What is a solid state relay?

Relays are common components that many people know or have heard of. There are characteristics that we must take into account when choosing the relay, and this is defined according to its application.

Knowing the many characteristics of a relay, the World of Electrical explains in this article what a relay is, what the characteristics of a relay are and the types of the relay. In addition, we explain how a solid-state relay works, the advantages and disadvantages of a solid-state relay. Come on, guys!

What is a relay and what is it for?

Relays are components that can be electromechanical or electronic, created with the function of producing sudden changes in one or more electrical output circuits. The relays have a control circuit, which, when powered, switches another pair of contacts connected to a secondary circuit or command. In short, we can say that every relay is configured as a contact that opens and closes according to a certain factor in its input.

Solid-state relay

The solid-state relay, also known by some as SSR (Solid State Relay), is a semiconductor component that also has the same conventional and a contactor, which is to drive a higher power load from a low power applied at its input.

The solid-state relay is different from an electromechanical relay. The main difference is that the solid-state relay has no mechanical elements or moving parts in its mechanisms. Therefore, its operation is based on semiconductor components instead of physical contacts such as MOSFET and SCR.

Various models of solid-state relays.

Solid-state relay: Advantages

Solid-state relays are electronic components with several advantages over electromechanical relays such as:

  • The solid-state relay does not generate an electric arc, due to the absence of physical contacts;
  • The service life of solid-state relays is longer, as they present less wear with use because the switching component is a semiconductor, that is, it does not have mechanical components;
  • The switching time of the solid-state relay is considerably shorter compared to the electromechanical relay, so they respond to high drive frequencies;
  • There is no problem of contact chatter, known as the bounce effect;
  • Solid-state relays do not generate noise, unlike electromechanical relays;
  • Solid-state relays do not generate electromagnetic interference (EMI) when switching occurs;
  • The operating voltage and current range of the solid-state relay is greater;
  • Smaller physical size;
  • Greater resistance to mechanical vibrations.

Solid State Relay: Disadvantages

Not everything is perfect, because, despite the many advantages of solid-state relays, they have some disadvantages that must be analyzed and taken into account in a project, such as:

  • The output circuit is sensitive and can be damaged by overvoltages;
  • The solid-state relay is more sensitive to transients;
  • The output of some solid-state relays needs a minimum operating current, this is because of the thyristor maintenance current;
  • Generally, the solid-state relay can have a higher cost, compared to conventional relays;
  • In the solid-state relay, there is heating when high currents are controlled or when switching at high frequencies, which requires heat sinks.

Solid-state relay: Care in application

As we mentioned earlier, the solid-state relay is a component that generates heat, so we must take care of its application. Therefore, it is necessary to use heat dissipation mechanisms, which are proportional to the power of the relay and considering the conditions of the environment where the solid-state relay will be installed.

When a solid-state relay is short-circuited at its output, it is damaged and enters an internal short-circuit state. This is dangerous because from there the mortar is energized continuously, requiring some care to avoid damage to the controlled processes. To avoid these eventual problems we recommend the use of ultra-fast fuses, to ensure that the relays are protected and therefore the circuit as well.

Solid-state relay: Applications

The use of solid-state relays has grown over the years, being increasingly used in industrial process control, especially in temperature control, motors, valves, solenoids and several other areas of the industry, such as machine manufacturers, food industries, industrial lighting, security systems, electronic instrumentation, elevator control, among others.

Because the solid state relay is made of semiconductors and has a good switching response, it is capable of switching at higher frequencies. This means that solid state relays are used as a switch in a pulse width modulation circuit, that is, used in PWM (Pulse Width Modulation) circuits.

To complement the subject, we leave below a video from the World of Electrical that talks about solid state relays, explaining their operation and the main applications.