Whasa Relay? Really... (sad I know..)

[AndyMan]

Guys,



I should have learned this in electronics class in high school, but I was too busy playing chess...



I want to know the real purpose of a relay, and when/why I should use one. I've hooked up kits (off-road lights for example) that came with relays, and have used them, but haven't been aware of the real purpose. How do you know what kind to choose, when to use it, etc?



Hope this doesn't fall into the "too stupid to be a post" category.



Thanks,

Andy

 

[datablitz]

im no expert, but basically a relay is a high voltage electronic switch, basically... .

it will prevent you from melting manual switches, and also sets you up to not have to run large high voltage lines into the cab of your truck.



but its basically an electronic switch.

 

[CChase]

A relay is a simple on/off switch which is controlled by a small electrical current rather than a mechanical button or lever.



Their purpose is to take the brunt of a given power requirement off whatever is switching it. Say for instance on the running lights you've installed, if you wanted them triggered off the headlight circuit, but the headlight circuit is not capable of supporting two additional 55W bulbs. You use a relay that takes somewhere in the neighborhood of 100mA (I think that's about what they use) off the headlight circuit to close the switch (relay) in the power wiring that actually provides the power to the lights and is connected directly to the battery in lieu of drawing power through the headlamp circuit..... make any sense?

 

[SRadke]

Basically a small version of a selenoid. It just uses power from a high current source (like the battery) and switches from a low current switch. They use them to prevent overloading switches.



-Scott

 

[dresslered]

Naw, nothing is too stupid as everyone has different specialties. The main purpose for using a relay is to pass larger amounts of current while utilizing smaller and safer amounts of current through the control switch (the switch toggles the relay whereas the relay toggles a bigger switch to turn on the lights). Using the smaller current for the controls allows for a substantial cost savings by using smaller wire for the longer runs, a much smaller and cheaper switch, and also eliminates the voltage drop associated with the long runs (lower voltage at the lights reduces candlepower). The current flow would rather be directly from the battery through the relay and to the lights rather than from the battery, to the switch inside the cab, then all the way back to the lights.



Let's use the off-road lights as an example. Ideally, you want to send large amounts of current directly from the battery to the lights themselves (minimal voltage drop). The relay acts as a switch for turning the lights on (closes the contacts on the heavy gauge wire). The smaller guage wire (can be extrememly thin) runs from the relay coil to the switch (probably located inside the cab). The smaller wire can be used because we only need to energize the relay coils to throw the contacts.



I suppose you could think of a relay as being the electrical counter part to hydrolics. Don't you apply very little pressure to your brake pedal to stop your 8000-lb truck? In a similar way, a relay uses some very small amounts of current to switch on some nice big accessories.



When choosing a relay, you need to consider the application. As a first guess-timate, choose the voltage rating and current rating. Then, you need to determine what is required to energize the coil--this can be a switch, a microprocessor, a photo cell, etc.

 

[AndyMan]

EXCELLENT! Thanks to everyone. That is exactly the information that I was looking for. It makes PERFECT sense. I appreciate each of you taking time to answer.



Speaking of different specialties, if anyone needs any serious PC/Server quesitons answered, I'd be glad to help. I do enterprise server support on the windows platform for a gi-mungous healthcare corporation... and a little pc support from home for sport.





Andy

 

[merryman]

A race where several people each run a portion of the distance carrying a stick!

Vaughn

 

[dresslered]

I think that Vaughn has the best definition yet! Must be watchin' the Olympics.

 

[klenger]

A relay can also be used to: (1) change logic states or (2) switch dis-similar types of power.



(1) 14 vdc applied to the coil of a relay to provide a ground at one of the relay contacts.



(2) 14 vdc applied to the coil of a relay to switch 115 vac at the relay contacts.



Working in the avionics industry (at least in the days of analong equipment), you could fix most any installation design problem with the addition of one more relay and two diodes.

 

[dresslered]

A relay can also be used to: (1) change logic states or (2) switch dis-similar types of power.

Excellent applications Klenger! I got a touch of this as well while I was at Boeing. I found that the avionics applications (i. e. hellicopters) were often too demanding for relays trying to switch the 270VDC Even spreading the contacts resulted in constant arcing conditions. I usually ended up going to solid state FET's instead!

 

[klenger]

(3) Another application is switching a signal from a remote location. Example: you want to switch a low level audio signal from a location from some distance away. Running the audio wires to a control switch may result in too much noise being induced into the wires. By installing a relay at the audio source, you can switch it using a control voltage that is not affected by noise.



(4) A relay can be used to switch multiple signal sources too numerous to do with a switch. Example, we would install 36 poll relays in airplanes to switch navigation signals. The 36 poll relay can be controlled with a single wire with a remote switch.