RELAYS TO MAKE SWITCHING AUTOMATIC

Relays and Solid State Relay
Relay and Solid State Relay

How Do Relays Work And Change A System Automatic?

Basically, the relay is an electromechanical device or switch that is open or close to the electrical current when flowing through it. In a relay, a magnetic force relays the effect of one circuit (control-circuit) to another circuit (load-circuit). Relays work like a very simple remote control switch, which character helps to develop a basic ON or OFF basic logic, is the very first step to change a system in automatic.

Relays are used in a wide variety of applications throughout industry, such in HVAC, Appliances, Security, Pool & Spa, Office Equipment, Telecommunications, Data-com, CP/OA, Office Automation, Process Control, Automotive, Energy Management Systems, Motor Drives/Controls, Appliance Controls, Industrial Control, Motors, Timers, Instrumentation, Test Equipment etc.

All relays actually contain basic two units- the sensing unit, the electric coil and the responsive unit, the switch contact. The coil is powered by AC (Alternating Current) or DC (Direct Current), when the applied current or voltage reached a pre-determined value then the coil generates a magnetic field which causes a moving force to close the open contract and/or open the closed contract.

In simply, there is no electrical connection between the control circuit and the load circuit, they are electrically insulated to each other; the control circuit transfers the signal to the load circuit to operate in a predetermined condition.

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Types of Relays

Relays may be classified from different point of views- like the construction of relays or purposes of the relay using etc. As per relay construction, there are basically two types of relays- Electromechanical Relays and Solid State Relay. Electromechanical relays are constructed with mechanical moving parts, whereas solid-state relays are integrated electronic devices.

  1. Mechanical timer relay- use mechanical elements such as gears and springs or sometimes fluids to limit the movement (and time) something takes to change position.
  2. Solid-state timer relay- utilizes solid-state components such as SCRs, transistors, thyristors, and even other microelectronics to time something. These have the same function as the mechanical timers, but have no moving parts, making them last much longer when used properly.


For general purposes electromechanical relays used in huge, due to their low cost, multiple poles, suitable for large power, no heating issues, suitable for AC/DC system.

Solid-state relays are used the light signal to control electronic devices. Advantages of solid-state relays are small sizes, long life, moving parts less. But it can only accomplish single-pole switching only.

For types of relays on the basis of mechanism, functions and characteristics are as below-

General Purpose Relay;
Latching Relay;
Ratchet Relay;
Timer Relay;
Safety Relay;
Interlock Relay;
Polarized Relay;

High-frequency Relay;
Limit Relay;
Differential Relay;
High Sensitivity Relay;
High Speed Relay;
Current Relay;
Make-before-break Relay.
For more details about relay types, the basis on using places and purposes, visit another complete post Lexis of Relays where described details. 

Relay Operations


Simple operation, the contacts of a relay switch when a voltage or current is applied to the relay coil. Electromechanical Relay operation actually determined by balancing the spring forces and attractive forces of the magnetic force generated in the relay coil.

Used Materials In Relays

 Relays normally used following material and their combination for contact, the ratio depends on the characteristic of load and sensitivity where it will be used.
Platinum, Gold, Silver, Cadmium Oxide, Nickel, Indium, Tin etc.

For magnetic materials, it’s important to consider improving electromagnet efficiency, high magnetic permeability & low-loss, stabilized and improved properties and no-oxidizing etc. and to do this soft iron, silicon steel, permalloy etc. are used.


Copper wires are used for relays winding and phenol resin, polyester resin, diallyl phthalate resin, polybutylene resin, polyacetal resin, polycarbonate resin, glass-reinforced materials are used as insulation materials.


polybutylene /terephthalate resin, polycarbonate resin and steel sheets are used for Relay Casing materials for transparent and hermit protection.

Pin Configuration Of Relays


Frequently used relays are 3 pins, 4 pins or 5 pins; all pin configurations are not the same. The variation of relays pin configurations is shown in the above figure.

Relay Checking Procedure

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How Check Relay is in Good Condition: If you find a relay is not labelled, how you sure which pin goes where? Then you must need to carry out a continuity check and first identify the relay pins before use. An Ohmmeter can use to check continuity.  For the coil or control circuit you will find 50 to 70 ohm’s, and zero and infinity/ OL for closed and open circuit respectably. Unless otherwise, you will sure your relay is faulty.

How Check Relay is in Good Operation Condition: Using a battery power energized the relay coil or control circuit, you will hear click/ operating sound; also connect the battery +ve to one side of the load circuit then put the test light to other side and disconnect, repeat the test. You will find the test light ON and OFF, which means the relay operating properly.

You also can carry out this relay operation condition check by a voltmeter. Just put a voltmeter in instating of the test light, the meter will show the battery voltage if it is in good condition.

Furthermore, you can carry out this test with an ohmmeter. Just energized the relay coil or control circuit by battery power. Then check the load circuit with an ohmmeter. The ohmmeter reading will show zero if the contact is closed, or it will show infinity/OL if the contact is open.

Finally, we can say a relay is just a device that can make or break a switch which is the primary condition to make a system automatic.


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