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#65806 - 05/13/06 02:41 PM What is the primary purpose of a relay?
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
I never understood what a relay actually does.

Could someone please enlighten me?

Thanks.

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#65807 - 05/13/06 03:11 PM Re: What is the primary purpose of a relay?
BigJohn Offline
Member

Registered: 03/06/04
Posts: 352
Loc: Boston, MA
A relay generally allows a low power signal to control a high power load. They're usually smaller, enclosed versions of contactors.

Relays are composed of a coil, and a set of magnetic contacts. When energized, the coil will open and/or close some of those contacts, depending on how the relay is designed.

For example, a relay in a heat pump may have a coil rated for a 24 volts at 0.5 an amp, but the relay contacts may be rated for 30 amps at 240 volts. When the 24 volt control signal from the thermostat flows to the relay coil, it closes the contacts which allows 240v to energize the heat pump.

-John

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#65808 - 05/13/06 03:13 PM Re: What is the primary purpose of a relay?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
There are many reasons for using relays.

The most common reason I install a relay or contactor is so I can switch a large circuit or many smaller circuits remotely with simple low cost low ampacity switch.

I might install a 200 amp 3 phase 600 volt contactor (relay) to control an entire lighting panel. By using the contactor I can use a small time clock to control the contactors coil and by doing that the entire panel.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#65809 - 05/13/06 03:22 PM Re: What is the primary purpose of a relay?
Celtic Offline
Member

Registered: 02/26/05
Posts: 367
Loc: NJ
Simply put...it's a switch that is controlled by another switch.

Here is an easy example:
You have parking lot lighting supplied by by a 3Ø 480v 40A circuit.
How do you control it?
- You could have the hired help turn it on/off at the desired times with a switch

- You could use a photocell (if you could find one that is rated)

etc

Each of these methods has it limitations:
- The store is closed on the Holidays, but you still want the lights on for security. Do you want to pay the help to turn the lights on/off?

- Still looking for the that rated photocell?

So you get yourself a contactor with a 120v coil and contacts rated at 3Ø 480v 40A.
Now you wire the photocell to the coil. When it gets dark, the photocell does it's thing and completes the circuit to the coil. The coil closes, the lights come on. When dawn comes, the opposite happens.

Here is a little animation of what happens: http://electronics.howstuffworks.com/relay1.htm

The photocell could be a switch or a time clock.

Think about how a sump pump works...how does it know when to start or stop?
On a grander scale, that would apply to a water treatment facility, car wash, etc.
_________________________
~~ CELTIC ~~
...-= NJ =-...

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#65810 - 05/13/06 04:19 PM Re: What is the primary purpose of a relay?
Alan Belson Offline
Member

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
Use a relay to;

Switch high voltage/current with safer/ more economic/ smaller/ lower voltage/ control circuit components, say in power transmission. Or, mixed voltage circuitry; controls= lvdc, power= higher vac.

Switch multiple circuits with no backfeeds.
As for example a heating boiler may be fired by signals from multiple programmable zone thermostats or timers, without backfeeding the thermostats/timers themselves or operating all the solenoid valves controlling water flow or all the circulators in the water circuits.

Switch applied logic circuits in machine controls, say Guard open = machine stops but a keyswitched relay allows running during tool setting. Largely displaced by PLC controllers and ICs.

Mechanically, a relay would just take a switched signal current and use it to energise a coil on a solenoid or a motor to operate another switch.

Alan
_________________________
Wood work but can't!

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#65811 - 05/13/06 06:52 PM Re: What is the primary purpose of a relay?
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
All these years I understood what a contactor did. Hell, I've wired more than a few. But I never understood that the relay itself was the guts of the contactor and how it physically worked. Now I know.

Thank You.

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#65812 - 05/13/06 07:10 PM Re: What is the primary purpose of a relay?
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
You guys are talking about contactors. These are relays http://electrical-contractor.net/ubb/Forum8/HTML/000194.html

We did lots of stuff with relays before the transistors got better and they started showing up on silicon chips.
_________________________
Greg Fretwell

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#65813 - 05/13/06 07:26 PM Re: What is the primary purpose of a relay?
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Relay, cintactor...the difference is only in scale!

Well....almost. Most 'contactors' have only "normally open" contacts. A relay typically will also have "normally closed" contacts. These contacts will let the relay operate something when the relay is "off". A common example is your typical alarm system; cut a wire, and you trigger the alarm.

Now...take a contactor, add overloads... and you now have a "starter." Add a disconnect switch, and it's now a "combination starter."

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#65814 - 05/13/06 07:43 PM Re: What is the primary purpose of a relay?
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
Wow, I feel like I opened a can of worms here. Seems that the ole relay has a million and one applications they can be used in. Cool pictures of those old computers being built way back when.

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#65815 - 05/13/06 07:56 PM Re: What is the primary purpose of a relay?
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Relays were really starting to go pretty fast when transistors took over. The basic wire contact relay had a 6ms operating time. The permissive make wire contact relay shaved that to 3ms. The reed relay got that down to about 1ms. You can do about as well with a mercury wetted relay. The slowest transistor gates were running about 1000 times that speed.
_________________________
Greg Fretwell

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