Types of Relays and their symbols

What is Relay?

A relay is a kind of switch that is electrically driven. It receives a signal from the source to open and close the contact. Relays built into electrical appliances transmit electrical signals to other equipment by turning a switch on and off.

Basic Relay Construction and operation

The relay can work both electrically and mechanically. The relay consists of a set of contacts and electromagnet and this helps to perform switching between ON and OFF.

The relay construction is mainly classified into four. They are

  • Contacts
  • Bearings
  • Electromechanical design
  • Terminations and housing.

Contacts: - This is considered the most vital part of the relay. It affects the reliability of the relay. Selecting the quality of contact is very important because only good contact can give limited contact resistance and reduced contact wear. The selection of the contact material depends upon several factors such as the nature of the current to be interrupted, the magnitude of the current to be interrupted, frequency and voltage of operation.

Bearings: - A single ball, multi-ball, pivot-ball, or jewel bearing are examples of bearings. For maximum sensitivity and minimum friction, a single ball bearing is used. The multi-ball bearing has a low friction coefficient and is more shock-resistant.

Electromechanical design: - Electromechanical construction includes the construction of magnetic circuits and the mechanical fixation of cores, yokes, and armatures. To increase the efficiency of the circuit, the resistance to the magnetic path is kept to a minimum.

Terminations and Housing: - With the help of a spring, an armature is assembled with the magnet and the base. Molded blocks give dimensional stability by isolating the spring from the armature. On the terminal link, the permanent contacts are normally spot welded.

Working Principle of relay

It is based on the electromagnetic attraction theory. The magnetic field will be energized when the relay circuit detects a fault current and this leads to the generation of a temporary magnetic field.

The working principle of the relay is based on electromagnetic attraction. It energizes the magnetic field and also produces a temporary magnetic field when the fault current passes through the relay. The magnetic field helps the ON or OFF the connections. Low power relays have only one contact, while high power relays have two contacts that open the switch.

When a power supply is flowing to the coil through the load contacts and control switch and the coil produces a magnetic field around it.

Due to the generation of the magnetic field, the upper arm tends to attract the lower arm and it makes the circuit close and allows the flow of current through the load. If the contact is already closed, it moves in the opposite direction, thus opening the contact.

Relay Terminals

COIL- This is referred to as the COIL terminal. We are giving supply voltage to this terminal when we need to close the switch. One side of the coil becomes positively charged and the other end gets the negative voltage. Polarity does not matter unless we use a diode.

NO- The NO or Normally Open terminal of a relay is also a load terminal that remains open when the relay is not in use.

When the relay is turned on, the NO and COM terminals are connected.

NC- This is the Normally Closed Switch. This terminal is used to attach the device you want to power ON when the relay isn’t powered and off when the relay is powered.

COM- This is a common terminal for relays. When the relay is turned on and the switch is closed, COM and NO are conducted. If the relay is unpowered and the switch is open, COM and NC are conducting.

Types of Relays

Relays are classified into various types based on their function, construction, terminals, etc. Different types of relays are

Based on Poles & Throw

Pole and Throw

The term “pole” refers to the number of circuits that a single switch may control in one action.

The term “throw” refers to the number of contact points.

The contacts NO and NC are single throws. Double-throw contacts are used for changeovers.

Based on poles and throws, relays are classified into the following types

  • SPST Relay
  • SPDT Relay
  • DPST Relay
  • DPDT Relay

SPST Relay

The SPST relay stands for Single Pole Single Throw. SPST has only one contact either NO or NC. It has two terminals that can be connected or disconnected. Including the coil terminals, the SPST relay has a total of four terminals.


SPDT Relay

SPDT relays or single-pole double-throw relays have a total of 5 terminals, 2 terminals for magnetic coils, and 3 terminals for contacts. The contact mechanism has one common terminal, one normally open terminal, and one normally closed terminal.


DPST Relay

The DPST relay stands for Double Pole Single Throw DPST relay. It is a combination of two SPST relay with 6 terminals. There are two scenarios depending on the relay type. No coil voltage: With NO, the load is disconnected because no current can flow. There are only 6 terminals.


DPDT Relay

DPDT stands for the double-pole double-throw relay. It is an electromagnetic device that is designed to separate two circuits electrically and connect them magnetically. They are often used to interface an electronic circuit, which works at a low voltage to an electrical circuit that works at a high voltage.

The parallel pairing of two SPDT relays with single magnetic coils is known as a DPDT relay.


Based on Operation Principle

  • Electromagnetic Relay
  • Electrothermal relay
  • Solid-State relay
  • Hybrid relay

Electromagnetic Relay

Electromagnetic relays operate on the principle of electromagnetic attraction. It is a magnetic switch. Electromagnetic relays have coils and contacts. When we apply a power supply to it, the coil becomes electromagnetic and attracts the contacts to close and open. image

Electrothermal relay:

A bimetallic strip is formed when we combine two different materials. The strip has a tendency to bend when we apply a power supply and this bending tendency is used to make a connection with the contacts


Solid-State relay:

In a Solid-state (SSR) relay we use semiconductors instead of mechanical parts as used in electrothermal and electromechanical relays. This makes the switching speed of the device increase. It has more life span and is faster than other relays.

Solid-state relays provide switching using solid-state components such as BJTs, thyristors, IGBTs, MOSFETs, and TRIACs.


Hybrid relay:

A hybrid relay is made up of a combination of both electromechanical and solid-state relays.

Based on Power Supply

  • AC Relay
  • DC Relay

AC relay

AC relays have two coils that form a magnetizing transformer for the core. The wire in the conventional coil is the primary winding of the transformer. The secondary winding of the transformer looks like a copper washer or D-ring. Some magnetism in the primary coil creates an electric current within the copper ring. The current in the ring or secondary winding of the transformer is actually lag compared to the current in the primary winding so that the core is always partially magnetized. The lever will not hit the core. Because as long as alternating current flows, a permanently magnetized core will never release it.


DC Relay

DC (Direct Current) is steady and never reverse. In the Dc relay, a single coil of wire around the iron core is used to make the electromagnet. When a power supply is passed through the DC relay, magnetism is generated in the core. As long as the DC is flowing, the steady magnetism keeps the lever attracted, and once stop the power supply, the iron core is demagnetized and the lever returns to its original position.


Based on Working Principle

  • Differential Relay
  • Distance Relay or Impedance Relay
  • Latching Relay
  • No Volt Relay
  • On/off time delay relay
  • Overcurrent Relay
  • Polarized Relay
  • Pulse Relay
  • Remanence Relay
  • Thermal Overload Relay

Differential Relay

A differential relay detects when there is a difference in two equal quantities of electricity. There are usually two magnetic coils connected in such a way that a differential relay is triggered when a difference in current flow in the two coils occurs. Here you can see the symbol for the differential relay.


Distance Relay or Impedance Relay

Distance relay is also called an Impedance Relay. Distance relay is mainly used to find faults in the electrical transmission lines. It is working based upon the distance of error on the transmission lines by calculating the impedance.


Latching Relay

A latching relay is an electrically actuated two-position switch. Holds all contact positions indefinitely without energizing the coil.

Controlled by two momentary-acting switches or sensors, one of which “sets” the relay and the other “resets” the relay. Latching relays perform basic memory functions by retaining their position even after the enable switch is released.


No Volt Relay

No Volt Relay or NVR is used to operate with a very low voltage. Just as the name implies it actuates when a low voltage is applied across its coil terminal. NVR is also called an under-voltage relay.


On/off time delay relay

The on/off delay relay has a timing mechanism. It does not turn off or turn on immediately. Always Switch on or off after a preset delay time.


Overcurrent Relay

As the name implies, an overcurrent relay is triggered when an overcurrent flows. This provides a continuous flow of current and triggers when the current exceeds a set limit.


Polarized Relay

Polarized relays use permanent magnetic flux in the electromagnetic section. This means that the working coil is polarized.


Pulse Relay

Pulse relay is also called a keep relay. This relay has a set (ON) and resets (OFF) by the input of pulse voltage. As the name implies, this relay will keep in its set or in reset position even the power supply breaks.


Remanence Relay

Remanence relays are designed as a set and reset mechanism. Easy to set or reset. Remanence relays can be used for both AC and DC.


Thermal Overload Relay

The thermal overload relay has a bi-metallic strip instead of a magnetic coil. The bi-metallic strip expands according to the magnitude of the current flow. This current flow causes the auxiliary contacts to get the switch open and closed.


Application of Relays

  • Relays can be used in both large electrical and electronics circuits
  • Used to control various applications such as signal controllers, temperature controllers, etc.
  • Can be used to perform both arithmetic and mathematical operations.
  • Used in automatic stabilizers
  • Used in both televisions, computers, etc.
  • Used to select circles when the system has multiple circuits.