Solid State Relay- Function, Design, Application, Pros, Cons and Testing

A solid state relay (SSR) is a kind of relay that is mainly used for switching purposes.  It is mainly used for switching the heating elements in the machines such as water bath, oven, incubator, etc. They are also used for the microcontroller, PIC, Arduino, etc. This is further classified into two types. They are DC SSR and AC SSR. Besides the SSR, you may even find a normal electromechanical relay or the contractor as a substitution.

Solid State Relay Vs Electromechanical Relay

Now, let us make a comparison between these two types of relays.

Solid State Relay Electromechanical Relay
There is no phenomenon of physical contact within, for the activation of the relay. There is the phenomenon of physical contact within, for the activation of the relay.
Since there is no physical contact, there is no problem of wear and tear of the relay Since there is physical contact, the relay can undergo wear and tear
It has got a high life span. It has got a low life span
It provides higher efficiency It provides lower efficiency
Comparatively expensive Comparatively cheap
Less vibration More vibration

Due to the superior nature of the solid state relay, it has replaced the electromechanical relay in a number of applications.

Design of Solid State Relay

Solid State Relay
External Structure of Solid State Relay

The figure shown here is the external structure of the DC SSR. You can see that it consist of 4 ports/ pins for the connection. Port 1 and 2 are the output ports. Similarly, the port 3 and 4 are the input ports. We need to supply 4 to 32 VDC in the input port. On supplying the input DC voltage of the given range, the relay activates. Thus, the LED present on the relay turns on. This input voltage is also known as the coil voltage or the pickup voltage.

On activating the relay, you can supply the AC voltage in the output port ranging from 24 to 480 volt. Also, the limiting current in the given relay is 16A. However, this value can alter from one solid state relay to another. Now the load connected to the output port operates.

In the case of AC SSR, you can supply AC voltage as an input and DC voltage as an output. Other things remain the same as that of the DC SSR shown in the figure.

Solid State Relay Working Principle

Solid state relay circuit diagram
Image Source:- https://www.allaboutcircuits.com/technical-articles/basics-of-ssr-solid-state-relay-the-switching-device/

The figure shown here is an example of DC SSR. So, you can see the dc voltage being supplied to the input terminal. So, when forward biased the LED gets turned on. Now it will emit light. This light goes to the photodetector. Based on the manufacturers, the photodetector can be a TRIAC, transistor, etc. This pair of LED and the photodetector is known as the optocoupler.

Once the photodetector receives light from the LED, it gets activated. Thus the flow of current begins to start in the output terminal.

Using this basic principle, it has been used in a number of applications such as zero crossings. The design or the construction of the zero-crossing circuit diagram is shown in the figure below.

Advantages of SSR

Solid state relay has got numerous advantages. Some of them are as follows.

  • Does not undergo wear and tear.
  • Has got a higher life span.
  • Highly effective.
  • Since there is no physical contact within, it does not undergo any kind of vibration.
  • Has got a higher switching capability with an ON time of 1 microsecond and 0.5 microseconds of OFF time.
  • It needs a low power consumption for operation (5 mA at 5 VDC).
  • Possible to operate in multiple modes such as zero switchings, instant-on, etc.
  • Since it can operate for small voltage input, it can be used for a number of applications such as from heater to microcontroller.

Disadvantages of SSR

Even though this type of relay is superior to the traditional relay, it too has got some defects. Some of them are as follows.

  • The voltage drop along the relay is the source of heat generation. So, you may need to add a heat sink along with the solid state relay.
  • There is a chance of leakage current even when the relay is turned off.
  • It is quite expensive.
  • Generally, a single relay provides an opportunity for a single contact to the load.
  • Even a small amount of input voltage can turn it on. Thus, sometimes there can be a risk.


There are a number of ways to check the condition of the solid state relay.

  • You can check the solid state relay using a multimeter only. For this, take a relay and the multimeter. Then set the multimeter to the resistance mode. Now check the resistance across the input and output ports. In both cases, the results should come in megaohm. Else, the relay can be faulty.
  • You can give voltage in the input port of the solid state relay. Then, the LED of the relay will turn on. That means it has been activated and may not be faulty. Now, on activation, the resistance across the output port will decrease. You can check it by measuring the resistance. The resistance will drop from megaohms to kilo-ohms.
  • The best option will be by adding load to the system.  For this, supply the suitable voltage to the input terminal. Then connect a load in the output port and check the condition of the load.







Related Articles

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button