Picking the right Solid-State Relay: A guide
Solid State Relays (SSR)

Picking the right Solid-State Relay: A guide

  • 2024-04-15

Relays are integral to any circuit- whether simple or complex. They can be used instead of switches or for purposes suited explicitly for them. Depending on their method of working, there are two main types of relays:

Electromechanical Relays (EMR)

  • Electromechanical Relays (EMR)
    The most common type of relay, it works on the principle of electromagnetism. Under the influence of the generated electromagnetic field, it controls the movement of mechanical contacts, allowing or interrupting the flow of current in a circuit. They are robust but can be slower and noisier compared tosolid-state relays.



  • Solid State Relays (SSR)
    These relays work using semiconductors and have no moving parts, making them more wear-resistant. They offer faster switching speeds, higher reliability, and longer lifespan as compared to electromechanical relays. They are compact, silent, and have lower power consumption.


Solid State Relays (SSR)

Electromechanical relay vs Solid state relay

Electromechanical relay vs Solid state relay

So, unless you need electromechanical relays for specific purposes or prefer them for economical reasons, solid state relays are easily the superior option. To better understand what you’ll be getting into, let’s discuss how exactly the relay functions.

Working principle of Solid State Relays

Asolid-state relay uses solid components such as semiconductors to control the flow of current in a circuit. The basic working principle of a solid-state relay involves two main components: an input control circuit and an output switching circuit.

 Working principle of Solid State Relays


The input circuit typically consists of an optocoupler, which is an LED (Light Emitting Diode). It triggers when a low voltage is applied as an input, emitting light that is sensed by the photo-sensitive device. Then, there is isolation between the input and output circuits. 


The output circuit incorporates a photo-coupler that captures the light and converts it into electrical energy to drive the load circuit, which can include components such as triacs, diodes, transistors, or thyristors. This allows the device to conduct or block the current flow.


It is the control signal, applied to the input side, that typically governs the operation of the solid-state relay. When the control signal is present, the input circuit allows the current to flow through the optocoupler's LED, activating the photosensitive device. As a result, current is allowed to flow through the main circuit.


Conversely, when the controlsignal is absent or removed, the input circuit prevents current from flowing through the optocoupler's LED. This deactivates the photosensitive device, causing the power semiconductor device in the output circuit to turn off, interrupting the current flow in the main circuit.


Opto-coupling technology is the reason the switching is swift and sensitive, having high insulation levels. The output voltage can be digital or analog, depending on the input and load-drive circuit.

Types of Solid State Relays

Having understood what sets apart SSRs from EMRs and their working, let’s now talk about the various types of SSRs available. This will help you decide which one will best suit your particular application. 


The two most common types of SSRs are:

  • Non-zero crossing
  • Zero Crossing

Non-Zero Crossing SSR

Also known as Random Turn ON-OFF relays, these relays immediately turn on the output voltage when the input voltage is turned on. Consequently, it doesn’t need synchronization with the AC waveform. They are best suited for applications where precise control of the load power is required. The downside is if the SSR switches on during a high-voltage phase of the AC cycle, it can cause a surge in the current or voltage spikes. This can interfere with the equipment and can cause damage to the appliances.

Non-Zero Crossing SSR

Zero Crossing SSR

Zero crossing SSRs are specially designed to combat the downside of random turn on-off SSRs. They switch on or off only when the AC voltage crosses zero (as the name suggests). So, even if the input voltage is supplied when the AC waveform is in between phases or at a high-voltage phase, it will only turn on when it passes zero. This minimizes the generation of electrical noise and voltage spikes during switching transitions. Thus, it reduces the probability of electrical disturbances occurring.


Zero Crossing SSR

Solid state relays can also be classified based on the type of isolation method (i.e. the method used to separate the control input circuit from the output switching circuit). Two common types of SSRs based on isolation methods are triac-based SSRs and transformer-isolated SSRs. 

Triac-based SSR

Here, the switching component in the output circuit is a triac. Triac-based SSRs are commonly used for AC switching applications. They are cost-effective, compact, and suitable for resistive loads or inductive loads with moderate inrush currents. However, they may have limitations in switching highly inductive or capacitive loads due to potential voltage spikes or current surges.

Triac-based SSR


Transformer-isolated SSR

Transformer-isolated SSRs utilize a small transformer for isolation between the control input and output circuits. It can either be a step up or a step-down as needed. Although similar in working to triac-based SSRs, it can protect against electrical noise, voltage spikes and transient events. Hence, they apply to a wide range of loads, including highly inductive or capacitive loads.


Point to remember while selecting an SSR

  • Load type
  • Electrical isolation
  • Switching speed
  • Protection against electrical disturbances

Still confused about which relay type to pick? Consult our manuals with specifications and guidelines here.

of manual and data sheet of Multispan


Solid state relays find applications across various industries due to their advantageous properties like fast switching, high reliability, long lifespan and electrical noise immunity. Let’s discuss a few here:

Industrial Automation

Finding one of their biggest applications in industrial automation, they can be found in motor control, heating and cooling systems, lighting control and power distribution panels. Most products come with dedicated inputs to connect SSRs to them.

HVAC Systems

SSRs play a crucial role in heating, ventilation, and air conditioning (HVAC) systems. They are used for controlling electric heaters, fans, pumps, and compressor motors, providing efficient and precise temperature regulation.

Energy Management

SSRs are utilized in energy management systems to control the switching of power to different loads based on demand, helping to optimize energy consumption and reduce costs.

Automotive Industry

SSRs are utilized

 in automotive systems for various functions, including lighting control, electric window motors, wiper control, and power seat adjustments. They provide efficient switching and reliable performance in harsh automotive environments.


Benefits of SSR

  • SSRs have fast switching speed for precise control, unlike any other component used for the same purpose - SCRs (Silicon Controlled Rectifier), EMRs, contactors.
  • They do not require gate triggering or control signals to maintain conduction whereas SCRs do. 
  • They provide full-wave switching capability for AC loads, while SCRs can only control one half-cycle of the AC waveform.
  • SSRs have no moving parts and hence have higher reliability and longer lifespan.
  • They generate less electrical noise and are immune to contact bounce issues.
  • They operate silently without humming or buzzing sounds.
  • SSRs are typically more compact in size


In comparison to their counterparts, solid state relays can serve several situations and are highly efficient. They even come in economical ranges, depending on their rating. Here at Multispan, we currently offer 25A, 40A, 50A, and 70A and are planning to expand this range in the future. They also come in different mounting methods, if you choose so, like din rail, PCB, bracket and panel for your convenience.

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