Solid State Relay (SSR) Guide: How It Works & When to Use One

Key Takeaways:

• A solid state relay switches electrical loads electronically and has no moving contacts.
• An SSR normally includes an input circuit, electrical isolation and a semiconductor output stage.
• Common options include AC-output, DC-output, single-phase and three-phase models.
• SSRs suit frequent switching, quiet operation and applications requiring long service life.
• Heat generation, leakage current and voltage drop must be considered.
• The input type, output type, load current, load voltage and thermal design must match the application.
• A mechanical relay may still be more suitable where very low off-state leakage or visible contact separation is required.

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What is a Solid State Relay?

A solid state relay (SSR) is an electronic switch that controls an electrical load without moving contacts. A small input signal activates internal semiconductor components, which switch the output circuit on or off. Because it has no mechanical contacts, an SSR can operate quickly, quietly and repeatedly with minimal physical wear.

Need to switch a heater, light, motor or other electrical load many times each day? A mechanical relay may wear, click or produce contact arcing under frequent operation.

A solid state relay offers a different approach. This guide explains how an SSR works, where it is used, what specifications matter and how to select one safely.

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Table of Contents

• Role of solid-state relay and how it works?
• What problems does an SSR solve?
• What types of solid state relays are available?
• Where are solid state relays used?
• Is an SSR better than a mechanical relay?
• Which solid state relay specifications matter?
• Why do solid-state relays generate heat?
• How do you choose the right Solid State Relay?
• Frequently asked questions

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Role of solid state relay and how does it work?

A solid state relay is a switching device that uses electronic components instead of moving contacts. It performs a similar basic task to an electromechanical relay: a low-power control signal operates a separate load circuit.

The main difference is what happens inside the device. A mechanical relay uses an energised coil to move metal contacts. An SSR uses semiconductor devices, such as thyristors, TRIACs, MOSFETs or transistors, depending on whether it is designed for an AC or DC load. Many models also use optical isolation. This allows the control side and load side to communicate through light while remaining electrically separated.

Isolation helps protect low-voltage control equipment, such as a programmable logic controller or temperature controller, from the higher-voltage load circuit. The exact isolation method and rating depend on the product.

Because it has no contacts physically opening or closing, an SSR does not produce contact bounce or an operating click. It can also handle frequent switching without the same mechanical wear found in conventional relays.

A Solid State Relay works in four main stages:

1. The controller sends an input signal. A PLC, thermostat, sensor or process controller applies the specified AC or DC control signal to the SSR input terminals.
2. The input circuit detects the signal. The device confirms that the input has reached the required turn-on level. The input voltage and current must match the SSR specifications.
3. The isolation stage transfers the command. In an optically isolated model, an internal LED activates a light-sensitive component. The command crosses the isolation barrier without a direct electrical connection between both sides.
4. The output semiconductor switches the load. The internal TRIAC, thyristor, MOSFET or transistor allows current to flow through the output circuit. When the control signal is removed, the output returns to its off state, depending on the switching design.

Some AC models use zero-cross switching. They turn on when the AC waveform is close to zero voltage, which can reduce electrical noise and switching stress with suitable resistive loads.

Random-turn-on models respond without waiting for a zero crossing. They may be preferred for phase control or loads requiring precise switching timing.

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What Problems Does an SSR Solve?

Industrial switching systems often face practical problems that a correctly selected solid state relay can help address.

Frequent Mechanical Relay Replacement
Mechanical contacts gradually wear, particularly when switching frequently or handling loads with high inrush current. An SSR has no moving contacts, making it useful for applications with rapid or repeated on-off cycles.

Noise in the Workplace
Traditional relays make an audible click whenever their contacts move. Solid-state switching is silent, which can be useful in laboratories, commercial equipment, building controls and other noise-sensitive environments.

Slow or Inconsistent Switching
Mechanical contacts can bounce briefly when closing. This may create several rapid electrical transitions instead of one clean change. A solid state relay provides contact-free switching without mechanical bounce.

Limited Space or Control Compatibility
Low-power electronic outputs can control many SSRs. This makes them practical for PLCs, temperature controllers and automated control systems. DIN rail, panel-mount and PCB formats can also support different enclosure layouts.

An SSR does not automatically solve every switching problem. Poor load matching, inadequate cooling or missing circuit protection can still cause failure. Correct engineering and product selection remain essential.

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What Types of Solid State Relays Are Available?

SSR type	Control input	Switched output	Common use
DC input, AC output	DC	AC	Heaters, lamps and suitable AC loads controlled by a PLC
AC input, AC output	AC	AC	AC control circuits switching AC loads
DC input, DC output	DC	DC	DC heaters, valves and other compatible DC loads
AC input, DC output	AC	DC	Applications requiring AC control of a DC load
Three-phase SSR	Varies	Three-phase AC	Three-phase heaters and industrial loads
Motor-control SSR	Varies	Motor circuit	Compatible motor switching or reversing applications

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Where Are Solid State Relays Used?

A solid state relay is often chosen when an electrical load must be switched frequently, accurately or quietly.

Common applications include:

• Industrial heating: Controlling ovens, furnaces, heat-sealing machines, moulding equipment and heating elements.
• Temperature control: Working with PID temperature controllers to maintain a set temperature through frequent switching.
• Lighting systems: Switching compatible commercial or industrial lighting loads.
• Process automation: Operating valves, actuators and other equipment from PLC or controller outputs.
• Food and packaging equipment: Supporting repeatable switching in production and packaging machinery.
• Building control: Managing suitable heating, ventilation and automated electrical loads.
• Motor applications: Switching compatible motors when the SSR is rated for the motor type, starting current and operating duty.

Load type matters as much as normal running current. Resistive heaters, incandescent lamps, transformers, solenoids and motors behave differently when switched.

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Is an SSR Better Than a Mechanical Relay?

Feature	Solid State Relay	Mechanical relay
Moving parts	None	Uses a coil and moving contacts
Operating sound	Silent	Produces an audible click
Switching speed	Generally fast	Generally slower
Contact bounce	None	Can occur
Wear during frequent switching	Low mechanical wear	Contacts and mechanism wear over time
Off-state leakage current	A small current may remain	Usually negligible across open contacts
On-state behaviour	Has a voltage drop and produces heat	Low contact resistance when healthy
Electrical isolation	Common, but product-dependent	Physical contact separation provides isolation
Typical failure mode	May fail short-circuit	Contacts may weld, burn or fail open
Cooling requirements	Often needs thermal management	Usually produces less contact heat

The main advantage of an SSR over a mechanical relay is reliable and silent operation during frequent switching. This makes it particularly useful for temperature control, where a controller may cycle a heater many times per minute.

A mechanical relay may be preferable for infrequent switching, very low-level signals, lower heat generation or applications requiring negligible leakage in the off state.

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Which Solid State Relay Specifications Matter?

Control Input
Confirm whether the controller provides an AC or DC signal. Check that the available input voltage and current fall within the SSR’s specified input range.

Load Voltage
The output voltage rating must suit the electrical supply. It should also provide an appropriate margin for normal voltage variations and transients.

Load Current
Consider the continuous current, starting current, switching frequency, duty cycle and ambient temperature. The relay’s permitted current may decrease as the surrounding temperature rises.

Output Type
Match the SSR to an AC or DC load. An AC-output Solid State Relay is not a general replacement for a DC-output device.

Switching Method
Choose zero-cross, random-turn-on, or another control method based on the load and application.

On-State Voltage Drop
A conducting semiconductor creates a voltage drop. This produces heat inside the Solid State Relay while current is flowing.

Off-State Leakage Current
A small current may pass through an SSR even when it is switched off. This can affect sensitive, high-impedance or very low-power loads.

Isolation Rating
Check the manufacturer’s specified isolation between the control and output circuits.

Surge Capability
Confirm that the relay can tolerate the load’s expected inrush current and electrical transients.

Mounting and Protection
Review heatsink, thermal compound, fuse, enclosure, ventilation and terminal requirements.

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Why Do Solid State Relays Generate Heat?

A Solid State Relay generates heat because its output semiconductor has an on-state voltage drop or resistance.

When load current flows, the device loses some electrical power and converts it into heat. Heat generation generally increases as the current rises.

If that heat cannot move away from the Solid State Relay, its internal junction temperature may exceed its safe operating limit. This can shorten the device’s life or cause sudden failure.

Higher-current applications often require:

• A correctly sized heatsink
• Suitable thermal interface material
• Adequate ventilation or airflow
• Appropriate spacing inside the enclosure
• Current derating for high ambient temperatures

The required thermal design depends on the load current, duty cycle, ambient temperature, mounting position and thermal resistance values given in the datasheet.

Fastron offers pre-drilled Solid State Relay heatsinks for suitable applications.

A suitably qualified person must complete electrical installation. An SSR and its heatsink may operate at dangerous voltages, and the heatsink can become hot during normal service.

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How Do You Choose the Right Solid State Relay?

Use this seven-step process before purchasing an SSR:

1. Identify the load. Record whether it is a heater, lamp, motor, transformer, solenoid or another load type.
2. Confirm AC or DC switching. Determine the output circuit voltage, frequency and normal operating current.
3. Check starting and surge demand. Do not select the relay using only the load’s running current.
4. Confirm the control signal. Match the SSR input to the output supplied by the PLC, temperature controller, sensor or switch.
5. Choose the switching method. Determine whether zero-cross, random switching or proportional control suits the process.
6. Complete the thermal design. Use the product datasheet to calculate heatsink and derating requirements for the actual enclosure temperature.
7. Add appropriate protection. Consider semiconductor fuses, surge suppression, circuit protection and safe isolation in accordance with the manufacturer’s instructions and relevant Australian requirements.

For three-phase loads, verify whether the system requires two-leg or three-leg switching. You must also confirm whether the selected unit suits the load connection and control method.

If you are uncertain, provide Fastron with the control voltage, supply voltage, load type, phase arrangement, current and expected switching frequency.

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Final Thoughts: Is a Solid State Relay Right for Your Application?

A Solid-State Relay provides fast, quiet, wear-free electronic switching for many industrial and commercial applications. It is particularly valuable where a controller must switch a load frequently, such as in temperature-control and process-control equipment.

The correct choice depends on more than the current printed on the case. Input compatibility, AC or DC output, load inrush, switching method, leakage current, voltage drop, cooling and protection can all affect performance.

If your application needs dependable and repetitive switching, explore Fastron’s range of solid state relays. The collection includes products for different control signals, output types, mounting arrangements and load requirements.

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