Educate me on relays

[shark92651]

I think I understand the basic premise of a relay. It is like a circuit that is activated with a switch and routes the high amperage through the relay rather than through the switch. Have I got that right? What I am not sure of though is when I should use these in my MKIV/Coyte build. I see 3 relays in the Ron Francis fuse box so I assume those are for things like the headlights, horn maybe? I have Lucas-style switches on my dash for hazard lights, wipers, and fog lights (LED). I also added some footbox lights, and a couple USB/12V plugs, but otherwise my electrical is pretty basic. I have swapped out all of the incandescent lights for LEDs. Should I add relays to any of these items?

Thanks,
David

[Papa]

You've got the concept. I added relays for the headlight dimmer (floor mounted), the coil power, and the starter trigger wire.

Dave

[edwardb]

As Dave said, you have the concept. The three relays in the RF harness are fuel pump, cooling fan, and horn. All three relatively high draw circuits, so use a relay. Flashers are a relay of sort, but that's off topic. Whether you need relays on the other functions you mention is a matter of the switch current rating and the current draw required. The wiper switch I wouldn't mess with. Those are routinely used without relays and you'd have some fun duplicating the high/low/park function. LED hazards and fog light are probably OK. Put a multimeter on the circuit and see what they draw. Footbox lights almost for sure OK without. The USB and aux outlet, you have them switched? Or are they connected to the circuit directly? If so, no relay should be necessary there either.

[shark92651]

Thanks for the replies. Is there a guideline as to what amperage reading through the switch would warrant a relay? It sounds like I may not need any additional ones, however. The USB and 12V adapter will be wired directly, no switch.

[Papa]

Each switch will (should) indicate its load rating in amps. Watts/volts=amps. So, a 60 watt headlight bulb draws 5 amps (60watt/12volts=5amps).

[Clueless]

Just a quick note:
I would give some margin. Taking the example from Papa above, if you have a 60 watt bulb, I would assume your switch should be capable of quite a bit more than 5 amps.
Does anyone have a good rule of thumb to use?

I usually double it if I can, but I think that is probably just my unreasonable paranoia.

[initiator]

I would put an inline fuse on your USB and 12V adapters if you don't already. I have a waterproof 2-in-1 adapter with USB and 12V - you want to protect against a short at the adapter or anything that's plugged in.

[FlyingCobra]

Generally relays are used for higher amperage items. If you're running LEDs, your current draws should be low enough such that you don't need relays for those. Headlights, fuel pump, and ECU are the main ones that come to mind.

[Avalanche325]

Yes, you have it. It allows you to use a little switch and run small wires to it, to control something that requires bigger wires and a bigger switch. They are a remote controlled switch.

Imagine having to run the battery cable up inside the dash, to a big Frankenstein switch, and then another big cable back out to the starter. A relay (in this case either in the starter itself, or the starter solenoid) keeps you from having to do this.

You only need relays when you will exceed what the switch and /or the wires going to the switch can handle. Only use them when you have to. If not needed, they unnecessarily complicate the circuit and add many failure points.

What relays are not, but are often confused for:
Circuit protection.
Switch protectors.
Magic voltage amplifiers.

Circuit protection, usually fuses, are something that everything should have. Fuses are there to keep your wires from melting or starting a fire when something goes wrong. You will notice that the headlight circuit does not have a fuse. WHAT?? The GM style switch that comes with the kit has a self resetting circuit breaker built in. This will stop the aforementioned melt down, but give you headlights as much as possible when something goes wrong.

[Fixit]

This is something I wrote up an eon ago for the guys in my chevelle club

http://www.northstarchevelles.com/NonSecure/Tech/Electrical/Basics2.pdf

[CraigS]

For me relays have a few other uses. In a modern car it's all done w/ ecus but we stick w/ relays in our machines. 1- you can have more than one thing to control a circuit. IE adding driving/fog lights. The magnet inside the relay needs power and a ground. For extra lights you could get the power from the parking or low beam circuit. The ground could be provided by your toggle switch. So the extra lights will only come on w/ a combination of the parking or low beams and the toggle switch. 2- relays can also be used as somewhat of a safety device by keeping the large current wiring out of the passenger compartment. The heavy power wire for a radiator fan or maybe headlights can go from a power source through the relay to the lights or fan. So that part of the circuit can be kept out in the engine compartment (and heavily insulated) and a small wire run into the dash for the switch. I like to run the ground wire for the relay in to the switch. That way, if it were to short, all that happens is that the fan or lights come on.

[Mark Eaton]

What relays are people using? Waterproof? Is it ok to get them off Amazon or should I go to Delcity?

Thanks,

Mark

[edwardb]

What relays are people using? Waterproof? Is it ok to get them off Amazon or should I go to Delcity?

Thanks,

Mark

Standard automotive style relays are basically a commodity item. I've personally never used waterproof ones, and wouldn't bother unless you have a specific location or concern where you think that's important. Amazon has a ton of choices and the ones at DelCity look OK. But they're also available at your local auto parts store. I try to stick with names brands I recognize, like Bosch, Hella, etc. But to be honest I've never had one fail from any brand. Make sure to match the rating of the relay to whatever it is you're switching. Some are rated for higher current than others.