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Basic Relay Operation

The Contacts

Before extending to the various types of relays, i will rst explain what and how the

basic relay operates. Each relay has two mechanical parts inside. The rst one is the contact(s)of the relay. The contacts operates similarly to the contacts of a simple switch or pushbutton.ou should consider the contacts as a pair of metals li!e the following diagram"

The two terminals operates as a switch. #hen the contacts are $in contact$ then thecurrent %ows from Terminal & to Terminal '. There are two types of contacts" the and the*. stands for Normal Open contact, while * stands for Normal Closed contact. Theormal pen is a contact li!e the one showed in the previous illustration. #hen the contact isstill, then no current %ows through it (because it is an +E circuit). n the other hand, aormal *losed contact allows the current to %ow when the contact is still. Bellow i illustrateboth of these contacts"

ou may notice that the * contact is turned upsidedown compared to the contact. This is done in purpose. This way, both contacts ( and *) will change state if aforce is applied to the left metal heading from -+ to #. The following animation showshow a contact operates by lighting a light bulb"

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/s for the * contacts, it wor!s exactly opposite as the contacts. 0oo! thefollowing animation"

A combination of contacts

/ relay may have a combination of the above contacts. 0oo! at the followingillustration

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1n this case, there is a 2 rdterminal called 3*443. The and * contacts arereferred to the *44 terminal. Between the * and the contact, there is no contact atany time5 The following animation shows how this pair operates"

And WHO defines the NORMAL state?

6, we have the 74/0 open and 74/0 closed contact. But which state isconsidered as 74/08 9oing one step closer to the relay operation, we nd the spring. Thisspring denes the 74/0 position of the *44 contact. 1f you see the above 2animations, you will notice that one time an : force is applied to the *44 terminal, andthe other time there is no force applied. #ell, this is actually wrong. There is indeed another

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force that pulls the contact towards -+ and this force is applied /00 the time. This force comesfrom the spring. 0oo! the following image"

ow you can see who is pulling the *44 terminal -+ all the time. ;o the springdenes what is the 74/0 state, and thus denes which contact is the 74/0 +E andwhich the 74/0 *0;E. 1n other words, the 74/0 state is dened as the state thatthere is other force applied to the *44 terminal except the one from the spring.

The last part - WHO moe the common contact of the relay?

This is the last part of the relay operation. The device that forces the terminal to move,is actually an electromagnet5 / coil is placed right under the contact. #hen current is %ownthrough this coil, a magnetism is created. This magnetism can overcome the force of thespring and can pull the contact towards it, thus it changes it$s position5 /nd due to the fact thatthe contact is usually a small piece of metal not capable to be pulled by the electromagnet,another piece of metal is attached to the common. This piece of metal is so called 3/rmature3.

:ollowing is (at last) the complete illustration of the basic relay"

ow, imagine that someone wants to control a '' =olts battery. / load7elay should be used for this application. The *oil ofthe relay is driven with the > =olts. The contacts from this relay () will be connected in

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series with the power supply of the load. Thus, the load will only operate when the relay isactuated. ur friend bellow will turn on an electric oven barehanded555