Simply Explained – Electrical Relays

A relay is an Electrically operated switch.
Relays can be used as switches or amplifiers.
Relays act as a bridge by making use of low currents to activate circuit with high current (where appliances or machines are connected).
There are two types of relays

Electromechanical relays: Contacts are closed or opened by a magnetic force.

Solid state relays: Having electronic parts with no mechanical contacts.

Electromechanical relays:
- Commonly have 4 parts: Electromagnet, Armature, Spring and Set of Electrical Contacts.
- When DC passes through the relay's input circuit, electromagnet gets excitation and attract the metal bar (a moving armature) towards stationary contact. This makes the circuit closed and activate the high-current appliance.
- Electromechanical force is used by these type of relays to control an electric circuit and so its name.

1.Input Circuit 2.Electromagnet 3.Metal bar 4.High- Current Appliance

Commonly have 6 (non-moving) parts : Resistor, LED, Photocoupler, Capacitor, Power transistor, Power MOS FET (not shown in image).

Lower current input passes through the LED and it emits light. This light is captured by a light sensor which actuates transistor present in higher electric circuit. When the transistor gets actuated it becomes a closed circuit and the load (appliance/machine) will turn on.

These type of Relays will make or break the contact in an electrical circuit by using signals.

1.Input Circuit 2.LED 3.Light Sensor 4.Transisitor 5.Trigger Circuit 6.High- Current appliance

Some relays are used with Thermostat (senses temperature), Pressure switch (senses pressure) etc.
Kind of electric lever: Switch on the relay with a small electric current and the relay will turn on the other appliances by using a bigger electric current.

For Example, Refrigerator's compressor keeps the interior cold. Most refrigerators have thermostat with 4 wires, one wire supplies 24V AC power to thermostat and other three will carry the controls signals from thermostat. When the temperature inside the refrigerator rises above or falls below the thermostat temperature settings, the thermostat sends an electrical signal to the relay switch to start/stop the compressor. Thus a low current activates the compressor which is using high current.
Relays protect an electronic device from various kind of faults like circuit failure, circuit overload, system failure etc.
Relays are widely used to control motors, high power industrial lamps, heaters, AC circuits, automobiles, machines used in industrial automation, military equipment etc.