How to Choose a Solid State Relay (SSR): Load Type, Current Rating, Heat Sink, and Protection

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An SSR datasheet tells you the maximum voltage and current under stated test conditions. The control panel adds the conditions that usually decide whether the device is reliable: motor starting current, capacitor-charging pulses, heat trapped inside the enclosure, closely grouped power devices, and transients that never appear in the nominal load rating.

That is why choosing a solid state relay (SSR) is not a catalog-number exercise. A reliable selection must answer four engineering questions:

決定	What must be established
1. Is the SSR electrically compatible?	AC or DC output, control-input range, polarity, voltage, frequency, phase arrangement, and switching mode
2. Can it survive the load?	Continuous RMS current, inrush magnitude and duration, power factor, switching frequency, and transient voltage
3. Can the installation remove its heat?	Conduction loss, local panel ambient, derating curve, thermal interface, heat sink, grouping, and airflow
4. What happens during a fault?	Leakage current, semiconductor-fuse coordination, surge suppression, overtemperature response, and safe shutdown after a shorted-output failure

Follow those four decisions in order. Do not compensate for missing load or thermal data by applying a universal current multiplier.

This guide assumes an industrial control-panel or OEM application. Product ratings, wiring, protective coordination, and thermal design must be verified against the exact SSR datasheet and the applicable equipment standard.

要点

• アン AC-output SSR cannot normally switch DC, because a triac or thyristor output relies on current zero-crossing to turn off. A DC-output SSR usually uses MOSFETs and must be selected for DC polarity and voltage.
• The printed SSR current is a conditional maximum, not a guaranteed usable panel current. The allowable current may fall sharply as ambient temperature rises or heat-sink performance worsens.
• Resistive load current alone is not enough for motor, transformer, lamp, solenoid, or capacitive loads. Their starting current and surge duration often control the selection.
• Heat-sink selection should be based on power loss and junction temperature, not a universal rule such as “use twice the load current.”
• Off-state does not mean open-circuit isolation. SSRs have leakage current and can fail shorted, so they should not be used as the sole maintenance disconnect or safety isolation device.
• A branch-circuit breaker may protect the cable without protecting the semiconductor. Fast semiconductor-fuse coordination must be checked separately when required.

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SSR Selection at a Glance

選定項目	What to determine	重要な理由
Output type	AC, DC, or AC/DC-capable architecture	The wrong output device may not turn off or may be damaged immediately
Control input	Input voltage range, current demand, polarity	A nominal 24 V PLC output must reliably drive the SSR under real voltage-drop conditions
Load category	Heater, motor, transformer, lamp, solenoid, LED driver, capacitor bank	Each load produces a different inrush, power factor, and switching stress
動作電圧	Normal, maximum, transient, and frequency	The SSR must withstand both steady voltage and repetitive disturbances
連続電流	RMS current at the worst operating point	Establishes the starting point for thermal selection
Surge current	Peak magnitude, waveform, and duration	Determines whether the output chip survives starting and fault transients
Switching mode	Zero-cross, random turn-on, or proportional control	Affects EMI, inrush behavior, and control method
熱設計	Power loss, ambient temperature, R-theta values, airflow	Excessive junction temperature is a primary cause of early SSR failure
Off-state behavior	Leakage current, dv/dt, minimum load	Small loads may glow, chatter, or appear to remain energized
保護	Semiconductor fuse, MOV, snubber, TVS, overtemperature trip	Different components address different failure mechanisms

If you first need the basic internal structure and operating principle, read Understanding Solid State Relays. This page focuses on sizing and specification rather than repeating the general definition.

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1. Collect the Load Data Before Choosing an SSR

An SSR cannot be selected correctly from “230 V, 10 A” alone. Build a load profile with at least the following information:

• AC or DC supply, including frequency and polarity
• Single-phase or three-phase circuit
• Maximum normal supply voltage, not only nominal voltage
• Load power or measured steady-state RMS current
• Power factor and efficiency where applicable
• Starting, charging, magnetizing, or cold-filament inrush current
• Surge duration and switching frequency
• デューティサイクルと1時間あたりの予想起動回数
• Local air temperature inside the enclosure
• Installation orientation, ventilation, and spacing from adjacent heat sources
• Required control method: simple on/off, burst firing, phase-angle control, or rapid cycling
• Consequence of an SSR failing shorted

For existing equipment, measured current and inrush waveforms are usually more useful than a power label alone. For a new design, obtain the load manufacturer’s maximum current and starting-current data rather than relying on generic multipliers.

Estimating normal load current

単相負荷の場合：

平衡三相負荷の場合：

For a resistive heater, power factor and efficiency are often close to 1. For a motor, use the nameplate current where available. If the stated power is mechanical output power, efficiency must be included.

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2. Choose the Correct Output Technology: AC SSR or DC SSR

The load supply determines the output stage. The controller voltage does not.

For example, a relay described as DC-to-AC commonly accepts a 3-32 V DC control signal but switches an AC load. The first term describes the input; the second describes the output.

SSR output	Typical semiconductor	Suitable load supply	重要な制限事項
AC output	Triac or inverse-parallel SCRs	AC loads	Normally turns off when load current crosses zero; not suitable for ordinary DC switching
DC出力	MOSFET, sometimes IGBT in specialized designs	DC loads	Observe polarity unless the output is explicitly bidirectional
Bidirectional DC output	Back-to-back MOSFETs	DC where reverse blocking is required	Higher conduction loss may result from the series devices
Three-phase AC output	Three controlled AC output poles	Three-phase AC loads	Thermal and phase-failure behavior must be assessed for all poles

Do not use the input description to infer output compatibility. Read the complete designation, terminal diagram, and output specification.

Can an AC SSR switch DC?

Usually no. A triac or silicon-controlled rectifier (SCR) remains latched while current stays above its holding current. DC has no natural current zero-crossing, so the device may remain on after the input signal is removed.

Can a DC SSR switch AC?

Only if its output is specifically designed for bidirectional current. A conventional polarized MOSFET output is not a substitute for an AC SSR.

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3. Match the Control Input to the PLC, Controller, or Sensor

A common SSR input marked 3-32 V DC includes an LED and current-limiting network. That broad label still does not prove compatibility with every controller.

確認する：

• guaranteed turn-on voltage and guaranteed turn-off voltage
• input current across the full voltage range
• input polarity
• controller output type: sourcing, sinking, relay, triac, or transistor
• PLC output current available per channel and per common group
• residual or leakage current from the controller when “off”
• voltage drop in long control wiring
• required isolation between channels and circuits

For AC-input SSRs, verify the input frequency, pickup range, release voltage, and input current. An AC-output PLC or two-wire sensor may leak enough current to keep a sensitive SSR input active; a properly engineered bleed network or interface relay may be needed.

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4. Select by Load Type, Not Just Running Current

The same 10 A running current can impose very different stress depending on the load.

負荷の種類	Main electrical stress	選択の焦点	Typical switching approach
抵抗ヒーター	Continuous current and cabinet heat	RMS current, duty cycle, heat sink	Zero-cross is commonly used for on/off or burst control
Motor or compressor	Starting and locked-rotor current, frequent cycling	Motor-rated data, start duration, stall protection, thermal margin	Follow the SSR manufacturer’s motor-load guidance
変圧器	Magnetizing inrush depends on switching angle and remanence	Transformer-specific surge curve and switching method	Use a purpose-rated solution; do not assume zero-cross is automatically best
Solenoid or valve	Inductive turn-off voltage	Surge suppression and release-time requirement	AC or DC output matched to coil; add appropriate suppression
Incandescent or infrared lamp	Cold-filament inrush	Cold resistance and repetitive surge capacity	Zero-cross often reduces disturbance, subject to application data
LED driver or capacitive input	Capacitor charging pulse, high crest factor, small steady current	Peak current, pulse duration, minimum load, leakage	Use capacitive-load data; nameplate watts alone are inadequate

抵抗加熱負荷

Heating loads are usually the simplest SSR application, but they can run at a high duty cycle for long periods. This makes conduction loss and enclosure temperature more important than starting current. Use the maximum hot-state current and the worst credible supply voltage.

Motors and compressors

Do not size a motor SSR from full-load current alone. Check starting current, acceleration time, starts per hour, stalled-rotor protection, phase loss, and whether reversing or braking is required. A general-purpose SSR amp rating is not equivalent to a motor utilization rating.

For applications requiring soft starting, current limiting, or controlled acceleration, a semiconductor motor controller or soft starter may be more appropriate than a simple all-or-nothing SSR. IEC 60947-4-2 covers semiconductor motor controllers, starters, and soft starters within its scope.

トランスフォーマー

Transformer magnetizing inrush is strongly affected by closing angle, residual flux, source impedance, and transformer design. Closing exactly at voltage zero can produce severe flux asymmetry in some transformer applications. Select an SSR or controller with explicit transformer-load guidance and a validated surge-current envelope.

LED drivers and capacitive loads

An LED power supply may draw only a fraction of an ampere after startup but produce a short charging pulse many times higher. Compare the driver’s inrush magnitude and pulse width with the SSR’s repetitive and non-repetitive surge data. Also check off-state leakage, because it can charge the input capacitor and cause flashing.

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5. Check Output Voltage and Transient Margin

The SSR output-voltage rating must exceed the highest steady-state voltage that can appear across the open output. It must also survive line disturbances, inductive switching, and repetitive transients.

Review:

• nominal and maximum continuous supply voltage
• AC RMS rating versus peak blocking voltage
• 直流最大動作電圧
• repetitive peak off-state voltage
• static and commutating dv/dt
• surge environment and upstream protection
• insulation rating between control and load circuits

Do not apply one universal voltage-margin percentage to every SSR technology. Use the manufacturer’s application curve and coordinate the SSR with the selected metal-oxide varistor (MOV), transient-voltage-suppression (TVS) device, or surge protective device.

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6. Size the SSR Current from the Derating Curve

The current printed on an SSR is normally established under specified thermal conditions. Those conditions may include a particular case temperature, heat sink, airflow, mounting orientation, or ambient temperature. A “40 A” SSR may therefore be unable to carry 40 A continuously inside a warm, sealed control panel.

Use the allowable-current factor from the exact datasheet curve:

次の場合：

• I_{load} is the maximum continuous RMS load current;
• F_{allowable} is the usable fraction of nameplate current under the actual ambient and mounting conditions.

If a datasheet permits only 50% of the nominal rating at the design condition, an 18 A continuous load requires an SSR rated at least 36 A before surge and load-category checks. This is not a universal 50% derating rule; it is an example of how to apply a manufacturer curve.

Current screening does not complete the selection. The candidate must still pass surge-current, thermal, switching-frequency, and protective-coordination checks.

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Solid State Relay Selection and Heatsink Calculator

Use the VIOX Solid State Relay Selection and Heatsink Calculator to estimate the required SSR current rating, conduction loss, heat-sink thermal resistance, junction temperature, surge margin, off-state leakage, and preliminary semiconductor-fuse I^2t coordination.

Enter the actual output technology, load type, system voltage, power or measured current, power factor, duty cycle, ambient temperature, and thermal parameters from the candidate SSR datasheet.

[Insert VIOX Solid State Relay Selection & Heatsink Calculator Here]

Preliminary selection only: The result does not replace the exact manufacturer derating curve, load-category rating, surge-current curve, mounting instructions, thermal-interface requirements, or approved semiconductor-fuse coordination table.

Open the full Solid State Relay Selection and Heatsink Calculator

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7. Zero-Cross vs Random Turn-On SSR

This choice applies mainly to AC-output SSRs.

Zero-cross switching

A zero-cross SSR waits until the AC voltage is near zero before turning on. It is widely used for resistive heating and burst-fire control because it can reduce conducted and radiated disturbance compared with arbitrary-angle switching.

It is not universally best. Transformer magnetizing behavior, motor starting, and specialized control methods require application-specific review.

Random turn-on switching

A random turn-on SSR begins conducting as soon as the input command and semiconductor conditions permit. It is required for phase-angle power control and applications where the controller must choose the firing angle.

の応用	Usual starting point	Check before approval
Resistive heater, simple on/off	Zero-cross	Cycling period, thermal load, EMI requirements
Heater, phase-angle control	Random turn-on	Controller compatibility, harmonics, EMC filtering
変圧器	Purpose-rated SSR/controller	Magnetizing inrush, remanence, firing strategy
Motor/compressor	Motor-rated SSR	Start/stall current, switching mode, starts per hour
DC load	DC-output SSR	Zero-cross terminology does not apply

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8. Single-Phase vs Three-Phase SSR

A single-phase SSR switches one controlled current path. A three-phase system may use:

• one integrated three-phase SSR;
• three single-phase SSRs controlled together; or
• two controlled poles in a three-wire circuit where the applicable design permits it.

An integrated unit simplifies mounting and coordinated control, but its combined heat can be substantial. Separate SSRs can distribute heat and simplify replacement, but the design must manage simultaneous commands, phase-loss detection, unequal cooling, and shared heat-sink temperature.

For motors, do not treat three independent general-purpose SSRs as a complete motor starter. Overload protection, short-circuit protection, phase-loss behavior, restart logic, and isolation still require a coordinated design.

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9. Calculate SSR Power Loss and Heat-Sink Requirement

AC SSR conduction loss

For a preliminary estimate using an on-state voltage drop:

where D is the on-duty ratio. If the manufacturer provides a power-loss curve, use that curve because the voltage drop is not perfectly constant.

DC MOSFET SSR conduction loss

For a MOSFET output:

Use R_{DS(on)} at the expected junction temperature, not only the low-temperature headline value. MOSFET resistance generally increases with temperature.

Heat-sink thermal resistance

For one SSR on one heat sink:

次の場合：

• T_{J,target} is the design junction-temperature limit, kept below the absolute maximum;
• T_A is the local air temperature around the heat sink;
• P is SSR power loss;
• R_{\theta JC} is junction-to-case thermal resistance;
• R_{\theta CS} is case-to-sink thermal resistance;
• R_{\theta SA} is the required sink-to-ambient thermal resistance in degrees Celsius per watt.

Lower R_{\theta SA} means a more effective heat sink.

Worked example

Assume an AC SSR carries 18 A continuously, has a 1.6 V on-state drop, and is installed where local ambient temperature may reach 45 degrees C.

P = 1.6 \times 18 = 28.8\ W

If the design targets a 100 degrees C junction temperature, R_{\theta JC}=0.5 degrees C/W, and R_{\theta CS}=0.2 degrees C/W:

R_{\theta SA,max} = ÷{100-45}{28.8} – 0.5 – 0.2 \approx 1.21\ ^\circ C/W

The selected heat sink must provide 1.21 degrees C/W or lower under the actual orientation, enclosure airflow, altitude, and mounting conditions. This example is illustrative; use the exact SSR loss data and thermal resistances.

Several SSRs on one heat sink

When several modules share a heat sink, total heat raises the sink temperature while each module retains its own junction-to-case path:

Do not calculate each SSR as if it had the entire heat sink to itself.

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10. Account for Panel Temperature, Grouping, and Mounting

Room temperature is not the design ambient. Measure or estimate the air temperature near the SSR and heat-sink inlet after the panel reaches thermal equilibrium.

Common causes of underestimating SSR temperature include:

• sealed enclosures exposed to sunlight;
• heat sinks mounted where natural convection is obstructed;
• several SSRs, contactors, or power supplies grouped closely;
• horizontal fins that cannot develop an upward airflow path;
• high altitude, dust filters, or reduced forced-air performance;
• excessive or poorly applied thermal compound;
• uneven mounting pressure or a warped heat-sink surface.

Use the SSR manufacturer’s torque, flatness, thermal-pad, and mounting instructions. More thermal compound is not better; the interface material should fill microscopic voids without creating a thick insulating layer.

Integrated heat sink or separate heat sink?

An SSR with an integrated DIN-rail heat sink can reduce part count and assembly time, while a separate heat sink gives the designer more freedom to manage larger losses, shared thermal loads, airflow, and replacement. Neither construction is inherently better.

構成	主な利点	主な制限事項	Best evaluation method
Integrated heat sink	Compact assembly and simplified installation	Fixed cooling capability and possible crowding inside the DIN-rail zone	Verify allowable current at the actual panel ambient and spacing
Separate heat sink	Wider choice of thermal resistance, size, and airflow	More mounting work and greater dependence on interface quality	Calculate the complete thermal path and validate case temperature
Shared heat sink	Can reduce panel space for several SSRs	Heat from every module raises the common sink temperature	Use total heat loss plus each module’s individual junction path

Do not select between these options from load current alone. Compare the resulting junction-temperature margin under the same enclosure conditions.

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11. Understand Off-State Leakage and Minimum Load

An SSR does not behave like a mechanical air gap. Its output semiconductor and any internal snubber pass a small current while off.

This leakage can cause:

• LED lamps to glow or flash;
• high-impedance meters to display “ghost” voltage;
• small solenoids to buzz or fail to release;
• electronic power supplies to charge and restart periodically;
• a load to appear energized during troubleshooting.

Compare the SSR’s maximum off-state leakage with the load’s release current and minimum operating current. A manufacturer-approved bleed resistor may solve some small-load problems, but it adds heat and must be rated for continuous voltage and safe discharge. An external RC snubber can also increase leakage.

Always isolate and verify absence of voltage before maintenance. The SSR’s off command is not a safe isolation method.

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12. Coordinate Protection Around the SSR

No single protective component addresses every SSR stress.

Protective measure	主な目的	選定時の注意
Branch breaker or general fuse	Cable and circuit protection	May be too slow to protect the semiconductor junction
Fast semiconductor fuse	Limits fault energy through the SSR	Use the SSR manufacturer’s coordination table or compare on a valid common test basis
MOV	Clamps repetitive or occasional overvoltage	Coordinate MCOV, clamping level, energy, aging, and follow current
RCスナバ	Limits dv/dt and ringing on AC inductive circuits	Adds off-state leakage and heat
フライバックダイオード	Suppresses a DC coil’s inductive kick	Slows coil release and must be polarity-correct
TVSダイオード	Clamps DC transients at a higher voltage	Faster load release but higher stress than a flyback diode
Thermal switch or sensor	Detects heat-sink overtemperature	Place it where it represents the critical thermal path
Mechanical contactor/disconnect	Provides isolation or redundant shutdown	Must be coordinated for sequencing and fault duty

Semiconductor-fuse coordination

A molded-case circuit breaker or miniature circuit breaker can clear a fault while still allowing enough energy to destroy an SSR. Where semiconductor protection is required, use an ultrafast fuse recommended by the SSR manufacturer.

A preliminary pulse-energy comparison may use:

However, fuse total-clearing I^2t and SSR withstand I^2t are meaningful only when their waveform, duration, initial temperature, system voltage, and test conditions are compatible. An approved coordination table is stronger evidence than an isolated catalog-number comparison.

MOV and snubber selection

The MOV maximum continuous operating voltage must remain above the highest normal line voltage, while its protective level should remain below the SSR’s relevant transient withstand. Repetitive transients, source impedance, and MOV end-of-life behavior also matter.

An RC snubber is useful where excessive dv/dt or inductive ringing threatens false turn-on or device stress. It should be engineered as part of the circuit, not added automatically, because it increases off-state current.

Use temperature trends, not only a fixed trip point

In continuously operating or high-consequence equipment, a heat-sink temperature sensor can provide more than emergency shutdown. Trending temperature together with load current and panel ambient can reveal a blocked air path, failed fan, contaminated heat sink, loose thermal interface, or increasing duty cycle before the SSR reaches its trip threshold.

A rising heat-sink temperature is not proof that the SSR itself is defective. Compare readings at similar load and ambient conditions, and investigate the complete thermal path. A thermal switch connected only to the control input also cannot interrupt a load after the SSR has already failed shorted; a separate mechanical switching or isolation device is required where the risk assessment demands positive interruption.

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13. Design for Common SSR Failure Modes

Power semiconductors often fail shorted, especially after overtemperature, surge current, overvoltage, or inadequate fault-energy limitation. The load may remain energized even when the control input is off.

症状	Likely causes	Engineering response
Load remains fully on	Output semiconductor failed shorted; wiring bypass	Isolate power, verify output resistance according to the manufacturer’s test method, replace the SSR, investigate thermal and surge causes
Load does not turn on	Open output, insufficient input drive, blown fuse, load fault	Measure control input, output voltage, fuse continuity, and load current safely
Works cold, fails hot	Inadequate heat sink, poor interface, excessive ambient, marginal input	Log case/heat-sink temperature and current after thermal stabilization
Nuisance fuse operation	Load inrush underestimated, wrong fuse, repetitive surge	Capture inrush waveform and review fuse/SSR coordination
False turn-on	Excessive dv/dt, control noise, wiring coupling	Improve routing, suppression, shielding, and input noise margin
Small load glows or chatters while off	Leakage current or snubber current	Check maximum leakage against load release characteristics; use an approved bleed network or different output architecture

Where continued energization could create fire, motion, pressure, or process hazards, do not rely on one SSR for the safe state. Use an appropriate mechanical disconnect or safety-rated architecture, independent overtemperature protection, and fault monitoring as required by the risk assessment.

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14. SSR Procurement Specification

A purchase description such as “40 A SSR, 24 V input” is incomplete. Use a specification that allows the supplier and panel builder to evaluate the real application.

Specification field	Information to request or provide
の応用	Load equipment, switching purpose, duty cycle, starts/cycles per hour
入力	Nominal/range, input current, pickup/dropout levels, polarity, PLC output type
Output technology	AC triac/SCR, DC MOSFET, bidirectional DC, or integrated three-phase
Load supply	Nominal and maximum voltage, frequency, phase arrangement
Load data	RMS current, power factor, efficiency, inrush waveform, surge duration
Switching mode	Zero-cross, random turn-on, or proportional controller compatibility
Current capability	Continuous current at the specified ambient and mounting condition
Thermal data	Power-loss curve, V_{ON} or R_{DS(on)}, R_{\theta JC}, interface requirement, derating curve
Off-state data	Leakage current, minimum load, blocking voltage, static and commutating dv/dt
Surge data	Peak current, waveform, pulse duration, repetition limits, I^2t basis
保護	Approved semiconductor fuse, MOV/snubber guidance, overtemperature option
メカニカル	Package, terminals, mounting, heat sink, torque, touch protection
ドキュメンテーション	Datasheet revision, test reports, traceability, applicable certification evidence

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15. Supplier Audit Checklist for OEM and Panel Projects

Before approving an SSR family, ask the supplier to provide:

• current-versus-ambient derating curves for the exact mounting arrangement;
• power-loss data across the intended current range;
• junction-to-case thermal resistance and interface instructions;
• surge-current curves with waveform and duration stated;
• recommended fast-fuse models or a documented coordination method;
• maximum leakage current and minimum load at operating temperature;
• dv/dt and transient-voltage data;
• terminal torque and conductor requirements;
• lot traceability and change-control process;
• production-lot data for critical characteristics such as off-state leakage, on-state voltage drop, blocking voltage, and input pickup where statistical process control is used;
• evidence for any claimed certification or standard compliance;
• samples for thermal-rise, inrush, leakage, and fault-response validation in the actual assembly.

For high-volume OEM sourcing, statistical capability data can help identify whether a parameter is consistently controlled rather than merely passing at the specification limits. If a supplier reports a process capability index such as Cpk, confirm the characteristic measured, sample size, control limits, test temperature, and lot period. A Cpk value without that context is not meaningful evidence of SSR reliability.

IEC 62314:2022 addresses requirements for all-or-nothing solid-state relays within its scope. AC semiconductor controllers and contactors for non-motor loads fall within IEC 60947-4-3, while semiconductor motor controllers and starters are addressed by IEC 60947-4-2. The applicable product and assembly requirements depend on how the SSR is supplied and integrated; a component certificate does not by itself certify the completed panel.

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16. Common SSR Selection Mistakes

Treating the nameplate current as usable panel current

The correction is to use the exact derating curve and verify junction temperature under the worst ambient condition.

Ignoring load inrush

Measure or obtain the inrush waveform. A peak-current value without pulse duration is insufficient.

Assuming zero-cross is always better

Zero-cross is a strong default for many resistive heating applications, not a universal rule for transformers, motors, or phase-angle control.

Selecting a heat sink by SSR amp rating

Calculate actual semiconductor loss and the complete thermal path. Two SSRs with the same current label can have different voltage drops and thermal requirements.

Relying on a general breaker to save the SSR

Protecting conductors and protecting semiconductor junctions are separate coordination tasks.

Treating “off” as isolated

Account for leakage current and short-circuit failure. Provide safe isolation and a risk-appropriate shutdown path.

Replacing an EMR without reviewing circuit behavior

An SSR changes leakage, heat, failure mode, voltage drop, and transient behavior. If the decision is still between technologies, review Electromechanical Relay vs Solid State Relay before finalizing the design.

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よくある質問

Is a larger SSR amp rating always safer?

Not automatically. A higher current class usually provides more chip area or thermal margin, but the benefit depends on the internal design, heat sink, ambient temperature, terminals, surge rating, and protection. Oversizing cannot compensate for the wrong output technology or inadequate thermal mounting.

Can two SSRs be connected in parallel to share current?

Do not parallel ordinary SSR outputs unless the manufacturer explicitly supports it. Small differences in on-state voltage, temperature, and wiring impedance can make one device carry disproportionate current, creating thermal runaway. Use one correctly rated SSR or a manufacturer-approved parallel assembly.

Can several SSRs share one heat sink?

Yes, if electrical clearances, isolation, mounting instructions, and the combined thermal calculation permit it. Use total module loss to calculate heat-sink temperature, then add each SSR’s own junction-to-case and case-to-sink temperature rise.

Why is voltage measured at the load when the SSR is off?

Off-state leakage through the semiconductor or internal snubber can produce a meter reading, especially with a high-impedance digital meter. Determine whether it is harmless ghost voltage or enough current to operate the load. Never assume the circuit is safe to touch based on the control command.

Is a zero-cross SSR always best for an AC load?

No. It is commonly preferred for resistive heater switching and burst control, but phase-angle control requires random turn-on. Transformer and motor applications require load-specific guidance because switching angle can change inrush and electromagnetic stress.

Why use a semiconductor fuse if the circuit already has a breaker?

The breaker primarily protects wiring and clears circuit faults. A semiconductor junction can be damaged before a standard breaker opens. A coordinated ultrafast fuse limits let-through energy to a level the SSR can withstand.

How hot is too hot for an SSR?

There is no reliable touch-temperature rule. Use the manufacturer’s maximum case or junction temperature, then design to a lower target that provides operating margin. Validate the local ambient, case temperature, heat-sink temperature, and current after the panel reaches steady state.

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Final Selection Rule

A reliable SSR is selected from the load backward, not from the catalog current forward. First match AC or DC output technology and control input. Then qualify the device against load inrush, datasheet derating, switching method, conduction loss, junction temperature, leakage behavior, transient protection, and fault response.

The best procurement record is not simply “SSR, 40 A.” It is a documented operating point showing the actual load, ambient temperature, heat sink, surge profile, protective devices, and safe response if the output fails shorted.

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Technical References

• IEC 62314:2022 – Solid-state relays
• IEC 60947-4-2:2020+AMD1:2024 – Semiconductor motor controllers, starters and soft-starters
• IEC 60947-4-3:2020 – AC semiconductor controllers and contactors for non-motor loads