Relays are simple electromagnetic switches. When energized, the electromagnet pulls on a lever to open or close a set of contacts.
Let’s go over some relay terminology:
coil – the electromagnet
contacts – the parts of the switch that touch to complete the circuit through the relay
form a contacts – contacts that are closed when the coil is energized. Also known as “normally open” contacts since they are open when the coil is not energized.
form b contacts – contacts that are closed when the coil is not energized. Also known as “normally closed” contacts.
form c contacts – a set of contacts that has normally open and normally closed positions.
single pole – a relay (or switch) that has only one set of contacts. Usually abbreviated as SP
double pole – a relay (or switch) that has two sets of contacts. Usually abbreviated as DP
single throw – a set of only form a or form b contacts. Usually abbreviated as ST
double throw – a set of form c contacts. Usually abbreviated as DT
wiper – the common terminal for form c contacts.
So a SPST relay would typically have one set of normally open contacts. A SPDT relay (the most common off-the-shelf automotive relay, though you can also find SPST relays) has one set of form c contacts. A DPDT relay will have two sets of form c contacts.
Why use relays?
Relays help relieve the burden on mechanical switches, especially if you are sending power out to several loads in parallel.
Look at the following circuit.
Let’s give the following values for the resistors:
R1 = 2 Ω
R2 = 3 Ω
R3 = 4 Ω
R4 = 6 Ω
The effective resistance of a parallel circuit is equal to the product of the resistances divided by the sum of the resistances.
Reff = 1/((1/R1)+(1/R2) +(1/R3) +(1/R4))
Reff = 1/((1/2)+(1/3) +(1/4) +(1/6))
Reff = 1/(15/12) = 1/1.25 = 0.8 Ω
For a 12 volt source, that means the current running through the switch is 15 amps. This means you have to run heavier gauge wire in the circuit, and there is more thermal stress on the switch.
Note: The parking light circuit in the S30 is one of the worst offenders. You have a lot of lights in parallel in this circuit, and it pulls around 9 amps or more. The wire gauge is also a little on the small side. If you replace the bulbs with bulbs of higher wattage, the circuit draws even more current. If corrosion develops in the sidemarkers between the positive and negative terminals, it can allow more current to flow through the circuit. The end result is that the circuit heats up more than many other circuits. That is part of the reason why this circuit fails so often (and frequently takes out the fusebox).
Relay coils have a higher resistance (83 Ω or so) than most loads in the car. That means they draw less power. You could have several coils in parallel, and they wouldn’t put much of a load on the switch.
In this circuit, the current would be about 0.14 amps.
Unfortunately that would require extensive re-wiring of the car. Even using one or two relays in a circuit to take the load off the switch means that you only have to worry about replacing a $5 relay (and it’s easy to keep spares in the glove compartment) as opposed to a $150 combo switch.
Let’s take a look at a typical automotive SPDT relay.
Terminals 85 and 86 are for the coil. They get wired to the switch circuit. The wiper is terminal 30. The normally open contact is terminal 87, and the normally closed contact is terminal 87a.
When the coil is energized, the wiper is in contact with terminal 87.
If you want one circuit to be hot when the switch is off, you wire your source to the wiper, terminal 30, and you wire the load to the normally closed contact, terminal 87a.
If you only want the circuit to be hot when the switch is on, wire the source to the normally open contact, terminal 87, and wire the load to the wiper. That way, you do not have an exposed 12 VDC source when the coil is denergized.
WARNING: Always make sure you have the proper fusing for any circuit you might add. The fuse should be rated such that the fuse will blow before the wire would melt and insulation burn in the case of a short circuit. Adding circuits without fusing increases the opportunity for an electrical fire in your car.
Looking at the base of the coil, you can see which terminal is which. The terminal numbers are usually embossed upon the base. Also, terminals 85 and 86 are on opposite sides and parallel to each other. Terminal 30 is parallel to 85 and 86. Terminal 87 is perpendicular to 85 and 86, and Terminal 87a is in the middle of the base.
Motorsport Auto sells some nicely crafted relay products for S30s that were designed and assembled by Dave Irwin. These products will keep your combo switch from deteriorating.
I plan on doing a relay project in the near future where I will be wiring the headlight relays to the steering column. I will post pictures on that project after its completion.