I of course use them to protect switches for all the devices mentioned above which are pretty much all really cheaply made nowadays. Bosch relays are inexpensive, really rugged, and come with pre-wired plugs that make them easy to install. Also like to use them for fun stuff like this circuit I put in all my builds that turns the car's headlight low beams to turn signals. Serve double duty by powering the headlight signal lights while protecting the turn signal switch and dimmer switch from high current. I have fried dimmer switches before with out relay protection. They aren't solely for the purpose of protecting circuits and switches. Relays aren't intrinsically good or bad. They are great where they serve a purpose. And on my Mysterion cloning project I used this relay based circuit to allow the top and chassis to be raised and lowered in several ways; by remote control; by switches outside the car in the 'trunk' (more like a glove box on this car!); or by switches on the dash. The circuit is hierarchical - the trunk switches over ride the remote, and the dash overrides both the others so the drive doesn't get locked in by some joker outside! One of these circuits control each of the two functions.
To relay or not to relay, that is the question! By all means relay where you need it, but don’t use one if not needed. Anytime you add contacts and connections, you add trouble. Bad connections are responsible for most electrical failures. “ Shorts” account for a smaller percentage of problems. If wired and connected properly, you shouldn’t have too much problem with “too many” relays. I always built a relay board for my one tons pulling trailers with a lot of lights to take the load off the turn signal and light switch and so my blinkers wouldn’t flash fast , back in the day. Like you I tended to “over relay”.Just my two cents, Bones
mike51merc disagree with almost everything you said about relays. We like to use them and they seldom fail. but hey there are lots of ways to do things. lots of good info. here Thanks
If you build without any accessories and use mechanical fuel pumps and fans, the old cars still had relays for the horn and the mechanical voltage regulator is either 2 or 3 relays. If you only have a generator capable of putting out 30 amps, you could wire without relays. The problem these days is that the ignition and other switches available today aren't able to carry much load. Better to transfer the loads to relays and use the switches to control the relays.
Higher amperage switches are not hard to come by, they just cost more then the weak ones. LOL I probably would not use a relay just for the sake of using a relay, something that I learned the hard way is that when you need one you really need one.
Which is why I like to use old switches, either used ones in good condition, or NOS or NORS. There are lots of ways to skin this cat.....
I run them simply to protect the switches. On my 71 GMC with H-4 Halogens I was going through dimmer switches (traditional on the floor) every couple of months and lost one headlight switch. I put in a pair of relays on the core support and no more burned out switches and the lights stay steady. That was when I was going to work at three in the morning and needed all the light I could get to spot the drunks staggering down the middle of the road in black (so the tribal cops don't spot them) clothes. On some accessory items that run off the switched side I don't that much draw running though the ignition switch.
I do believe this is one of those situations that the only thing we will all agree on is to disagree. A place for relays and a time not for. Ben
The great thing about this thread is everyone is right! There is no wrong answer. Unless you blow a switch.
Why the necessity to place the relay as close as possible to the item being controlled? Cons of not doing this? Sent from my toilet
One reason off the top of my head is to reduce voltage loss due to the length of a run or wire gauge. In my case, (I don't have any runs over 5') is use small switches that won't carry much current.
There is no necessity... A relay is nothing more than a switch, and where it's placed in the circuit doesn't matter. But many relays are used to correct deficiencies in the harness (almost always undersized wire), so in those cases you want to make the circuit path as short as possible to reduce voltage drop.
Having done a bit of industrial machine wiring, (before PLCs) relay ladder logic can do some interesting things for you. Most are off topic in a strict sense, but if/then, and/nand, or/nor, are the fore runner of computer control. Nothing like having a bank of a hundred or more relays in a cabinet going clickety, clickety, clickety while a machine is running and having to find the one that isn't doing the clack sound. I will be using relays to control headlights, (on/off and hi/low beam) along with horns, power windows and anything else that I feel takes more current than my switches can carry. I'd much rather have a relay protecting a switch that doesn't need protecting than having a switch that needed protection burning my truck to the ground! Anyone remember the Ford pick-ups from the 80s and 90s that got the reputation for burning garages, switches weren't heavy enough, would get hot, burn the insulation around the terminals, then when you least expected it, burst into flames under the dash.
Try opening up a cabinet with none of the relays identified, and then to put icing on the cake, they wired the whole thing with purple wire.... BTDT... Relays can be wonderful things. But don't exactly fit into the KISS (keep it simple stupid) ideal. Adding unneeded components adds a layer of complexity and introduces yet more failure points. As I've said before, in too many cases relays are added to 'fix' a design flaw that should have been addressed when the harness was being constructed and/or components selected. One place relays could be used effectively is if you're towing. Running an auxiliary circuit (sized for the full trailer lights load) to the rear of the towing vehicle, then using relays powered by the turn/brake and taillight circuits to power the trailer lights would eliminate any issues with flasher sizes or circuit overloads. Of course, with the now-widespread use of LED lights, not a big deal anymore. But if you want 'old school' lighting (and I'm sorry, but LEDs just don't look right on vintage vehicles), this would offer another layer of protection for your harness.
My '32 isn't meant to be vintage and I'm designing my own harness to take advantage of newer components/technology. The worst offender? I'll be running LED headlamps and while they won't give that warm incandescent glow, it's REALLY nice only having to run 16ga wire and no headlight relay (other than for the hi/lo beams). As a matter of fact, the only incandescents in the car will be the gauge lighting. Hell, I could run 18ga for most of the lighting runs if I wanted to. It would look pretty much vintage until I turn a light on though. --------------------------
14 ga for the majority of circuits in a car is massive overkill, but other than an alternator charge you'd be hard pressed to fry it.
That chart is an invitation to a fire..... And will also give you large voltage drops at most of the lengths shown, some well above 5%. It's this sort of misinformation is why guys need relays to 'fix' issues in their harnesses. Maximum wire ampacities: #18 - 6A max fuse size, 5A max load. #16 - 8A and 6A, #14 - 15A and 12A, #12 - 20A and 16A, #10 - 30A and 24A, #8 - 40A and 32A, #6 - 55A and 44A, #4 - 70A and 56A, #3 - 85A and 68A, #2 - 95A and 76A. And while these amp numbers will give you a 'safe' install, they still don't guarantee that you won't have excessive voltage drops on longer runs. I do a voltage drop calc for virtually every circuit/wire, and keep the drops to below 3% max, preferably less. If I have to bump a wire size up to reduce drop, that's easier than adding a relay and yet another circuit. It is possible to 'cheat' a small wire in sometimes, but only in very short lengths. As an example, a #10 wire can carry 50A in a 6" length and only have a drop of .14%. This will produce almost no heat, and while I wouldn't recommend it, will work. But by the time the length gets out to 10 feet, the drop is up to 3.6%, enough to start affecting circuit performance. At 16 feet, it's now up to nearly 6% and you WILL notice the voltage drop on most circuits. And voltage drops add up; if you lose 3% between the battery and the fuse panel, everything downstream from the panel is short the 3%. Lose 3% from the panel to the device, and now you're down 6%. Use that chart and you could have drops in the 10-14% range or more by the time you get to the last device. And the larger the load, the bigger the drop. Checking this with a meter without the load connected and energized will tell you nothing. Voltage drop calcs can be involved, but luckily there's calculators available that make it easy.... I like this one best... http://www.calculator.net/voltage-d...nce=10&distanceunit=feet&eres=50&x=76&y=25 Plug in your wire size, material (copper), voltage (I use 13.8 as that seems to be a common value when the system is charging), DC (not AC), single set of conductors, circuit length, and load amps. Note that this assumes the distance is one-way, so it doubles the wire length. For use here where your ground path is probably through the body/chassis, cut the length in half. If you have a fiberglass car and/or are running ground wires, don't cut it in half. Personally, I wouldn't use anything smaller than #14 in a harness. The cost difference is minimal, it will reduce the number of different connectors you'll need, and the wire is physically stronger compared to 18/16.
I’m planning on using a relay to wire the electric fuel pump in my Coupe. And because I’m anal, I don’t want to stick one of those modern plastic 4 wire relays in my mid 60’s correct build….it would look way outta place next to my glass fuses. Would a good old fashioned horn relay be able to effectively handle the fuel pump load?
Definitely NOT. Horn relays are rated for momentary contact. If you put a continuous load on one it's gonna cook itself to death in about two minutes. Just get a continuous duty relay (headlight relay) and stuff it inside an old OEM relay or voltage regulator housing.
Not what I was hoping for, but good to know so I don't burn it up. I guess I will just have to hide the modern relay.
I don't know what the draw is. It's a Holley red 12-801-1. Their instructions show you can wire it without a relay, but they recommend that you do use one.
Old school electro-mechanical relay, good for 20 amps. Even uses a glass fuse. I use one to power my rad cooling fan. http://ecatalog.smpcorp.com/STD/#/vehicles/parts/num/LR-32?type=s
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