What is a Static Relay : Working & Its Applications

The solid-state relay or static relay was first launched in the year 1960. As the name suggests, the term static in the static relay implies that this relay has no moving parts in it. As compared to an electromechanical relay, the lifespan of this relay is longer and its response speed is faster. These relays were designed as semiconductor devices which include integrated circuits, transistors, small microprocessors, capacitors, etc. So these types of relays replace almost all the functions which were being accomplished earlier through an electromechanical relay. This article discusses an overview of a static relay – working with applications.

What is a Static Relay?

An electrically operated switch that has no moving parts is known as a static relay. In this type of relay, the output is simply attained through the stationary components such as magnetic & electronic circuits. Static relays are compared with electromechanical type relays because these relays utilize moving parts to perform a switching action. But both relays are used to control electrical circuits using a switch that is open or closed based on an electrical input.

Static Relay
Static Relay

These types of relays are mainly designed to perform similar functions using electronic circuit control like an electromechanical relay performs by using elements or moving parts. A static relay mainly depends on the designs of microprocessors, analog solid-state circuits, or digital logic circuits.

Static Relay Block Diagram

The static relay block diagram is shown below. The static relay components in this block diagram mainly include a rectifier, amplifier, o/p unit & relay measuring circuit. Here, the measuring circuit of the relay includes the level detectors, logic gate & the comparators like amplitude & phase.

Static Relay Block Diagram
Static Relay Block Diagram

In the above block diagram, the transmission line is simply connected to the current transformer (CT) or potential transformer (PT) so that the transmission line provides the input to the CT/PT.

The output of the current transformer is given as an input to the rectifier which rectifies the input AC signal into the DC signal. This DC signal is given to the measuring unit of a relay.

The measuring unit relay performs the most significant action necessary within the static relay system by detecting the input signal level throughout the level detectors and evaluating the magnitude & phase of the signal throughout the comparators to perform the logic gate operations.

In this relay, two kinds of comparators are used amplitude and phase comparators. The main function of the amplitude comparator is to compare the input signal’s magnitude whereas the phase comparator is used to compare the phase variation of the input quantity.

The relay measuring unit o/p is given to the amplifier so that it amplifies the signal’s magnitude & transmits it to the o/p device. So this device will strengthen the trip coil so that it trips the CB (circuit breaker).

For the operation of the amplifier, the measuring unit of the relay & the o/p device requires an extra DC supply. So this is the main drawback of this static relay.

Static Relay Working Principle

The working of the static relay is, first, the current transformer/potential transformer receives the input voltage/current signal from the transmission line & gives it to the rectifier. After that, this rectifier changes the AC signal into DC and this is given to the measuring unit of the relay.

Now, this measuring unit identifies the input signal level after that it comparators the magnitude & the phase of the signal with the available comparator in the measuring unit. This comparator compares the i/p signal to make sure whether the signal is defective or not. After that, this amplifier amplifies the signal’s magnitude & transmits it to the o/p device to activate the trip coil to trip the circuit breaker.

Static Relay Types

There are different types of static relays available which are discussed below.

  • Electronic relays.
  • Transductor relays.
  • Transistor relays.
  • Rectifier bridge relays.
  • Gauss effect relays.

Electronic Relay

An electronic relay is one kind of electronic switch used to operate the circuit contacts by opening & closing without any mechanical action. So, in this type of relay, the current carrier pilot relaying method is utilized to protect the transmission line. In this type of relay, electronic valves are mainly utilized as measuring units.

Electronic Relay
Electronic Relay

Transductor Relay

Transductor Relay is also known as magnetic amplifier relay which is very simple mechanically & even though some of them may be electrically little complicated so this does not change their reliability. As their operation is mostly dependent on stationary components whose characteristics are simply predetermined & verified. Thus they are very easy to design & test as compared to electromechanical relays. The maintenance of these relays is negligible practically.

Transductor Type
Transductor Type

Transistor Relay

A transistor relay is the most generally used static relay where the transistor in this relay works like a triode to overcome the limitations caused by the electronic valves. In this relay, a transistor is used as an amplifying device & a switching device which makes it suitable for attaining any functional characteristic. Generally, transistor circuits cannot perform only the necessary relay functions but also provide the required flexibility to suit different relay requirements.

Transistor Relay
Transistor Relay

Rectifier Bridge Relays

Rectifier bridge relays are very famous due to semiconductor diode development. This kind of relay includes a polarized moving iron relay & moving coil and also two rectifier bridges. The most common are relay comparators based on rectifier bridges, which can be arranged as either amplitude or phase comparators.

Rectifier Bridge
Rectifier Bridge

Gauss Effect Relays

Some metals as well as semiconductors resistivity changes at less temperatures once they are exposed to the magnetic field in relays which is known as the Gauss effect relay. This effect mainly depends on the ratio of depth to width & increases with the increase within this ratio. This effect is simply observed in some metals at room temperature like bismuth, Indium Magneto, indium arsenide, etc. This type of relay is better as compared to the Hall Effect relay due to simpler circuitry & construction. But the gauss effect within static relays is limited due to the high cost of crystal. So, the polarizing current is not necessary & output is comparatively higher.

How to Connect a Static Relay to a Microcontroller

The interfacing of a solid-state relay or static relay with a microcontroller-like Arduino board is shown below. The main difference between normal relays and SSR is; a normal relay is mechanical whereas SSR is not mechanical. This static relay utilizes the mechanism of an optocoupler to control high-power loads. Similar to mechanical relays, these relays simply provide electrical isolation between two circuits as well as an optoisolator works like a switch between two circuits.

Static relays have some benefits as compared with mechanical relays like they can be turned ON with very lower dc voltage like 3V DC. These relays control high power loads, its switching speed is higher as compared to mechanical relays. During switching, it doesn’t generate any sound as there is no mechanical component within the relay.

The main intention of this interfacing is to measure the room temperature & it will turn ON/OFF the AC based on the room temperature. For that, a DHT22 temperature sensor is used which is a fundamental and low-cost humidity & temperature sensor.

The required components of this interfacing mainly include a Crydom SSR, Arduino, DHT22 temperature sensor, etc. Give the connections as per the interfacing given below.

Connect a Static Relay to a Microcontroller
Connect a Static Relay to a Microcontroller

This sensor uses a thermistor & capacitive humidity sensor to measure the surrounding temperature. It provides a digital output signal on the data pin. This sensor has one drawback; you can get new data only from it after every two seconds. The DHT22 temperature sensor is an upgrade of the DHT11 sensor but the humidity range of this DHT22 sensor is more precise as compared to dht11.

In the above interfacing, the solid-state relay works directly from Arduino’s digital pins. This relay needs 3 to 32 volts dc to activate the other circuit. At the output side, you can simply connect a maximum load with 240 volts AC & up to 40A of current.

Arduino Code

Upload the following code into the Arduino board.

#include “DHT.h”
#define DHTPIN 2 //DHT22 digital pin to Arduino pin connection
// Uncomment the sensor you are using i am using DHT22
//#define DHTTYPE DHT11 // DHT 11
#define DHTTYPE DHT22 // DHT 22 (AM2302), AM2321
//#define DHTTYPE DHT21 // DHT 21 (AM2301)
// Initialize DHT sensor.
void setup() {
Serial.println(“DHT22 test!”);
pinMode(7, OUTPUT); //SSR switching on/off pin
dht.begin(); //Begin sensor operation
void loop() {
delay(2000); //2 seconds delay
// Reading temperature or humidity takes about 250 milliseconds!
// Sensor readings may also be up to 2 seconds ‘old’ (its a very slow sensor)
// Read temperature as Celsius (the default)
float t = dht.readTemperature();
Serial.print(“Temperature: “);
Serial.print(t); //Print temperature on serial monitor
Serial.print(” *C “);
if(t<=22){ //Temperature less than 22 *C switch off AC(Air Conditioner)
digitalWrite(7, LOW);
if(t>=23){ //Temperature greater than 22 *C switch on AC(Air Conditioner)
digitalWrite(7, HIGH);

In the above Arduino code, the library of the DHT temperature sensor is included first. This library is valid especially for different temperature sensors like DHT11, DHT21 & DHT22 so we can utilize these three sensors with a similar library.

Here, the AC is switched ON/OFF at the centigrade temperature. If the room temperature is below 22-degree centigrade then the relay will be turned OFF and if the room temperature increases then the relay will be turned ON and makes the AC turn ON automatically. Between every reading, there is two seconds delay to make sure that the temperature sensor has updated the reading or not which is not the same to before the reading.

Here the main drawback is whenever the room temperature increases to 30 degrees centigrade then the relay will get hot. So heat sink needs to install with the relay.

Static Relay Vs Electromagnetic Relay

The difference between static relay and electromagnetic relay includes the following.

Static Relay

Electromagnetic Relay

A static relay uses different solid-state semiconductor devices like MOSFETs, transistors, SCRs, and many more to achieve the function of switching. An electromagnetic relay uses an electromagnet to achieve the switching function.
An alternate name for this static relay is the solid-state relay. An alternate name for this electromagnetic relay is an electromechanical relay.
This relay works on the electrical & optical semiconductor properties. This relay works on the electromagnetic induction principle.
The static relay includes different components like a semiconductor switching device, a set of i/p & switching terminals, and an optocoupler. The electromagnetic relay includes different components like an Electromagnet, Moving armature & set of i/p & switching terminals.
This relay doesn’t have any moving parts. This relay includes moving parts.
It does not generate switching noise. It generates switching noise.
It consumes extremely less power than in mW. It consumes more power
These relays do not need a substitute for contact terminals. These relays need the substitute of contact terminals.
This relay is installed at any location and in any place. This relay is installed always in a straight position & at any place away from the magnetic fields.
These relays have a compact size. These relays have a large size.
These are highly accurate. These are less accurate.
These are very fast. These are slow.
These are costlier. These are not costlier.

Advantages and Disadvantages

The advantages of static relay include the following.

  • These relays consume very less power.
  • This relay gives very quick response, high reliability, accuracy, and long life & it is shockproof.
  • It doesn’t include any thermal storage troubles
  • This type of relay amplifies the i/p signal which enhances their sensitivity.
  • The unwanted tripping chance is less.
  • These relays have maximum resistance to shock, so they can operate easily in earthquake-prone regions.
  • It needs less maintenance.
  • It has a very quick response time.
  • These types of relays give resistance to shock & vibrations.
  • It has a very quick resetting time.
  • It operates for an extremely long period
  • It consumes very less power & draws power from a secondary dc supply

The disadvantages of static relays include the following.

  • The components used in this relay are extremely responsive to the electrostatic discharges which mean unexpected electron flows between the charged objects. So, special maintenance is necessary to the components so that it does not affect the electrostatic discharges.
  • This relay is affected easily by high voltage surges. So, precautions must be taken to avoid damage throughout voltage spikes.
  • The relay working mainly depends on the used components in the circuit.
  • This relay has less overloading capacity.
  • Compared to the electromagnetic relay, this relay is extremely costly.
  • This relay construction is simply affected by the surrounding interference.
  • These are responsive to voltage transients.
  • The semiconductors devices’ characteristics like diodes, transistors, etc. utilized in these relays change by temperature & aging.
  • The reliability of these relays mainly depends on a number of small components & their connections.
  • These relays have less short-time overload capacity as compared to electromechanical relays.
  • The operation of this relay can be simply affected because of the aging of the components.
  • This relay operation speed is limited by the component’s mechanical inertia.
  • These are not applicable for commercial purposes.


The applications of static relay include the following.

  • These relays are widely used in very high-speed-based protection systems of EHV-A.C transmission lines with distance protection.
  • These are also used in earth fault & overcurrent protection systems.
  • These are used in long & medium transmission protection.
  • It is used to guard parallel feeders.
  • It gives backup safety to the unit.
  • These are used in interconnected & T-connected lines.

Thus, this is all about an overview of a static relay – working with applications. These relays are also called solid state switch which is used to control the load by turning ON & OFF once the exterior voltage supply is given across the device’s input terminals. These relays are semiconductor devices that utilize solid-state semiconductor electrical properties such as MOSFET, transistors, and TRIAC to perform input & output switching operations. Here is a question for you, what is an electromagnetic relay?