Why relays?

[joshuabardwell]

By using relays you can design circuits that would take a very complcated switch to duplicate. One of my favorites is a circuit that won't let you switch off an engine while the lights are still turned on. Turn off the key and remove it, the engine still runs until you turn off the lights, at which time everything shuts down and won't restart without the key. On an old 4x4 I could push a button which would let the engine run without the key, but shut everything down as soon as anyone touched the brakes-great for leaving it running with the heater on while you went in for coffee in an area where someone else might try to drive away.

That's really clever. Having done some more research, I see that you can build, basically, logic arrays out of relays--using them like boolean gates and stuff. The simple examples I have been thinking of (turn on X when Y is turned on) really don't begin to tap the possibilities with relays.

 

[LD1]

Im a bit late to the thread, but I'll gibe my :2cents:

Sure, there are high amp switches out there like you mention, and you can use them without a relay if you want, but nothing inside modern vehichles are rated that high. Think about you turnsignal lever for example. And most of those have the wiper control and WWF control built in to that. Now imagine having to run 4 or 5 12 guage wires in that little lever:confused2:

Same goes for the headlights. Ones like also on the turnsignal lever. The horn is another example.

Just too many small switches. But another example is controlling multiple circuits. For example, your keyswitch. Just clicking it to the "on" position usually powers up several relays. Mostly associated with fuel, ECM, and Ignition. Now imagine having to have a seperate wire for each of those things all comming off the keyswitch. Plus all of the items that ONLY work when the key is on, like the wipers, headlights sometimes, radio, windows, etc. Without relays, ALL of that current would be going through the ignition switch.

And a final benefit (and this is only becoming more popular in newer vehicles), is called ground switching. In older vehichles, one side of the relay was always grounded, and the other received the low current 12v from your switch to activate. Nothing wrong with this at all, but now they are evolving to having the relay energized with 12v at all times, but without a path to ground, it wont activate. Connect it to ground (through your switch), and the relay will energize.

One of the reasons for this is that it cuts down (bigtime) on the amount of hot wires running everywhere. So in the even of a failure (and/or) short in the wire, instead of causing a burnt wire/fuse/or even a fire, a short to ground would simply allow that circuit to turn on. Much safer, and IMO a little easier to diagnose.

The only issue now is that you HAVE to know what kind of system it is before you start.

 

[dstig1]

It's funny but one of the key things I was taught years ago was "always switch the hot". I learned that one based on my experience wiring one switch backwards (120v) and there was a leakage path to ground that left the circuit partly energized when off. Now there they go doing that exact thing on purpose

 

[LD1]

Residential wiring and 120v is a different animal. I am not an electrician, and dont know much of the code and stuff, so I dont know a good reason as to why you cannot switch the neutral. Maybe someone else does???

But on a car, and when switching relays, you can clearly see the benefit for having the ground do the switching. Sure, you will still show a 12v potential in the ground comming to the switch if the switch is in the off position. But in the even of a failure, or short to ground, it simply activates the relay (without you turning the switch). Thats it. No melted wire, no blown fuses, no fire risk. If the horn is honking and your not laying on it, it is a good chance you have a short to ground (east troubleshooting). Whereas in a positive switching setup, the same short to ground would have blown fuses. Too many hack-job mechanics that dont know how to fix problem, and just throws in a larger fuse. Now fuse dont blow, but wire melts and catches fire instead:confused2: you get the idea

 

[boggen]

all the little dash lights and and lights in switches that light up. would be a pain to try and use a larger size wire to run to all of them. being able to use say a 5amp fuse, and running smaller size wire. to each of those small lights help, but then each switch most likely controls something that uses more amps and watts. so another 10amp, 15amp, 20amp, 30amp 40amp fuse is used. to be placed in line to what ever the switch controls.

if you are already running a small size wire to the little lights within the switches when the switch is in the on position why run a heavier size wire to each switch, when a 5amp fuse for the lights can still be used. and then a relay.

it is nice to be able to pull off a fuse panel lid, and have relays right there, that you can just pull out and test, vs taking 1/2 to the full dash of a vehicle apart. just to be able to get to the backside of a switch.

it all comes down to $$$ and manufacturers buying "bulk wire" in the 1,000's of feet at a time.

i have a tendency to by pass relays, and just buy a bigger switch able to handle the amps, and just run directly to battery, using a heavier duty wire to reduce any loss in the electricity, but it gets costly doing so at times. vs just using a relay and smaller gauge wire.

exception to above, you get into switches that 3 to 8 different settings the switch could be placed in. and each setting controls something different some how. trying to mess with larger size wire, that does not bend easily and trying to insert the ends of wires into squeeze clamp inside the switch can be a bear more so when you only have any were from 1/2" to couple inches of extra length of wire to work with. and barely enough room to squeeze your hand in around a switch to do anything. a relay would allow for smaller wires at the switch, and more likely place all the larger size wires some place else were if ever need be, easier working and replacing them.

 

[dstig1]

Oh I totally get the difference. Cars are DC and only have hot and ground. Houses are AC, higher voltage and have 2 paths to ground - neutral and ground wires. In normal use, the ground never carries current, but if you hose something up it is possible to have current leak to the ground and cause a hazard or at least odd behavior in electronics as they can be partially energized. I'm not an EE, but once you see it happen, it makes intuitive sense not to switch the neutral on house wiring.

 

[kebo]

My definition is that a relay allows a control circuit using a small amount of current to control another circuit operating at a much higher current capacity. You can use a control circuit (for example your ignition switch) operating at a couple hundred milliamps to energize a relay and close a pair of heavy duty contacts capable of handling several dozen amps (or even higher amounts for brief periods of time).

 

[sparc]

That is one use for a relay yes. That's more in line with a contactor which is a relay with a special purpose.

Another use for relays is logic circuits. Long before computers and and semi-conductor devices entered the field relays were the way to perform logic functions in control circuitry. Want something to turn off when something else comes on, use a relay. Want to progress through a series of conditions to start or stop a piece of equipment, use relays.

I've worked on relay logic chains that are four and five levels or more deep. All built with relays with four sets of NO-C-NC contacts, some with AC coils and some DC, some normally-energized and some not. You can get lost pretty fast tracking your way through a circuit with 20 or so of these relays in it in a series-parallel layout, especially when they cross over into other circuits for permissives or alarm or trip functions.

I had a job where I had to figure out how to replace as many of a population of 860+ relays as I could with the plant staying online, the rest would be replaced when the plant was shut down. Took a lot of picking through the logic drawings to figure out how to get each relay in a condition where it could be replaced (these were socketed relays) without tripping any equipment and still operate the plant, then test the relays output functions to verify it would work as intended.

 

[firemanpat2910]

one more way relays are used, saftey. if you have plain switch in a circiut with a interlock safety switch,you open the safety cover motor shuts off. close safety cover and the motor starts right back. add a relay and a momentary switch in the circiut. open safety cover motor shuts off. close cover and motor will not start again until you cycle the switch.

 

[weedpharma]

Oh I totally get the difference. Cars are DC and only have hot and ground. Houses are AC, higher voltage and have 2 paths to ground - neutral and ground wires. In normal use, the ground never carries current, but if you hose something up it is possible to have current leak to the ground and cause a hazard or at least odd behavior in electronics as they can be partially energized. I'm not an EE, but once you see it happen, it makes intuitive sense not to switch the neutral on house wiring.

Switching neutral in a mains AC situation is simply dangerous. Power is always available at the load. Switch off the power switch to work on the machine and it can still kill you. (Anyone with any sense would also pull the plug first.)

In a 12v situation it is different. You may blow fuses or melt your best screwdriver, but it will not kill you to come in contact.

Weedpharma