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Solid State Relays: 3 Reasons to Make the Switch

TecTalk blog

What is a relay?

Relays were first used in long-distance telegraph circuits as amplifiers back in the 1800s. They repeated the signal coming in on one circuit and re-transmitted it on another circuit. They were also used in telephone exchanges and early computers to perform logical operations. Nowadays, relays are widely used to switch starting coils, heating elements, pilot lights and audible alarms.

telegraph      telegraph up close
The basic purpose of a relay is similar to that of a valve. While one controls the flow of liquid, gas and air, the other controls the flow of electricity. In electromechanical relays, contacts are opened and closed by a magnetic force. Solid state relays have no contacts and the switching is done electrically.


What is an electromechanical relay?Electromechanical Relay

Electromechanical relays (EMRs) are switches used to isolate circuits or batteries, detect faults on transmission and distribution lines, and control a high powered circuit using a low power signal. EMRs are composed of a frame, coil, armature, and contacts. The relay’s contacts are opened and closed by a magnetic force which is provided by the coil when it is energized. Compared to solid state relays, EMRs provide a cleaner on and off state since there is a large distance between the contacts that acts as insulation.


Types of electromechanical relays:

General purpose relays: usually operated by a magnetic coil and operated with AC or DC current.

Machine control relays: operated by a magnetic coil. These are heavy duty relays used for starters and industrial components.

Reed relays: small, compact, with a fast-operating switch.


What is a solid state relay?

CG SSRSolid state relays (SSRs) can be thought of as the modern, 21st century version of the EMR. These relays are composed of a sensor, electronic switching device, and coupling mechanism. With the use of a semi-conductor, the relay switches on and off when a small external voltage is applied across its control terminals. It consists of three circuits: an input circuit, control circuit, and output circuit. The input circuit serves the same function as the coil on an electromechanical relay. The circuit is activated when a voltage higher than the pickup voltage is applied to the circuit and deactivated when the voltage is less than the minimum drop-out voltage of the relay. The control circuit determines when the output component is energized or de-energized. So, it functions as the coupling mechanism between the input and output circuits. The output circuit switches on a load which would be done by the contacts on an EMR. Solid state relays can be found in medical equipment, HVAC systems, professional food equipment, theatrical lighting, and industrial machinery.

SSR uses-01

When choosing between SSRs and EMRs consider the application’s electrical requirements, cost constraints and life expectancy. EMRs are ideal for heavy duty environments and capable of operating in AC or DC current, while SSRs can only operate in one or the other. They are also a more economical choice if there are cost constraints. However, it may be wise to invest in a higher quality solid state relay than having to frequently spend money on replacement parts for an electromechanical relay. Below are three reasons to make the switch to solid state relays…

  1. Fast and energy-savingSolid State Relays are fast and energy-saving. 

The main difference between solid state relays (SSRs) and electromechanical relays (EMRs) is that SSRs do not have any movable parts. This feature creates a number of great advantages to using this type of relay. Since the relay does not have to energize a coil and physically open and close contacts, it actually consumes 75% less power than EMRs. This also means that the relay switches at a much faster rate. While an EMR averages between 5-15ms to switch and settle, an SSR averages between .5-1ms.

  1. Silent, compact, and powerful

    They are silent, compact, and powerful.

Without movable parts, SSRs are completely silent electrical devices. Solid state relays do not have large, clunky electromagnets attached to the them, and therefore are a small and compact alternative. Their small size also saves valuable installation space. Being small, however, does not make them any less powerful than electromechanical relays. In fact, because optical coupling completely isolates the circuits of the relay, there’s no need to worry about the relay being fried by too much voltage.

  1. Infinite lifespanThey have an infinite lifespan.

One downside to solid state relays is that when they stop working, the entire relay needs to be replaced. With electromechanical relays, contacts are able to be replaced individually. However, while this may be true, an SSR will outlast an EMR by many, many years. Again, without the presence of movable contacts, there are no parts to be worn out and carbon doesn’t build up. An EMR has an average lifespan of one million cycles, whereas an SSR has a lifespan of roughly 100 times that.

Now in order to benefit from an solid state relay’s practically infinite lifespan, the SSR needs to be maintained and used properly. One of the problems that arises when using solid state relays is the substantial amount of residual electrical resistance. This resistance generates heat which can be combatted by the use of a heat sink. Without the heatsink the lifespan of the SSR can be diminished dramatically, so it is very important to consider purchasing a heatsink and SSR together. Fortunately, several of the Carlo Gavazzi solid state relays sold in our webstore already come equipped with heatsinks.

Types of Solid State Relays

Zero-switching solid state relays are the most common. With this type of relay, the circuit begins conducting current when the voltage is near the point of crossing zero volts on an AC cycle. If the input signal is applied at any other point, the relay will wait to output the signal until the voltage is near zero on the AC cycle.

Instant-on solid state relays turn on the load instantly when the control voltage is applied. So the load can be turned on at any point in the AC cycle. This allows for greater control for the user.

Peak switching solid state relays turn on the load when the control voltage is applied, but only when the AC cycle is at its peak.

Analog switching solid state relays have an unlimited amount of output voltages. These relays have a built-in synchronizing circuit that controls the amount of output voltage in relation to the input voltage.

While we focused on the benefits of solid state relays in this blog, the choice between EMRs and SSRs ultimately depends on the application in which they will be used. There are both benefits and disadvantages to each type of relay. Yet, it is undeniable that solid state relays have been a great technological advancement for the industry.

Carlo Gavazzi SSRrgc1 60 rec2brf1carlogavazz

Trimantec carries a wide variety of Carlo Gavazzi solid state relays and accessories. We also carry solid state relays already equipped with heatsinks. Please, do not hesitate to contact us if you have questions!

So, do you prefer EMRs or SSRs? Let us know in the comments below!

Download Carlo Gavazzi’s solid state relay brochure

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