This article introduces electromechanical and solid-state relays and discusses their differences and uses.

Relays are electronic components that, like transistors, can control the flow of electricity. However, unlike transistors, they are either fully on or fully off and are often more suited for high power situations.

Relays come in a wide variety of packages, are often easy to identify, and can even be worth salvaging depending on the relay type. Let’s dive into the characteristics of the two main categories of relays: electromechanical and solid-state.

Electromechanical vs. Solid-State Relays

Relays fall under two main categories: electromechanical relays (EMR) and solid-state relays (SSR).

Electromechanical relays (also often referred simply mechanical relays) have small contacts that connect together in the presence of an electromagnetic field. The magnetic field that connects the two contacts together is derived from a small electromagnetic coil that can be switched on and off by external circuits. Such relays can have multiple configurations including normally open, normally closed, and multiple contacts in parallel.

Solid-state relays, however, have no moving parts and are often made with special configurations of MOSFETs and BJTs to provide power switching. Solid-state relays have some advantages over electromechanical types including the lack of moving parts which both reduces wear and tear as well as providing much higher switching speed. But electromechanical relays often have lower resistance and therefore can control much higher power devices (to be specific, their size to power control ratio is better).

Electromechanical relays are also considerably cheaper than their electronic counterparts and are arguably easier to drive. 

Electromechanical Relays

Electromechanical relays are through-hole parts, meaning they have pins that go through a hole on the top of the PCB and are soldered on the underside. They are often found in high-power switching applications where cost is a consideration such as consumer electronics that use mains power.

While they’re always square in shape, relays can be found in many different types of housing including transparent and opaque plastic.

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An example of an EMR in opaque housing. Image courtesy of Mouser.

Transparent designs make it easy to identify them as the electromagnet and contacts can be seen, while the opaque varieties may require inspection of the printed part numbers for identification.

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An example of an EMR in transparent housing. Image courtesy of Mouser.

Some relays have diagrams on the underside that show what each pin does which can be very helpful for hobbyists especially when salvaging relays. 

Solid-State Relays

As stated earlier in the article, solid-state relays do not have internal moving parts or contacts. Instead, they have solid-state semiconductors, such as thyristors and transistors, to switch currents.

Solid-state relays come in different shapes and sizes and are available in both through-hole and surface mount packages. However, because relays are found in power switching applications, through-hole varieties are much more common. Through-hole technology is very good at handling power switching currents. 

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Through-hole SSR. Image courtesy of Mouser.

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SMD SSR. Image courtesy of Mouser.

These relays are most often solid black with identification numbers printed on them and are more expensive than electromechanical relays. While less common in consumer electronics, some solid-state relays can be found with screw terminals. Solid-state relays can be incredibly small which can be useful for low power devices.

Robin Mitchell
Graduated from the University Of Warwick in Electronics with a BEng 2:1 and currently runs MitchElectronics.

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