In a previous video we have discussed the ins and outs
of the Electromechanical relays We have learned why we still
better use the relays in general regardless of the great
advancements in technology In this video let’s learn about the other type of relays the Solid-State Relays or SSRs You’ll also learn how to choose among all different types of SSRs
depending on a given application As you see the look
is quite a bit different the same as its manufacturing technology Although, we’re not going to talk about
the manufacturing technologies here and for now this is enough to know that There is no mechanical moving part and all are made of semiconductors such as diodes Transistors Thyristors Triacs and so on There are different designs
for different usages For example when you’re designing the internal layout of an electrical control panel you always in need of more space And this relay as it has a slim design would be an appropriate choice for you You can use it as an interface
between your PLC output cards and the loads out there in the process However, as you’ll see in the future videos 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 They also have different names
depending on their manufacturer For instance Photo Relays MOSFET Relays Solid State Modules Solid State Drives, and so on I hope you’re excited
about the rest of the video So, keep watching as there are
lots of key practical points to discuss! First of all, let’s see how an SSR works 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 As you see the input is ranging from 3 to 32 volts DC Let’s see what it means You can check this by doing a continuity test using a power supply and a voltage tester or multimeter to make sure of the functionality of your relay I adjust the tester for the continuity test and 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 so the electronic output
contact has been closed On the output side, 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 that we want to use as the load and control the temperature using
a control signal coming from a PLC In a future video you’ll see that SSRs are usually
used with another type of controller known as the PID 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 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 Of course, there is a sensor to feedback
the temperature of the tank to the PLC To talk about the advantages
and disadvantages 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 SSRs as they are WAY faster
than electromechanical relays 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 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 Due to all of these advantages and so many more we believe you’ll see the Solid-State
relays more and more over time Apart from single-phase Three-phase Slim or PCB types of SSRs they fall into three main categories according to their output switching modes Watch the rest of the video to get a better understanding of different types of Solid-State Relays and how to choose them for a given application 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 The second and the most common type is
the “Zero-Crossing” or “Synchronous” type What is zero-crossing? In an AC sine-wave, wherever the
wave crosses the horizontal axis we’ll have a zero-crossing point 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 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 The third type of Solid State Relays are proportional Control SSRs and have their own different types.
The most common types of them are “Phase-angle” and “Burst-fire” relays Proportional Control SSRs are used for
extremely precise control of the output especially in heating
and lighting applications 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 For instance, 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 volts control signal to the SSR input for transmitting
50% of the power to the load Then the corresponding output
AC voltage will be like this As you see, the SSR output,
which is a Triac has turned on at the peak of every AC half-cycle and therefore conducting 50%
of the power to the load 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 be like this Say, from every 10 cycles of the AC voltage only seven cycles will pass toward the load These were the simplified waveform
for almost all common types of SSRs Selecting the correct
type of solid-state relay allows great precision in process control Let’s quickly see which SSR is
appropriate for which application 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 In a future video, 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 That’s it! Thanks for watching and
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