All component comparisons
Relay vs Solid-State Relay (SSR)
Relay (electromechanical) and Solid-State Relay (SSR) compared — what each is better at, and which one the job actually calls for.
An electromechanical relay gives you a genuine air gap, essentially zero on-resistance, and it shrugs off inrush — but the contacts wear, it is slow, and it clicks. An SSR has no moving parts, switches silently and lasts far longer, yet it drops around a volt (so it needs a heatsink at any real current), leaks a small current when it is supposed to be off, and gives isolation through its optocoupler rather than through air.
What each one is
Relay (electromechanical)
An electrically operated switch: a small coil current magnetically flips heavy-duty contacts, letting a low-power signal control a high-power load with isolation.
Solid-State Relay (SSR)
A semiconductor-based switch that controls AC or DC loads without moving parts, using optocoupler isolation and a triac/MOSFET output.
Key specs
Relay (electromechanical)
- Coil voltage(V)
- What energises it: 5V, 12V, 24V DC (or AC types).
- Coil current/resistance(mA / Ω)
- Drive current the coil needs - usually needs a transistor.
- Contact rating(A @ V)
- Load it can switch, e.g. 10A 250VAC / 10A 30VDC.
- Contact form(-)
- SPST-NO, SPDT (COM/NO/NC), DPDT.
Solid-State Relay (SSR)
- Input voltage(V DC)
- Control voltage required to activate the SSR (typically 3-32V DC).
- Output current(A)
- Maximum current the SSR can handle on the output side (e.g., 10A, 25A, 40A).
- Output voltage(V AC/DC)
- Maximum voltage the SSR can switch (e.g., 24-380V AC for AC SSRs).
- Isolation voltage(V)
- Voltage rating between input and output circuits (typically 1500-4000V).
- On-state resistance(Ohm (Ω))
- Resistance when conducting (typically 0.05-0.2Ω for power SSRs).
Types you will meet
Relay (electromechanical)
- Electromechanical SPDT/DPDT
- Sugar-cube PCB relay
- Automotive (12V)
- Solid-state relay (SSR)
- Reed relay
Solid-State Relay (SSR)
- AC SSR (triac output)
- DC SSR (MOSFET output)
- Zero-crossing SSR
- Instant-on SSR
- High-current SSR
- Low-voltage control SSR
How to choose
Relay (electromechanical)
Match coil voltage to your control rail; contact rating above the load (derate for DC/inductive). Choose contact form (NO/NC). Use an SSR for fast, silent, frequent switching.
Solid-State Relay (SSR)
1) Match input voltage to your control signal (3.3V/5V/12V/24V). 2) Select output current > max load current. 3) Choose output voltage matching your AC/DC supply. 4) Consider zero-crossing vs. instant-on for your application. 5) Check isolation voltage for safety.
How each one fails
Relay (electromechanical)
Contacts arc, pit or weld shut on overload/inductive loads; coil opens; chatter from a weak supply; MCU damage if flyback diode is missing.
Solid-State Relay (SSR)
Fails short-circuit (output stuck ON) or open-circuit (output stuck OFF). Visible signs: burn marks, discoloration, or physical damage. Can fail from overcurrent, overvoltage, or voltage spikes.
Full reference for each part
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