.FLYINGHEAD AUTO ESSENTIALS
.TITLE Introducing the relay
.AUTHOR Stephen Amontis
.SUMMARY In an effort to expand our coverage of the unplugged world of the automobile, author Stephen Amontis gives an introduction to world of the 12v relay. In this example article, he details how to wire a set of fog lights to turn on and off with your headlights.
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.H1 About this article
One of the areas we’ve been rather light in here at Computing Unplugged Magazine is really one of the cores of the unplugged world, the automobile. In this article, author Stephen Amontis introduces us to the automobile’s nervous system with an explanation of how a wiring relay operates. In the future, we hope to bring you more coverage of the unplugged world of the automobile.
Now, take it away Stephen…
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Ok, here’s a basic explanation of how a relay works in a 12 volt environment. The example I’ll use here is wiring up a set of fog lights so that they turn on automatically when you turn on your headlights.
.H1 Making connections and safety
Let me say first of all that if you’re uncomfortable with wiring your car, or if you aren’t sure what you’re doing, please seek the help of an expert. Improper wiring can lead to damage to electronic equipment, damage to your car’s electrical system, or even fires!
.CALLOUT Have you noticed I’ve mentioned fires four times now?
It’s very important to make sure you make good sound connections, I recommend soldering all electrical connections. The twist and tape method or the use crimp on connectors will eventually lead to problems. It’s also VERY important that you insulate all connections.
Improperly insulated connections can cause severe electrical problems and even fires. I suggest using heat shrink tubing for all connections. In cases where you can’t use heat shrink tubing and are forced to use electrical tape, please use good quality tape.
Make sure you use fuses for all electrical connections. They can prevent damage to your electrical system, damage to your equipment equipment, and they can also prevent fires.
Fuses should always be installed as close as possible to the power source. If you’re going to be adding a lot of accessories to your car, it might be a good idea to add a second fuse box. Tapping power from an existing fuse box is a bad idea. This also gives you an easy place to tap power without running multiple wires through the firewall.
.BREAK_EMAIL Click here to read the rest of the article if you want to find out more about relays and how to wire them.
When running wires through the engine compartment make sure the wires are well secured, and cover them with split loom. It’s available from almost any car stereo store. Any wires being run directly from the battery MUST BE FUSED AT THE BATTERY! Make sure you use a fuse holder that is waterproof and meant for outdoor use.
.H1 Relay essentials
The purpose of a relay is to use a low amperage circuit to trigger a device on a high amperage circuit. For example, if you want your headlight switch to turn on a set of fog lights, hooking the fog lights directly to the headlight wire would cause you to exceed the maximum amperage rating of your headlight switch and headlight circuit. The end result would be a burned out switch, melted wires, or possibly an electrical fire.
Have you noticed I’ve mentioned fires four times now? Use caution.
The average headlight circuit is about 20 amps; your fog lights will probably draw something close to 30 amps. If you connect your new fog lamps to your headlight circuit you’re now trying to draw 50 amps from a 20-amp circuit. THIS IS BAD!
The relay fixes this problem as it has a low amperage side (the trigger side) and a high amperage side (usually drawing power from the battery). The trigger side uses the headlight power as a trigger. When you turn your headlights on, the relay only draws 0.05 amps to trigger a small electro-magnetic switch. The switch closes a contact that now sends power to your fog lights from the high amperage side of your relay, in this example, your battery.
Take a look at the diagram in Figure A.
.FIGPAIR A The wiring diagram of a relay.
Pins 85 and 86 are the low amperage trigger wires. Sending (+) power to 85 and (-) power to 86 will trigger the relay. This will cause the high amperage contact to move from contact 87a to 87. High amperage power is now flowing through your relay from pin 30 to pin 87. Once you remove power from the trigger, the contact returns to pin 87a. I’ll talk more about the importance of pin 87a later.
.H1 Wiring your fog lights.
So in our fog light scenario, pin 86 would be grounded. Pin 85 would get connected to one of your (+) headlight low beam wires. Pin 30 would get connected to the battery. Pin 87 would get connected to the (+) side of your new fog lights.
When you turn on your headlights, pin 85 would receive (+) power and trigger the switch, sending power from your battery to your fog lights. Now your fog lights turn on and off automatically with your headlights, and because you connected them to the low beam wire, they’ll now turn off automatically when you turn on your high beams!
.H1 Being creative
You can take this simple example and apply it to anything. You already know that to trigger the relay you need to apply a ground and positive power to pins 85 and 86. In our fog light example we used a positive trigger (the headlights) to turn on our fog lights. But what if you want to turn on something using a negative trigger?
For example, what if you want add more interior lights to your vehicle? Most vehicles use a negative trigger (-) to turn on the dome light. When you open your door there is a small pin switch that grounds a wire to the chassis of your car and turns on the light. Here’s how you’d add lights using a negative trigger.
Find a good source for constant power (power that does not turn off when you turn off your key). The best source is the battery or a secondary fuse box that you’ve installed. Hook the constant power wire to pins 30 and 85 on your relay. Now hook the (-) side of your new lights to a good ground.
If you’re grounding to a painted surface, grind off the paint first so you get a good ground. Run a wire from your factory pin switch to pin 86 on the relay and BLAM! Your new lights will now turn on when you open your door!
Ok, let’s look at what we just did. Pins 85 and 30 were both hooked to constant power. When we open the door, it sends (-) power to pin 86 on the relay. The (-) power now trips the low amperage side of the relay and sends your constant power to your new lights, turning them on.
.H1 Pin 87a
Pin 87a is sometime called the "at rest" pin. When the relay is NOT triggered, power from pin 30 goes to pin 87a. This can actually be used to do some very creative things. Here’s a really cool example. Most newer cars have a buzzer that’ll sound if you leave your headlights on after removing the key from the ignition. Many older cars don’t have this feature.
This example will use pin 87a to create a headlight-warning buzzer. The first thing you’ll need is a small 12v buzzer. You should be able to pick one up at Radio Shack.
First, mount the buzzer under the dash and ground the (-) wire from the buzzer. Now connect the (+) wire from the buzzer to pin 87a. Use an electrical tester to find the parking light (+) wire coming from your headlight switch. Connect that wire to pin 30 on the relay. In this case, the parking light wire can power our buzzer because it draws so little power.
Now ground pin 86 on the relay and connect pin 85 to an ignition wire. This is a wire that has power ONLY when the ignition is on. Now, if you have the car turned off and the lights on, the buzzer will sound!
Let’s look at how this works. Pin 86 is grounded and pin 85 is connected to ignition power. When the car is turned on, the relay will be triggered and the contact will be sending parking light power to pin 87. In this example, no wires are connected to pin87 so our buzzer won’t sound.
When you turn the ignition off, the relay is no longer triggered so the contact flips to pin 87a. If the parking lights are on, the power from the parking lights will power the buzzer and you’ll be reminded to turn off your lights!
You could even take this example one step further and hook the (-) side of the buzzer to the (-) pin switch on your door. This would cause the buzzer to sound only if:
Your ignition switch was off, and
Your lights were on, and
Your door was open!
Cool huh?
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.H1 Product availability and resources
For more information on Radio Shack and the products they carry, visit http://www.radioshack.com.
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