This is an informational post. This is to explain and teach the basics relay function and how to wire up a relay with either an off trigger or on trigger set up. Of course this information does not mean that you will be a genius when it comes to wiring up things with relays, but will give the basic knowledge of relays and their function.
What is a relay?
A relay is a simple electromagnetic switch that when the coil is energized it causes the switch to either open or close. In Automotive it is used to allow smaller wires at the trigger source, such as headlight switch, and higher amperage wire to carry the load of the work a shorter distance. Relays can be used in either on or off coil states and can power almost anything in a car from a horn to hydraulics.
It is important to understand what you are dealing with and as such you need to learn the basics of automotive electricity and how it works. I will give a brief description of the basics below before I continue on the relay section this is going to be a good refresher for some and new stuff for others.
Battery: Big ugly thing under the hood that holds a charge and has a + and a – printed on it. The battery is the key to electricity to a car; when you do any electronic work on a car please disconnect the battery so you don’t blow fuses or burn wires.
Positive: +, hot, positive are all terms that mean the same thing it is the power side of a circuit.
Ground: -,Ground, cold are also terms that are the return side of the circuit (complicated don’t ask)
Short circuit: When two opposing poles of electricity come in contact ie.. positive and negative touch. This can be bad as it will melt wires at the worst if it continues or blow fuses at the best.
Trigger: It is something that causes a reaction. For example pushing the horn button is pushing a trigger. Triggers can be either + or -.
Welcome back to the relay portion of the show. Here is a basic diagram of a 5 prong relay. I choose the five prong relay because it is a little less common but it can show you a different sort of relay option which is the off trigger relay.
One thing you should first notice is the numbers on the diagram. These numbers are the generic numbers used on almost every relay out there. It is a sort of generic coding if you will.
#30 Power wire
#87 feeds the device
#87a feeds the device
For this example I made the trigger wire that feeds #85 positive, but it can be negative and if you make the trigger negative then the #86 must be a positive.
Here is a basic wiring diagram to illustrate the use of a relay to light up a set of fog lights.
You will see how the #30 comes straight from the battery (it has a fuse in between battery and relay) this will be the power that supplies the physical fog lights.
#85 is to a ground source (remember car metal is ground).
#87 connects the fog lights to the relay.
#86 is fed by the switch, toggle, push button, light sensitive, etc.
With this set up here is what should happen. Switch is clicked power goes to 86 the power of 86 and ground of 85 causes a coil inside to move the internal switch of the relay allowing power to travel from #30 to #87 and the fog lights come on.
Now #87a works in reverse when the switch is flipped off then no power goes 86 hence the coil releases and moves the internal switch to pass power from #30 to 87a and power a device that only works when power is absent.
I hope this helps you understand the basics of relays and the flow of power in a car.
If you have questions please pm me or post here and I will answer plus add to this post to help others in this forum.