Types, Applications and Testing
We use the relays in home automation systems, automobiles/motorcycles, industrial applications, DIY (Do It Yourself) projects, test and measurement equipment and many other areas. What is a relay? How does the relay work? What are the areas of application of relays? We will learn more about relays in this study.
What is a Relay?
The relay is a simple electromechanical switch. While the regular switches are used to turn off/on a circuit manually, a relay is a switch that connects or separates two circuits. However, instead of manual operation, a relay uses an electrical signal in order to control an electromagnet to connect or disconnect another circuit.
Relays can be of different types, such as electromechanical, solid state. Electromechanical relays are widely used. Now, let’s have a look at the internal parts of this relay before learning the principle of operation. While there are many different types of relays, their working principles are the same.
Internal Components of Electromechanical Relays
Electromechanical relays consist of the following parts:
- Coil
- Mobile and Fixed Contact
- Core
- Conductive Wire
- Spring
The electromagnet is formed by a copper coil wound on a metal core. The two ends of the coil are connected to the two pins of the relay. These two ends are used as DC supply ends.
Usually there are two more contacts for connecting a high ampere load, and they are called “switching points”. There is also a common contact for connecting these switching points.
These contacts are called normally open (NO), normally closed (NC) and common (COM) contacts.
We can use a relay in an AC or DC circuit. In AC relays, at each zero current position the relay coil loses its magnetism and the circuit may get disconnected anytime.
Therefore, AC relays are engineered with special mechanisms that will provide continuous magnetism to avoid this problem. These mechanisms include electronic circuit layouts or a shaded coil mechanism.
Working Principles of Relays
A relay works based on electromagnetic induction principle.
- When a current is applied to the electromagnet, it creates a magnetic field around it.
- The above picture shows the working principle of a relay. A switch is used to apply DC current to the load.
- Within the relay, the copper coil and the iron core act as electromagnets.
- When DC current is applied to the coil, it starts to pull the contact as shown. This is called the “energization” of the relay.
- When the supply is cut off, it returns to its original position. This is called the “de-energization” of the relay.
- There are also relays whose contacts are closed at the beginning and opened when energized (i.e. the opposite of the relay shown above).
Types of Relay Contacts
Just as we learned that a relay is also a switch, the terms such as ”Poles and Throws” also apply to relays. Relays can be classified based on the number of contacts that the relay changes, as well as the number of circuits.
Before we learn this classification of contacts, we need to know the poles and throws of a relay switch.
Poles and Throws
Relays can change one or more circuits. Each switch in the relay is called a “pole”. The number of circuits connected by a relay is indicated by “throws”.
* Depending on the poles and throws, the relays are classified as follows:
Single Pole Single Throw (SPST)
A single-pole single-throw relay can control a circuit and connect to an output. It is used only for applications that require an ON or OFF state.
Single Pole Double Throw (SPDT)
A single-pole double-throw relay connects an input circuit to one of two outputs. This relay is also called “switching relay”.
Although the SPDT has two output positions, it may contain more than two throws based on the configuration and application requirements.
Double Pole Single Throw (SPST)
A double-pole single-throw relay has two poles and a single throw and can be used to connect two terminals of a circuit concurrently. This relay is used, for instance, to connect both phase and neutral terminals to the load concurrently.
Double Pole Double Throw (DPDT)
A double-pole double-throw relay has two poles and two throws for each pole. They are used for engine directional control, as well as phase or polarity reversal.
For all these relays, the switching between the contacts is carried out when the coil is energized.
COM or Common Terminal
COM refers to the common terminal of the relay.
This is the output terminal of the relay and one end of the load circuit is connected here.
This terminal is connected internally with one of the other two terminals, depending on the state of the relay.
NO Terminal: The NO (Normally Open) terminal is a load terminal that remains open when the relay is not active.
When the relay is activated, the NO terminal closes along with the COM terminal.
NC Terminal: The NC (Normally Closed) terminal is the other load terminal of a relay. This terminal is normally connected with the COM terminal of the relay when there is no control input.
When the relay is activated, the NC terminal is disconnected from the COM terminal and remains open until the relay is deactivated.
Types of Relays
There are different kinds of relays available according to different areas of use. The types of relays that are commonly used are as follows;
General Type Relays
Solid State Relay
The solid state relays, unlike the magnetic relays, are not equipped with mechanical parts such as a contact, coil, spring, etc. The most important feature of the solid state relay is that it can withstand high currents.
Reed Relay
This is a type of relay with two contacts in a gas-filled glass tube. Reed relay contacts work by the magnetic field effect. When a magnet is brought closer to the reed relay, the contact’s states are switched “on” and “off”. These relays are mostly used in security systems.
Thermal Relay
This is a type of relay that opens and closes the contacts based on the ambient temperature.
Leakage Current Relay
The leakage current relays are used to cut off the energy in the circuits that have an insulation fault that causes a current higher than normal values flows.
Time Relay
This is the control circuit element that operates or stops a control circuit when the set time expires.
Areas of Use of Relays
Relays are used to protect the electrical system and minimize the damage that may occur to the equipment connected to the system due to overcurrent/overvoltage. A relay is used for protecting the equipment connected to it.
Relays are used to control the high voltage circuit with a low voltage signal in applications such as audio amplifiers and some modem types.
They are used to control the high current circuit with a low current signal in applications such as the starter motor solenoids used in automobiles. They can detect and isolate errors that occur in power transmission and distribution systems.
Typical application areas of relays are:
• Lighting control systems
• Telecommunications
• Alarm Systems
• Traffic control
• Electrical power system protection systems
• Computer interfaces
• Automotive
• Household appliances
