PCB Relays vs Solid-State Relays: Which Is Better for Your Design?

PCB Relays vs Solid-State Relays: Which Is Better for Your Design?

Relays look like a simple choice—until you ship products. Choosing the wrong relay type causes overheating, contact welding, nuisance failures, leakage current problems, or short lifetime under real loads.

There are two main choices:
• PCB (electromechanical) relays: a mechanical switch controlled by a coil
• Solid-state relays (SSR): an electronic switch (usually MOSFET/triac-based)

This guide explains which one to choose and what specs matter in real products.

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What each relay type actually is

PCB Electromechanical Relay (EMR)

A coil creates a magnetic field and physically moves contacts to open/close the circuit.

Key characteristics:
• True mechanical isolation when open
• Very low “on resistance” when closed
• Audible click
• Limited mechanical lifetime (but can be long if used correctly)

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Solid-State Relay (SSR)

An SSR uses semiconductor devices to switch (for AC often a triac, for DC usually MOSFETs). No moving parts.

Key characteristics:
• Silent switching
• Fast switching
• Very high cycle life
• Has on-state voltage drop and off-state leakage (important!)

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The #1 decision: AC load or DC load

If you are switching AC mains loads

Both EMR and SSR can work, but the tradeoffs are different:
• EMR gives true off (almost no leakage), low loss, cheaper
• SSR gives silent, fast switching, no contact wear, but has leakage and heat

If you are switching DC loads

Be careful:
• Many “SSR” products are AC-only (triac-based) and won’t work for DC.
• For DC, you need a MOSFET-output SSR or a DC-rated EMR.

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Key differences that matter most

1) Leakage current (SSR issue)

SSRs almost always leak some current when “off.”

Why it matters:
• LED lamps may glow when “off”
• sensitive loads may not fully turn off
• measurement circuits can behave strangely

EMRs have near-zero leakage when open (true isolation).

If your product needs a “true off,” EMR is usually safer.

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2) Heat and power loss

EMR:
• typically very low loss through the contacts
• but coil consumes power while energized (unless latching relay)

SSR:
• loses power as heat when on
• AC SSR: voltage drop across triac
• DC SSR: Rds(on) causes I²R loss

If you switch high current for long periods, SSR heat becomes a design problem.

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3) Lifetime and switching cycles

EMR:
• mechanical wear and contact wear
• lifetime depends heavily on load type (inductive loads reduce life)

SSR:
• no mechanical wear
• very high switching cycle life
• great for frequent switching

If you’re switching many times per second or very frequently, SSR is usually the right choice.

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4) Switching speed and noise

EMR:
• slower (milliseconds)
• makes clicking noise

SSR:
• very fast (microseconds to milliseconds depending on type)
• silent

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5) Load type sensitivity (especially inductive loads)

Inductive loads: motors, solenoids, transformers.

EMR issues:
• contacts can arc and wear quickly
• may need snubber/RC network, MOV, or flyback diode (DC)

SSR issues:
• can be more tolerant, but still needs surge protection
• AC SSR often has “zero-cross” versions that reduce EMI

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When an EMR (PCB relay) is usually better

Choose an electromechanical relay if:
• you need true off (near-zero leakage)
• you want low heat at high current
• cost is important
• switching frequency is low (on/off occasionally)
• you need strong isolation behavior

Common EMR applications:
• switching AC mains loads (heaters, pumps, appliances)
• power routing where leakage is unacceptable
• safety isolation functions

Be aware:
• coil power consumption (especially always-on)
• contact wear with inductive loads

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When an SSR is usually better

Choose a solid-state relay if:
• you need silent switching
• you switch very frequently
• you want long cycle life (no moving parts)
• you want faster switching and less mechanical failure risk
• your design can tolerate leakage current and you can manage heat

Common SSR applications:
• frequent AC switching (temperature control, automation)
• noise-sensitive products
• applications where mechanical wear is the main risk

Be aware:
• leakage current when off
• heat sinking / PCB thermal design for on-state loss
• correct SSR type for AC vs DC

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AC SSR: “Zero-cross” vs “Random turn-on”

If you choose an AC SSR, you’ll see these types:

Zero-cross SSR
• turns on near AC zero voltage
• reduces EMI and inrush stress
• best for resistive loads (heaters)

Random turn-on SSR
• turns on immediately when commanded
• needed for phase control / dimming applications
• can cause more EMI if not designed carefully

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Key specs to check (EMR)
• Contact rating (AC and DC ratings are different)
• Inrush current rating (motors and lamps have huge inrush)
• Coil voltage and coil power
• Contact type (SPST, SPDT, etc.)
• Isolation rating and creepage/clearance requirements
• Mechanical life and electrical life

Common mistake:
Using contact “10A” rating without checking inrush—lamp/motor inrush can weld contacts.

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Key specs to check (SSR)
• Load type: AC SSR vs DC SSR (don’t mix)
• On-state loss:
• AC SSR: on-state voltage drop
• DC SSR: Rds(on)
• Off-state leakage current
• Surge current rating
• Isolation rating
• Thermal resistance / heat dissipation needs

Common mistake:
Ignoring leakage current → load never fully turns off.

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Practical “pick this” rules (fast)

Pick EMR if:
• low switching frequency
• true off required
• high current with low heat
• lowest cost desired

Pick SSR if:
• frequent switching
• silent operation needed
• long lifetime under switching cycles
• you can tolerate leakage + manage heat

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Real-world examples

Example 1: Smart plug switching a lamp

EMR often better:
• true off (no glow)
• low heat
• cost-effective
Add MOV/snubber for surge protection.

Example 2: Temperature controller switching a heater many times

SSR often better:
• high cycling
• silent
• long life
Use zero-cross SSR for less EMI.

Example 3: Switching a DC solenoid

EMR or DC SSR (MOSFET SSR) can work:
• if EMR: add flyback diode or snubber
• if SSR: check Rds(on) loss and leakage behavior

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Common mistakes (what causes field failures)
• Treating AC and DC ratings as the same
• Not accounting for inrush current
• No snubber/MOV on inductive or mains loads
• SSR heat not managed → thermal shutdown or failure
• SSR leakage causing “off” load behavior

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Quick checklist before you lock a relay choice
• Is the load AC or DC?
• What is the steady-state current and inrush current?
• How often will it switch (cycles per day)?
• Is leakage acceptable when “off”?
• Can you dissipate heat (especially SSR)?
• Do you need silent operation?
• Are creepage/clearance and isolation requirements satisfied?