How Solid State Relay Works

What are Solid State Relays and how should we use them in practice? These are some of the questions we’ll answer in this article.

In a previous article, we have discussed the ins and outs of the Electromechanical relays which I strongly suggest you to check that first from here.

In that article, you’ve learned why we still better use the relays in general, regardless of the great advancements in technology.

In this article, we investigate a fully electronic type of relay; the Solid State Relay or SSR for short.

We check to see how to test a Solid State relay with multimeter and then you learn about Solid State relay wiring. After we checked differences between various types of SSRs you’ll also learn how to choose among all different types of SSRs depending on a given application.

What is a Solid State Relay and Why do we Use Them?

The look is quite a bit different from electromechanical relays; the same as its manufacturing technology.

Solid State Relays

As it’s mentioned, there is no mechanical moving part used in Solid State relays manufacturing and all are made of semiconductors such as diodes, Transistors, Thyristors, Triacs, and so on.

Solid State Relays

There are different designs for different usages.

For example, when you’re designing an electrical control panel’s internal layout, you always need more space.

And there is a slim design for Solid State Relays that would be an appropriate choice for you!

Solid state relays din rail mount

If you replace the EMRs with the slim design of the SSRs then you’ll have more spare space in your panel to add more hardware.

Power Protection Relays on Medium Voltage Panels

You can use Solid State Relays as an interface between your PLC output cards and the loads out there in the process.

What is the purpose of solid state relay

However, as you’ll learn in the future articles, the Thyristors, and Triacs are more intended to drive the resistive heating elements and therefore the Solid-State Relays which use these electronic parts in their output circuits, are also more applicable for these purposes.

What is a solid state relay

They also have different names depending on their manufacturer. For instance;

– Photo Relays,

– MOSFET Relays,

– Solid State Modules,

– Solid State Drives,

and so on.

How to Test a Solid State Relay with Multimeter

The relay we have chosen for this example is a single-phase relay that accepts a fixed DC voltage on its input terminals and has only a Normally Open contact on its output.

It’s input is ranging from 3 to 32 volts DC.

How to check Solid state relay

By doing a continuity test using a power supply and a voltage tester (or Multimeter) we make sure of the functionality of the Solid State relay.

First off, we should adjust the tester for the continuity test.

How to test a solid state relay with multimeter

Then, I place the probes on the output terminals of the relay.

As soon as the input voltage reaches the 3 volts or above, you can hear a sound from your tester saying the resistance between the probes is almost zero so the electronic output contact has been closed.

How to test solid state relay using multimeter

How does a Solid State Relay Work? (Solid State Relay Wiring)

On the output side of the relay, we see that we can connect a 24 to 480 volts AC load.

Let’s assume that there is a 600-watt/230-Volt heater (Heat Tracing) that we want to use as the load and control the temperature using a control signal coming from a PLC.

NOTE: In a future article, you’ll learn that SSRs are usually used with another type of controller, known as the PID controller.

SSR PID temperature controller

The heater will get its power from the AC power source but via the SSR. We transfer the live wire to the heater via the SSR.

So, we connect the Live wire from the power source to one of the output terminals of the SSR and will wire its other terminal to the heater.

The Neutral wire will be directly connected to the heater from the power source.

Here, you have to make sure to cover the terminals of the SSR as it has electrical power all the time; even when the relay output is switched off.

Solid state relay wiring

As soon as the PLC sends the command, the SSR LED turns on, showing the output of the relay is closed.

So, the heater turns on and starts warming up to increase the temperature.

How to wire a solid state relay

Of course, there is a sensor to feedback the temperature of the tank to the PLC.

How to use solid state relay

Solid State Relay vs. Mechanical Relays

1. High-Speed Switching SSRs

Consider a process in which we’re gonna send commands to a load in a matter of milliseconds.

In this process, the speed of switching gets a major parameter for us.

So, we’ll benefit from the semiconductor technology of the Solid State relays as they are WAY faster than electromechanical relays.

Solid State Relay Advantages

2. No Sparks, Low Noise!

As you may already know, the millivoltage signals, such as the signals from the Thermocouples can be corrupted by the electrical noise.

Whenever an electromechanical relay switches on or off, it produces some electrical noise in the panel, and the more the electromechanical relays are, the more could be the noise and the chance of corrupting our signals in the control system.

So, we better use the SSRs as they emit far less electrical noise.

Solid State Relays produce no Sparks

3. Hazardous Area Compatibility

In a hazardous area, you HAVE TO use the SSRs;

Because the generated sparks from an EMR switching, could be very dangerous and lead to an explosion.

Solid State Relays for Hazardous Areas

Apart from single-phase, three-phase, slim, or PCB types of SSRs, they fall into three main categories according to their output switching modes.

Types of Solid State Relays

1. Random Turn On Solid State Relay (Asynchronous)

The first one is the “Random Turn-On” SSR or “Asynchronous” Solid State Relay.

When the controller applies the control voltage to the input terminals of the relay, the relay output will turn on immediately after that and fully pass the current toward the load.

Random Turn On Solid State Relay

2. Zero Crossing Solid State Relay (Synchronous)

The second and the most common type is the “Zero-Crossing” or “Synchronous” type.

What is zero-crossing?

In an Ac sine wave, whenever the wave crosses the horizontal axis, we’ll have a zero-crossing point.

Zero Crossing Point Definition

So, in this type, as opposed to a “Random Turn-On” type relay, when the input is active, it does not conduct the load current immediately;

but the output will wait for the first zero-crossing point of the AC load voltage, to pass the whole electrical current toward the load.

Zero Crossing Solid State Relays

In both “Random Turn-On” and “Zero-crossing” types of SSRs, when the control voltage is removed from the input terminals, the output will not stop passing the load current until the next zero-crossing point of the wave reaches.

It is a characteristic of all types of solid-state relays regardless of their switching type.

Zero crossing vs random turn on

3. Proportional Control Solid State Relay

The third type of SSRs are named “Proportional Control” Solid State Relays and have their own different types. The most common types of them are:

– Phase-angle Relays

– Burst-fire Relays

Proportional Control SSRs are used for extremely precise control of the output (especially in heating and lighting applications).

Proportional solid state relay

In Proportional Control SSRs, the controller will apply an ANALOG control signal to the input of the SSR instead of a fixed DC or AC control signal.

So, the control signal could be an analog voltage signal such as 0-5- or 0-10 volts DC or it could be an electrical DC current such as 4-20 mA.

The output will vary the amount of the load current depending on the amount of the control signal on the input side.

Types of solid state relays

3.1. Phase-Angle Proportional Control Solid State Relay

Let’s assume we have a “phase-angle” SSR that accepts a 0-10 volts signal on its input terminals.

The controller applies a 5-volt control signal to the SSR input for transmitting 50% of the power to the load.

As a result the SSR output (which is a Triac) will turn on at the peak of every AC half cycle and therefore conducting 50% of the power to the load.

Phase Angle Solid State Relay

3.2. Burst-Fire Proportional Control Solid State Relay

As another example, this time, we have a “burst-fire” SSR with again a 0-10 volts DC analog control signal.

If the controller applies 70% of the input signal (which is 7 volts here), then the output AC voltage would conduct 70% of the total power to the load.

It means, from every 10 cycles of the AC voltage, only 7 cycles will pass toward the load.

Burst Fire Solid State Relay

These were the simplified waveform for almost all common types of SSRs.

Which SSR for Which Application?

Selecting the correct type of solid-state relay allows great precision in process control.

For “Resistive loads” like heating elements the Zero-crossing and Proportional Control SSRs are perfectly suited.

For “Inductive loads” such as electric motors, contactors, and so on, the Turn-On type SSRs are usually a better fit.

Solid State Relay Applications

In the future lesson, we’ll show you an example of a Zero-Cross type SSR in practice and you’ll understand why it is essential to choose the correct type of SSR depending on your application.

And… that’s it! This time YOU let us know what you would like us to cover for the next articles.

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