You open up a modern, high-tech smart home controller. It’s packed with microscopic surface-mount components, powerful microprocessors, and Wi-Fi chips.
And then, sitting right in the middle of all that silicon, is a big, blocky, plastic cube. When it activates, it makes a loud CLICK.
It’s a mechanical relay. Technology from the 1830s.
This begs a “soul-searching” question for any engineer: In a world where MOSFETs and IGBTs are cheap, microscopic, and silent, why haven’t we killed off the relay?
Why rely on a moving metal arm held by a spring when we have solid-state physics?
The answer isn’t nostalgia—it is cold, hard engineering reality. It turns out, the “clumsy” relay has a superpower that silicon just can’t replicate.
Let’s break down the battle between the Hard Switch (Relay) et le Soft Switch (Transistor).
1. The “Air Gap” Security: Why Relays Are the Ultimate Firewall
The #1 reason relays are still king is a concept called Galvanic Isolation.
Think of a MOSFET (transistor). Even when it is “OFF,” there is still a physical, chemical connection between the high-voltage load and your sensitive microcontroller. They are sharing a piece of silicon. Often, they have to share a “Ground” reference.
If that MOSFET fails catastrophically (say, a voltage spike punches through the gate oxide), that 240V mains power doesn’t just stay on the load side. It travels backwards, straight into your 5V Arduino or Raspberry Pi.
The result? Your microprocessor is instantly fried.
The Relay Advantage
A relay has no electrical connection between the coil (control side) and the contacts (load side). They are coupled only by a magnetic field. Inside the box, there is a physical Air Gap.
- The Scenario: Your 240V motor shorts out and sends a massive surge back up the line.
- The Relay: The contacts might weld shut. The plastic case might melt. But your microcontroller? It’s safe. The surge cannot jump the air gap to the coil.
Pro-Tip: We call this the “Moat.” If you are designing a circuit where the control logic must survive even if the load side explodes, you need a relay. It is the ultimate sacrificial layer.
There is a classic engineering maxim: “You can use a 12V coil to switch a 240V mains line, and never worry about the voltage difference.” This is the power of the Dry Contact.
2. The “Brainless” Switch: AC, DC, It Doesn’t Care
Transistors are finicky. They are semiconductor devices, which means they have rules.
- BJTs/MOSFETs are inherently CC (courant continu) devices. They allow current to flow in one direction (Drain to Source).
- Le Problème: If you want to switch 120V AC (Alternating Current) with a MOSFET, you have a headache. The current reverses direction 60 times a second. A single MOSFET will block half the wave and act like a diode on the other half. You need two MOSFETs back-to-back, or a Triac, plus complex drive circuitry.
The Relay Advantage
A relay is just two pieces of metal touching each other.
- Polarity: It doesn’t care.
- Direction: It doesn’t care.
- Voltage Type: AC? DC? Audio signals? Data? It doesn’t care.
When you give a customer a relay output, you are giving them a universal key. They can hook up a 24V DC solenoid, a 120V AC fan, or a millivolt-level audio signal. The relay handles them all with zero voltage drop and zero “leakage” current.
Pro-Tip: If you don’t know ce the user is going to connect to your output, use a relay. A transistor output requires the user to match voltage and polarity perfectly. A relay just says, “I connect A to B.”
3. Where the Transistor “Anti-Kills” the Relay
So, if relays are so great, why don’t we use them in our phones or computers?
Because relays have two fatal flaws: Vitesse et Wear.
The Speed Limit
A relay is a mechanical arm moving through space.
- Relay Speed: ~50 to 100 milliseconds. Max switching frequency: maybe 10 times per second (10 Hz).
- Transistor Speed: Nanoseconds. Max switching frequency: Millions of times per second (MHz).
If you need to dim an LED using PWM (Pulse Width Modulation), where you switch the power on and off 1,000 times a second, a relay is useless. It would sound like a machine gun for about 10 minutes before it disintegrated.
The Death Count
A relay has a limited lifespan.
- Durée de vie mécanique : Every time it clicks, the spring fatigues and the pivot wears. A good relay might last 1 million cycles.
- Durée de vie électrique : Every time it opens under load, a tiny arc pits the contacts. At full load, it might only last 100,000 cycles.
A MOSFET, if kept cool and within spec, has a theoretically infinite lifespan. It does not wear out.
4. The Middle Ground: The Solid State Relay (SSR)
“But wait,” you say. “What about Solid State Relays?”
The SSR is the “hybrid.” It uses an internal LED to trigger a photosensitive semiconductor.
- It has Isolation: Yes (Optical isolation).
- It has Speed: Yes (Faster than mechanical, slower than bare MOSFET).
- It has Silence: Oui.
The Catch: Heat.
A mechanical relay has near-zero resistance (milliohms). An SSR has a voltage drop (usually 0.7V to 1.5V) across its output.
Push 10 Amps through a mechanical relay? It stays cool.
Push 10 Amps through an SSR? It generates 15 Watts of heat. You need a massive heatsink to keep it from melting.
Summary: The Engineer’s Decision Matrix
So, the “clumsy” click isn’t going away. It’s a deliberate engineering choice. Here is your cheat sheet for when to stick with the old tech:
| Scenario | Use a Relay | Use a Transistor/MOSFET |
|---|---|---|
| Safety Priority | HAUT (Need Galvanic Isolation) | LOW (Shared ground is OK) |
| Le Type De Charge | AC or Unknown (Universal) | DC Only (Known Load) |
| Vitesse de commutation | Slow (On/Off occasionally) | Fast (PWM / High Frequency) |
| Lifespan Needed | Finite (<100k cycles) | Infinite (Millions of cycles) |
| Audio/Noise | Click is OK | Must be Silent |
In engineering, “Newer” isn’t always “Better.” Sometimes, the best solution is still a copper coil, a steel spring, and a satisfying click.
Précision Technique Note
Résistance de contact : Mechanical relays typically have contact resistance in the range of 50mΩ to 100mΩ, which is negligible for power loss but can be an issue for very low-voltage signals (wetting current required).
Leakage: Transistors/SSRs always have a tiny leakage current when OFF. Relays have zéro leakage (infinite resistance) when open.
Actualité: Principles of electromechanical vs. solid-state switching are fundamental physics and remain current as of November 2025.
