I’m confused about when to use a relay versus just using a switch when wiring electrical accessories. Isn’t a relay and a switch pretty much the same thing? It seems like a relay just adds an extra level of unnecessary complication to wiring. Why do the car companies use both, and sometime fuses and other times circuit breakers?
You are correct that a relay is basically just a switch, but it’s a switch operated via remote control, so to speak. A regular switch controls electrical current by connecting or breaking the path of either the positive side of a circuit (most common) or the negative side of a circuit. A relay does the same thing, but instead of a physical toggle or lever, the contacts that connect or break the circuit are controlled by a small coil inside the relay that is energized or de-energized by a switch that can be mounted elsewhere (hence the term “remote control”). While a relay does add an extra step to the switching process, the main advantage is that you can leave the actual switching of high-power circuits for things like off-road lights or air compressors in the engine compartment, and control that switching with a much smaller amperage circuit that energizes the relay’s coil via a switch mounted inside the passenger compartment. It’s always good to have high-amperage circuits separated from the passenger compartment by a firewall; should something go wrong with the circuit, there’s a physical barrier between vehicle occupants and the potential fire. Smaller amperage circuits also allow the use of physically smaller switches, so packaging is easier. You will often see circuits carrying less than 20 amps switched directly, while circuits of 30-40 amps can sometimes be directly switched but will often be controlled by relays. Anything above 40 amps is pretty much always on a relay.
Much like switches and relays, fuses and circuit breakers do the same thing, but they go about it in different ways. A fuse contains a heat-sensitive filament inside that is designed to heat up and melt if the current of the circuit exceeds the rating of the fuse. This breaks the path of electricity before damage occurs to the wires or other components of the circuit. A fuse is one-time use and must be replaced. A circuit breaker also uses a heat-sensitive element, but instead of burning up, it trips a spring-loaded mechanism that breaks the path of electricity. Some breakers must be physically reset (much like breakers on your house), but the breakers used most commonly in vehicles will reset automatically after a certain amount of time. If the current fault is still present, it will trip again shortly after reconnecting the current path. You will often see circuit breakers used in important circuits like headlights and wiper motors, but we’ve also seen them used to control ABS servos, window regulators, and door lock circuits. The vehicle’s owner’s manual will typically tell you which circuits are on a fuse and which are on a breaker. Fuses are cheaper to wire and use, but a breaker is sometimes preferable for a troublesome circuit or something that periodically gets wet when it shouldn’t and shorts out.