What Is Solid State Relay | Pros and Cons | Practical Application

Table of Contents

What Is Solid State Relay?

Introduction:

• A solid-state relay is a substitute for Electromagnetic relay.
• It is referred to as optocoupler due to its construction.
• It consists of a light source (e.g. LED) and a light sensor.
• It is known by the acronym “SSR”.
• It mainly used to provide isolation than switching high current.

• It is a semiconductor package.
• It switches power ON or OFF between its output terminals with the response to small current and voltage between input its terminals.
• These relays use LEDs, photodiodes, SCRs and thyristors for switching a circuit ON/OFF.
• The figure depicts the solid-state relay.
• It is available in AC and DC variants.
• It usually functions as SPST switch with either normally open or normally closed versions.

Solid State Relay

The black color device on the Relay Controller Board above is SSR (Solid State Relay). We all know less about the relay. But this relay is a little different from the electromechanical relays.

The relays we usually see consist of coils, metallic cores, armature bodies, etc. But using a special sensor in this relay senses the desired input signal and helps in the switching of the automatic power system.

Its main components include a thyristor, a mosquito-switching device that allows the control section of the device to receive up to 8-20 amperes.

The advantages of other relays are commonly used

It’s light in weight
Maybe switching fast.
Less sensitive to external factors such as heat, relative humidity, external electromagnetic field.
Prevention does not change that much.

Advantage(Pros) Of Solid State Realy

There is some advantage of a solid-state relay:

It offers a very fast response usually about 1µs ON and 0.5 µs OFF.
It has a very high lifetime.
It is smaller in size
There are no physical moving parts involved in the design of a solid-state relay. Hence there is no mechanical noise. Moreover, there will be no contact bounce and a clean output signal is available.
Due to less current requirement (5 mA at 5V DC) it offers low power consumption.
It is highly durable.
There are no coils in the design which can introduce back EMF into the circuit.
It is insensitive to vibration.
As there is a complete internal separation between input and output terminals, it can be used to switch high voltages.
Some variants can work as low as 1.5V DC, dissimilar electromagnetic relay which requires at least 3V DC.

Disadvantage(Cons) Of Solid State Realy

There is some disadvantage of a solid-state relay:

Its cost is very high.
It is bound by specific current/voltage characteristics.
There is a fixed value voltage drop on output due to internal impedance. Hence it is less efficient. This is negligible for high switching voltage operation.
There is leakage current at the output even if the relay is in OFF mode.
It generates waste heat during ON mode. This is proportional to voltage drop.
It is triggered by brief voltage spikes on the input side, unlike the electromagnetic relay.
It is more vincible to surges and spikes in the current which is switched on output side unlike EM relay

Solid State Relay Practical Application

Application 1: Smart control of traffic signals

Controlling traffic signals can be done using data acquired from vibration sensors on crossroads. SPST relays act as simple switches for controlling red-yellow-green signals, which work on the higher power supply.

Application 2: Temperature-based control of table fan

Application 2 shows how to control the speed of a table fan working on a 230V AC supply. Speed of DC motors and fans can be controlled by using simple potentiometer or pulse-width modulated (PWM) signals. For AC equivalents, controlling can be done by using power resistors or power semiconductor devices. A power resistor can stand against the rated wattage on it. Resistance across power resistor varies from 120 ohms to 60 ohms to 0 ohms, left to right with the right-most terminal fixed. Depending on the temperature, an SPDT relay can be switched on to provide resistance of 120 ohms or 60 ohms in the circuit, leading to different fan speeds

Application 3: Precision agriculture

Application 3 shows how to water a field in accordance with the water content in the soil. Arduino settles upon the amount of time and, hence, the amount of water to be pumped in the field, using a DPST relay. A siren in parallel sounds to indicates the pump in on condition. This application is helpful in saving water.

Application 4: Unmanned ground vehicle for inhumane environments

Application 4 explains how to control a simple unmanned ground vehicle (UGV) that moves forward and backward depending on the input. A DC motor’s direction of rotation is controlled by alternating the polarities of the input voltage. A sensor such as an ultrasonic sensor observes the obstacles and decides whether to move forward or backward. Switches with the polarity of 12V are required for the motor. The direction of motion of the UGV changes accordingly.

These applications indicate stable methods to employ relays to generate real-world applications. Relays, either electromagnetic, reed or solid-state, execute the same operation, that is, switching. Electromagnetic relays are spontaneously implementable for any voltages or any devices and, hence, should help people in demonstrating projects successfully for environmental, medical, military, agriculture, societal, automobile, aviation, and other applications.