hi all

can anybody explain, why we need relays?

and if a instrument is giving potential free contact, like pressure switch, then in this case we do not need any relay? am i wrong ?

please clarify me

thanks
pandu

 

Relays are electrically operated switches that are used for controlling a circuit by a low power signal. Such as using one signal to control several circuits.

elecTRONica
http://www.bldcmotors.co.uk

 

"can anybody explain, why we need relays?"

Lots of reasons, many of which have been discussed on this site in the past.

"if an instrument is giving potential free contact ... then in this case we do not need any relay?"

You are correct. In this case, the instrument already HAS a relay. (I've oversimplified here; there are other ways to have potential free contacts, but the effect is the same.)

You've somewhat answered your own question; one use for a relay is to isolate voltages. Another is to provide multiple signals as a result of one. Another is current capacity; often I/O is limited to perhaps 0.5 amps to 2 amps, which would be enough to fire a relay that has a capacity of say 10 amps.

To extent, we don't "need" relays any more than we "need" airplanes. But they sure do make life a lot easier.

-James Ingraham
Sage Automation, Inc.

 

>>Why a relay is required?

By definition, a relay is an electrically operated switch. It "relays" (sends) a contact position.

This "sending" could be as follows
Open contact -> open Contact
Open contact -> closed contact
Level Increase/decrease in Voltage
Level Increase/decrease in current
One or more combinations can be used in a circuit.

By design a relay is able to "galvanically" isolate two circuits. Meaning, there is no flow of current between circuits. This is also one of the applications of a relay, providing "potential" free contacts.

Now, a pressure switch is itself providing contacts. Thus a relay is not required. But circuit design requirements might require you to use a relay, e.g. obtaining extra contacts.

Regards,
Tomy Zacharia