Question About Relays

[quantumtangles]

I cannot seem to get the practical benefit of relays (even after googling and checking them out in electronics encyclopaedias).

A small voltage/current is used to control a larger voltage/current. So they seem to be switches, sort of like MOSFETs. Small voltage controls larger one. Stop press.

Why not just turn something on and off with a regular switch? Why bother using relays?
Can anyone provide a practical example where the use of a relay would have an obvious practical benefit in a circuit (isolation/safety?, convenience?). I would like to get this once and for all and move on.

Thanks in advance for any responses. I am sure the answer is straightforward but I cant seem to get it as I do not really ever make mains AC stuff.

 

[davenn]

well total isolation is the obvious advantage
you dont get any isolation with a FET, huh !

a small DC votage say 6V switching 240VAC mains
to some extent SSR's have now fulfilled that job, but even then for many applications good old mechanical relays can often be physically smaller


Dave

 

[Harald Kapp]

Why not just turn something on and off with a regular switch? Why bother using relays?

Well, you don't have to use a relay. Take the dishwasher in you kitchen. It has a microcontroller to control the cleansing program. Now if the microcontroller wants to turn on the motor (just for example), the controller could light a small lamp and an operator (you) sitting in front of the dishwasher would react to the lamp's signal by pushing the appropriate switch.

Personally, I prefer to sit in my living room, reading or listening to music instead of in front of the dishwasher while a small relay in the dishwasher does the trick for me.

 

[davenn]

nice example Harald

Dave

 

[Arouse1973]

Well, you don't have to use a relay. Take the dishwasher in you kitchen. It has a microcontroller to control the cleansing program. Now if the microcontroller wants to turn on the motor (just for example), the controller could light a small lamp and an operator (you) sitting in front of the dishwasher would react to the lamp's signal by pushing the appropriate switch.

Personally, I prefer to sit in my living room, reading or listening to music instead of in front of the dishwasher while a small relay in the dishwasher does the trick for me.

Like it

 

[Shadow1976]

Automotive use relays as a means to reduce cable size under dash and to control larger currents like headlights, indicators etc. trying to use a switch without a relay would result in burnt switches and possibly wiring. It would have to be an ugly switch with baulky contacts. Not a lot of nice looking switches are high rated in my opinion.

 

[KrisBlueNZ]

A switch needs to be mechanically flipped by a person. A relay changes state in response to voltage across its coil, which can be controlled by a circuit.

A common example is the relays used in washing machines to activate the pump and other heavy duty devices. These relays are controlled by the microcontroller that handles the wash cycle, so that the appropriate heavy devices in the machine start and stop at the appropriate times.

Edit: Snap! A dishwasher is a good example too.

 

[quantumtangles]

Thanks so much. Wonderful examples

 

[jcurrie]

motor contactors in industral equipment , like a 24v to control a 4150v motor on a miner that way the opperator isn't exposed to high voltage, the only real diff in a contactor and a relay is the contactors have replacable contacts and are realy heavy duty.
jc

 

[quantumtangles]

Thanks JC. I understand it intuitively now (because of the examples above) and I am looking forward to making a useful device containing relays.

 

[flippineck]

I didn't know contactors had replaceable contacts.. that's pretty neat

 

[jcurrie]

some of the smaller ones may not but all of the ones i'v worked with have, and some of the contacts were silver alloy.
jc