PWM switching for DC loads; SSRs combine relay isolation with solid-state reliability for AC mains switching. This comparison guide helps you choose the right device for every application.

Table of Contents

• Quick Comparison Table
• Mechanical Relay: Pros and Cons
• MOSFET Switch: Pros and Cons
• Solid State Relay: Pros and Cons
• Decision Guide: Which to Use
• Arduino Wiring Examples
• Frequently Asked Questions
• Conclusion

Quick Comparison Table

Mechanical Relay: Pros and Cons

Pros: Handles AC and DC, complete electrical isolation between control and load circuits, can switch high voltages (up to 250V AC), very low contact resistance when closed, and works with any load type (resistive, inductive, capacitive).

Cons: Audible clicking noise, limited lifespan (mechanical wear), slow switching (cannot do PWM), generates EMI at contact closure, requires flyback diode protection for the coil, and coil draws continuous current (50-80mA for 5V relay).

Best for: Mains appliance control, infrequent switching, applications needing complete isolation.

MOSFET Switch: Pros and Cons

Pros: Microsecond switching speed enables PWM for motor speed and LED dimming, zero power consumption in steady state (voltage-driven gate), unlimited switching lifespan, completely silent, very low on-resistance.

Cons: DC only (cannot switch AC), no galvanic isolation (load and controller share ground), static-sensitive (ESD can damage gate), requires gate driver for high-side switching, heat management for high currents.

Best for: DC motor control, LED strip dimming, PWM applications, high-frequency switching, battery-powered devices.

Solid State Relay: Pros and Cons

Pros: Silent operation, optical isolation (3.75-5kV), no mechanical wear, handles AC mains, zero-cross switching reduces EMI, fast enough for on/off control.

Cons: Higher on-state voltage drop (1-2V for SCR/TRIAC type) generates heat, needs heat sink for loads above 2A, more expensive than relays, limited DC switching capability (DC SSRs are special and expensive), false triggering possible in high-noise environments.

Best for: Frequent AC mains switching (heaters, lights, ovens), factory automation, applications requiring silence and long life.

Decision Guide: Which to Use

• AC load, infrequent switching: Relay (cheapest, simplest)
• AC load, frequent switching: SSR (no wear, no noise)
• DC load, on/off only: Relay or MOSFET (relay for isolation, MOSFET for silent operation)
• DC load, PWM needed: MOSFET (only option)
• Safety-critical isolation: Relay (true galvanic) or SSR (optical isolation)
• Battery powered: MOSFET (zero quiescent current)

Arduino Wiring Examples

// 1. Relay Module (active LOW)
// VCC→5V, GND→GND, IN→D7
// Load: Connect to NO (normally open) and COM terminals
digitalWrite(7, LOW);  // Relay ON
digitalWrite(7, HIGH); // Relay OFF

// 2. MOSFET (IRLZ44N)
// Gate→220Ω→D9, Source→GND, Drain→Load(-)
// Load(+)→12V supply(+), 10kΩ pull-down Gate to GND
analogWrite(9, 128);  // 50% PWM

// 3. SSR (Fotek SSR-25DA)
// Control (+)→D7, Control (-)→GND
// Load: L→SSR output, N→appliance
// Requires DC 3-32V control, switches AC 24-380V
digitalWrite(7, HIGH); // SSR ON (AC load receives power)

Frequently Asked Questions

Can I use a relay for PWM motor control?

No. Relays cannot switch fast enough for PWM (minimum 5ms vs PWM’s 1-2ms period). The contacts would arc and weld together. Use a MOSFET for DC motor PWM or a VFD (variable frequency drive) for AC motors.

Are cheap SSRs from online marketplaces reliable?

Many cheap SSRs labelled “25A” or “40A” are counterfeits that cannot handle more than 5-10A. The internal TRIAC is often undersized. Buy from reputable brands (Omron, Crydom, Fotek genuine) or derate cheap SSRs by 50-75%. Always use a heat sink and verify temperature under load.

Can a MOSFET replace a relay in all cases?

No. MOSFETs cannot switch AC mains, do not provide galvanic isolation, and require a common ground between control and load. For 230V AC appliance control, a relay or SSR is the correct choice.

How do I switch both DC and AC loads from one Arduino?

Use MOSFETs for DC loads (motors, LEDs) and relays or SSRs for AC loads (lights, fans, heaters). A single Arduino can control multiple MOSFETs and relays simultaneously using different digital pins.

Conclusion

Each switching device — relay, MOSFET, and SSR — has its place in electronics projects. Understanding the strengths and limitations of each ensures you choose the right device for reliability, safety, and performance. For most Arduino home automation projects, you will likely use all three types for different loads. Find relays, MOSFETs, SSR modules, and Arduino boards at Zbotic.