How Do Solid State Relays Work?

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Solid state relays come in a wide range of types and are used in different applications and industries. It is important to identify the type of solid state relay that will serve the purpose intended when using these devices in your project. This in-depth guide to solid state relay types will help you do that.

Basics of Solid State Relays

There are a variety of applications for solid state relays, including computers, consumer electronics, office machines, and industrial controls. Solid state relays are non-contact switches that use semiconductors for the ON/OFF action.

Solid state relays work by generating a small amount of current through a LED. The light generated activates a photovoltaic (PV) cell, which generates enough voltage to activate a solid state relay.

As solid state relays do not produce sparks and have long lifespans, they are ideal for applications in explosive environments.

We now have a basic understanding of solid state relays, so let's go over the different types that are available.

Types of Solid State Relays by Isolation Method

In solid state relays, isolation refers to the electrical separation between the input and output sides of the device. Three main kinds of solid state relay isolation are used in SSRs, namely optical, transformer, and reed.

SSR with photo coupled sensor

A phototransistor and an infrared LED are used in optical isolation to separate the input and output sides of a relay. The LED emits infrared light that triggers the phototransistor, which triggers the relay's output.

Solid state relays are able to achieve very high isolation voltage differences, enabling engineers to design very compact relays without compromising isolation.

SSRs isolated from transformers

A transformer-isolated solid state relay uses a high-frequency transformer to electrically isolate the input and output sides of the device. The transformer is usually coupled with an isolation amplifier to provide the required level of protection.

These SSRs are mainly used in industrial applications where high voltages and currents are involved. They are also used where high levels of protection are required.

Reed isolated SSR

The reed-isolated solid state relay uses a hermetically sealed reed switch as the isolating element. A reed switch is a type of switch or sensing device that is activated by a magnetic field generated by an external coil. The read switch in SSRs closes thermistors to produce switching.

Hybrid SSR

In essence, the hybrid SSR combines two (or more) relay technologies: an electromagnetic switch and a solid state relay.

A hybrid solid state relay combines the advantages of solid state switching with those of electromechanical relays by combining their input and output SSR parts in parallel.

It does not wear out its contacts, and the SSR can switch very quickly, while the relay can handle very high currents.