SOLID STATE relays: what are they?

Published the 24.02.2026

SSR (solid state relay):

In industrial automation, there are many types of relays, including those known as ‘ SOLID STATE’ relays, or more commonly known as ‘SSR’ (SOLID STATE RELAY). Why is this type of relay so sought after and widely used? To find out, we must start from the historical beginnings of the ‘common electromechanical relay’ and understand its primary and essential function and how it works.

A LITTLE HISTORY

The common electromechanical relay as we know it today was invented in the early 19th century when, in 1825, a certain American inventor and scientist named Joseph Henry improved and perfected the operation of Sturgeon's electromagnet.

Henry exploited the principle discovered by his colleague Sturgeon, namely that passing a certain electric current through a conductor wire generates a magnetic field; if this wire is then wound in a spiral around an iron bar, the magnetic field increases even , causing the bar itself to exert a force of “attraction” towards any other ferrous object. This is the principle of the electromagnetic magnet.

What does Henry do that his colleague Sturgeon does not?

Henry's improvement to this device invented by his colleague Sturgeon is simply to apply a conductor wire covered with an insulating layer , so that as many coils as possible can be placed side by side (even touching without creating short circuits) around the iron core, thus exponentially increasing the intensity of the magnetic field developed and the resulting force of ‘attraction’ generated by the iron core inside the coils.

Invention of the ELECTROMECHANICAL RELAY

At this point, Henry observed the behaviour of the magnet, namely its property of developing/generating a force of “attraction” towards another object made of conductive metallic material, and understood that

At this point, Henry observed the behaviour of the magnet, namely its ability to develop/generate a force of “attraction” towards another object made of conductive metallic material, and realised that this force could be exploited to activate an additional mechanical device, which in turn could activate a common electrical circuit. To do this, he needed a force of attraction capable of “moving” these two parts of the system (lever + contact), but this was not a problem for Henry, who had discovered how to increase (safely) the level of force of attraction developed by the magnet.

Once he understood the principle, he turned the idea into reality. He placed the magnet (coil), powered by its own independent circuit, next to a sort of metal “mechanical lever ” (armature) that would respond to the force of attraction generated by the magnet itself when it was powered by electric current. He mechanically connected this ‘armature’ to a system of ‘contacts’ electrical which were ‘closed’ if the armature connected to it, excited by the coil in attraction, literally moved the contact from the closed pole to the “open” pole, and consequently ‘reopened’ once the coil was no longer powered, losing its force of attraction generated towards the armature, which caused the electrical contact to return to its initial resting position. As can be seen, terms familiar to us begin to appear, such as ‘ COIL’, ‘ARMATURE’ and ‘ELECTRICAL CONTACT’, i.e. the names representing the three fundamental components of the common RELAY.

The electromechanical relay was invented.

The ‘cons’ of classic electromechanical relays

We have seen the operating principle of the classic electromechanical relay, discovered about 200 years ago by the American scientist Joseph Henry:

- a component (COIL) powered by a certain electric current coming from a first circuit, closes or opens an ELECTRICAL CONTACT via a mechanical lever (ARMATURA), which, in turn, is crossed/powered by a second electrical circuit, thus able to electrically control a device connected to it.

From this context, it is easy to deduce that the mechanism, although simple in operation, involves considerable mechanical wear , as the moving part represented by the moving armature can break rarely if the relay is used infrequently, but at the same time can break often if moved frequently. Another weak point of the classic relay is the solidity of the coil when the control current passes through it; if the latter is poorly calibrated or the relay is used frequently, thus causing a lot of current to circulate current inside the coil itself, the latter can overheat and short-circuit, effectively interrupting the operation of the entire relay.

SOLID STATE relays SOLID STATE (SSR) relay; operation and main difference with classic electromechanical relays:

In the 1960s and 1970s, industrial production began to increase exponentially, requiring ever more advanced machinery and applied technologies. It was thanks to the invention of the ‘Transistor’ (1947) and the ‘Thyristor’ that it became possible to discover electrical switching without moving mechanical parts. In the case of the ‘solid state’ relay, in fact, through a ‘optocoupler’, an electrical current is transmitted and supplied to the output contact, but WITHOUT the aid of a ‘moving’ part that opens or closes the output contact itself. This eliminates any problems associated with mechanical wear , as there are no moving parts. This latter feature is the main difference between an electromechanical relay and an SSR relay.

Operation:

The ‘solid state’ relay essentially consists of three main parts:

- input circuit

- drive circuit

- output circuit

In the input circuit , the control voltage is applied to the device, i.e. the voltage that gives the drive command to the device itself; it is usually supplied in a range of values starting from 3 volts to 32 volts (DC or AC).

The drive circuit is the part of the circuit that activates the ‘optocoupler’, which in turn activates the ‘transistor’ of the output circuit.

The output circuit of the relay, consisting of a transistor, is the part that activates or deactivates the so-called ‘load power supplies’. After receiving the signal from the drive circuit, the transistor turns on, allowing electric current to flow to the load connected to it via the relay's output contacts.

More specifically, it can be said that an SSR relay consists of three internal microcircuits defined as:

of ‘isolation’
‘function’
‘trigger’

The first circuit, called ‘isolation’, electrically isolates the two sides of the relay, i.e. the control side ( control) from the load side. This function ensures electrical isolation between the two parts. The galvanic isolation of the relay is achieved with the aid of an LED or an infrared diode to activate the transistor on the load side (trigger).

It should be noted that SSR relays, if of the ‘AC’ type, can be single-phase and three-phase, thus adapting to all types of load.

HEAT SINKS: what are they used for?

It is worth mentioning some special accessories related to SSR relays with SOLID STATE. We are talking about HEAT SINKS.

An SSR relay offers numerous advantages over a classic electromechanical relay, such as: no mechanical wear, high current capacity with reduced dimensions, silent operation, no electric arcs between internal components.

On the other hand, however, there are some drawbacks such as overheating of the internal semiconductor component (TRIAC) which, especially in the presence of high electrical current absorption loads, tends to overheat considerably. If this heat is not properly “dissipated”, it can cause the relay itself to overheat, causing it to break and/or even melt.

So how can this problem be reduced, if not avoided ?

Simply by applying a special HEAT SINK to the relay, which, being made of metallic material, has the particularity of dissipating the heat of the relay itself. Often, this latter component is not considered in the application of an SSR relay, but it is good practice to take it into account as it significantly increases the life of the relay itself.

Advantages of SSR relays:

The main advantages of an SSR relay compared to a traditional electromechanical relay are as follows:

- silent operation

- long operating life

- significant electrical capacity with small dimensions

- very high frequency switching; 1/5 milliseconds, compared to 15 milliseconds for a classic electromechanical relay

When to use SSR relays:

‘ SOLID STATE’ relays are mainly used with loads RESISTIVE loads, where high switching frequencies and high inrush currents are expected .