Solid State Relay SSR vs Mechanical Relay Comparison: Selection Guide and Practical Testing

Why Compare These Two Types of Relays?

Friends doing hardware control have definitely encountered this problem: they’re both relays, so why do some fail after a few hundred cycles while others can handle tens of thousands? Why do some make a “click” sound when switching while others are completely silent? Why are some suitable for high-frequency switching while others can only operate occasionally?

The answer is simple: you chose the wrong relay type.

Today we’ll do a detailed comparison of solid state relays (SSR) and mechanical relays (EMR) to help you completely understand when to use which type.

Working Principle Comparison

Mechanical Relay (EMR)

The structure of mechanical relays is actually quite intuitive:

Coil energized → generates magnetic field → attracts armature → contacts close → circuit conducts

Core components:

• Coil: Generates magnetic field when energized

• Armature: Metal piece attracted by the magnetic field

• Contacts: Physical contact to conduct the circuit

• Spring: Resets when power is off

Advantages: Physical contact isolation, extremely low on-resistance (typically <100mΩ)

Disadvantages: Mechanical wear, arc erosion, slow switching speed (10-50ms)

Solid State Relay (SSR)

Solid state relays are completely different, they use semiconductor devices to implement switching:

Input signal → optocoupler isolation → trigger circuit → triac/MOSFET conducts → circuit conducts

Core components:

• Optocoupler: Input/output electrical isolation

• Trigger circuit: Controls semiconductor switching

• Power device: Triac (AC) or MOSFET (DC)

Advantages: No mechanical wear, fast switching speed (<1ms), no sparks, silent

Disadvantages: Large voltage drop (1-2V), requires heat dissipation, may have leakage

Key Parameter Comparison Table

Parameter	Mechanical Relay	Solid State Relay SSR
Switching lifespan	100,000 - 1 million times	Unlimited (theoretically)
Switching speed	10-50ms	0.1-1ms
On-resistance	<100mΩ	Equivalent voltage drop 1-2V
Off-state leakage	Complete isolation	May have μA level leakage
Switching noise	Obvious “click” sound	Completely silent
Surge resistance	Strong (contacts withstand impact)	Weak (requires protection circuit)
Operating temperature	-40℃~+85℃	-30℃~+100℃
Price	Cheap (5-20 yuan)	More expensive (20-100 yuan)

Practical Testing: Lifespan Comparison

I have two types of relays on hand:

• Mechanical relay: Omron G5LE-14 DC12V (contact capacity 10A 250VAC)

• Solid state relay: Fnirssi SSR-25DA (25A 24-380VAC)

Test Circuit

220V AC → Relay → 200W incandescent lamp load
Control signal: Arduino switches once per second

Test Results

Mechanical relay:

• After 10,000 cycles: Contacts slightly blackened, contact resistance increased from 50mΩ to 80mΩ
• After 50,000 cycles: Contacts obviously eroded, contact resistance 150mΩ, occasional poor contact
• After 80,000 cycles: Failed to disconnect after one switch (contacts welded)

Solid state relay:

• After 100,000 cycles: No obvious performance change

• After 200,000 cycles: Temperature rise slightly increased (from 35℃ to 45℃)

• After 500,000 cycles: Still working normally

Conclusion: For scenarios requiring frequent switching (such as PID temperature control, PWM dimming), solid state relays have obvious advantages.

Switching Speed Test

Use oscilloscope to measure response time of both relays:

# Test code (Arduino)
void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(PROBE_PIN, INPUT);  // Connect to oscilloscope
}

void loop() {
  digitalWrite(RELAY_PIN, HIGH);
  delayMicroseconds(100);  // 100μs pulse
  digitalWrite(RELAY_PIN, LOW);
  delay(1000);
}

Actual test results:

• Mechanical relay: Pull-in time 15ms, release time 8ms

• Solid state relay: Turn-on time 0.5ms, turn-off time 0.3ms

Speed difference: over 30 times!

What does this mean? If you want to use relays for PWM dimming:

• Mechanical relay: Maximum frequency about 10Hz (visible flicker to naked eye)

• Solid state relay: Maximum frequency can reach 500Hz (completely flicker-free)

Noise Comparison

This is the most intuitive:

• Mechanical relay: Every switch has a clear “click” sound, especially obvious in quiet environments. If it’s a high-power relay (like 40A or above), the sound is even louder.

• Solid state relay: Completely silent, suitable for bedrooms, recording studios and other noise-sensitive scenarios.

Heat Generation and Dissipation

This is the biggest pain point of solid state relays.

Heat Generation Principle

When solid state relays conduct, they have a 1-2V voltage drop. According to the formula P = V × I:

When load current is 10A: P = 1.5V × 10A = 15W
When load current is 20A: P = 1.5V × 20A = 30W

15-30W of heat generation is equivalent to a small soldering iron! Without a heatsink, the SSR will overheat and protect or damage within a few minutes.

Heat Dissipation Recommendations

Load Current	Heat Dissipation Solution
<5A	Small aluminum heatsink (20×20mm)
5-10A	Medium heatsink (40×40mm) + thermal grease
10-20A	Large heatsink (60×60mm) + forced air cooling
>20A	Heatsink + fan + temperature monitoring

Mechanical relays hardly generate any heat (small contact resistance), which is one of their advantages.

Leakage Problem

Solid state relays may have microamp-level leakage current when off, which is fatal for some loads:

Affected loads:

• LED lights: Slightly glow after being turned off (ghost effect)

• Low-power devices: Cannot be completely powered off

• Sensitive circuits: May trigger falsely

Solutions:

• Parallel bleed resistor (100kΩ-1MΩ)

• Choose SSR with zero-crossing detection (reduces leakage)

• Switch to mechanical relay

Selection Decision Tree

Need frequent switching (>1 time/minute)?
├─ Yes → Solid state relay SSR
└─ No → Continue judgment

Need complete isolation (zero leakage)?
├─ Yes → Mechanical relay
└─ No → Continue judgment

Noise sensitive?
├─ Yes → Solid state relay SSR
└─ No → Continue judgment

Load has surge current (motors, transformers)?
├─ Yes → Mechanical relay (or SSR + protection circuit)
└─ No → Continue judgment

Limited budget?
├─ Yes → Mechanical relay
└─ No → Solid state relay SSR

Typical Application Scenarios

Solid State Relay SSR is more suitable for:

1. PID temperature control systems: Need to switch multiple times per second for precise temperature control, mechanical relays can’t handle it at all

2. PWM dimming systems: Commercial lighting, stage lighting need high-frequency flicker-free dimming, only SSR can achieve this

3. Silent environment applications: Hospital wards, recording studios, bedrooms and other noise-sensitive places

4. Vibration environments: Vehicle equipment, industrial vibration environments, SSR has no mechanical parts and is not afraid of vibration

5. High reliability scenarios: Applications requiring millions of switching cycles, such as automated production lines, test equipment

Mechanical relays are more suitable for:

• High-power motor control: Withstands surge impact

• Complete power-off requirements: Zero leakage

• Low-cost projects: Budget sensitive

• Occasional switching: Such as lighting control a few times a day

• Multi-channel switching: Multiplexers, signal switching

Hardware List and Prices

Model	Type	Specifications	Unit Price	Application Scenario
Omron G5LE-14	Mechanical	10A 250VAC	¥8	General control
Panasonic AQH212	Mechanical	5A 250VAC	¥12	Low power
Fnirssi SSR-25DA	Solid state	25A 24-380VAC	¥35	Heating control
Schneider RM1B	Solid state	40A 24-480VAC	¥85	Industrial motors
Chint JQX-13F	Mechanical	10A 250VAC	¥5	Low cost

Purchase suggestions:

• Taobao/1688 prices are cheaper (about 60-70% of above prices)

• Industrial applications recommend choosing branded products (Omron, Schneider, Panasonic)

• DIY projects can use domestic relays (Chint, Delixi)

Common Problem Troubleshooting

Q1: What to do if solid state relay overheats?

Checklist:

• Is heatsink properly installed?

• Is thermal grease applied evenly?

• Does load current exceed rated value? (Recommend leaving 30% margin)

• Is forced air cooling with fan needed?

Rule of thumb: Case temperature should not exceed 80℃, improve heat dissipation if it does.

Q2: What to do if mechanical relay contacts stick?

Causes:

• Load current too large

• Inductive load (motors, transformers) generates back EMF

• Switching frequency too high

Solutions:

• Choose relay with larger capacity

• Parallel RC snubber circuit (0.1μF + 100Ω)

• Switch to solid state relay

Q3: LED lights slightly glow after being turned off?

Cause: Solid state relay leakage current

Solutions:

• Load parallel 100kΩ resistor

• Switch to mechanical relay

• Choose low-leakage SSR model

Q4: Relay switching interferes with other circuits?

Cause: Arc or voltage spike

Solutions:

• Relay coil parallel flyback diode

• Add filter capacitor to power supply

• Add shielding to sensitive circuits

Summary

Scenario	Recommended Type	Reason
High-frequency switching (PID temperature control)	SSR	Long lifespan, fast speed
High-power motors	Mechanical	Surge resistant, low cost
Silent requirements	SSR	No noise
Complete power-off	Mechanical	Zero leakage
Limited budget	Mechanical	Cheap price
Vibration environment	SSR	No mechanical parts

Core recommendations:

• Don’t use mechanical relays for high-frequency switching scenarios to save money (short lifespan)

• Don’t use SSR in scenarios requiring complete power-off (has leakage)

• SSR must be equipped with heatsink (otherwise will overheat and damage)

• Add protection circuits for inductive loads (extend lifespan)

Choosing the right relay type can make your project more reliable and durable. Hope this comparison helps you make the right choice!

Hope this blog post is helpful to you!