Relay has wide varieties of applications in different industries. We have discussed many circuit applications making use of relay like automatic water tank level controller and clap switch using 555 timer.
What is Relay?
- Usually a relay is used in a circuit as a magnetic switch to turn on a second circuit.
- A relay is an electromagnetic switch, which is activated when a small current is passed through its coil.
- The interesting fact is that this small current is capable of turning on a secondary circuit which works on much larger current.
- There are varieties of relay available in the electronics market, each of them operates at different voltages.
- When you construct your own circuit you must consider the voltage ratings that will energize (trigger) it.
Why we need a relay? Use of a Relay
Why relays are useful to us? As the name indicates, many sensors are extremely sensitive type of electronic components and produce only small output currents. But we need to drive bigger electrical machines using those sensors! Relays, makes it possible for small currents to trigger bigger current circuits.
What is inside of a relay?
- The important part of a relay is the coil. This coil makes the relay as an electromagnetic switch.
- What is an electromagnet? We are familiar with the term ‘Electro Magnet’ i.e. any current carrying conductor that induces a magnetic field.
- If the conductor is covered around an iron core, the iron core becomes magnetized. This is called an electromagnet.
- The same principle is applied in relays too. When a current passes via the relay coil, an electromagnet is set up which pulls one switch contact away from another.
Types of Relay contacts (NO, NC & Common)
An ordinary relay has two contacts called Normally Open (NO) and Normally Closed (NC) contacts. We can switch the common terminal to NC or NO. Please look at the working animation of relay below.
Working animation of relay
- Normally relays are driven by relay drivers. What is a relay driver? It is nothing but a transistor.
- This transistor is turned on by some sensors.
- When this transistor is activated, it feeds current to an electromagnet that pulls a metal switch closed and activates the second high current.
- The relatively small current in the input driver circuit activates the superior current in the output circuit.
[Example]Real time application of relays
- Suppose you want to build an electronically operated street light system that switches an electrical bulb ON or OFF with respect to the intensity of surrounding light.
- You could use some kind of electronic light sensor circuit (LDR) to sense dark or light conditions, but LDR produces only small resistance variations according to light intensity.
- You could connect the LDR circuit to the input circuit of a relay via relay driver.
- When a small current from sensor flow through this circuit, the relay will activate its output circuit, allowing a much bigger current to flow. Thus turning ON the electrical bulb.
Relay connection diagram
- Identify the two terminals of coil (just open the relay, you can easily find it).
- Connect Vcc to one terminal and fed the other terminal to the collector of driver transistor.
- Connect the high current neutral line to common terminal of the relay.
- Connect the NO terminal to any terminals of the load (Bulb, Motor etc.).
- Connect the phase line directly to another terminal of the load.