I have a kind of dumb question. I'm no stranger to simple electronics, but I'm relatively inexperienced with automotive electronics, and I'm trying to figure out the proper usage of a relay. My understanding is that a relay is used when the current draw of the load is more than the switch that controls the load would be rated to handle. But what's confusing me is, I see switches all over the Internet that are rated to handle 15-20 amps at 12 volts, which is plenty for many applications. So why not just run the hot wire directly through the switch and be done with? And then I look under the hood of my tractors, and there's relays all over the place. So there must be something I'm missing. There must be some reason to use a relay other than that the switch isn't rated to handle the current draw of the load.
The example I'm pondering right now is, I'm thinking about putting a light set on my ROPS. I was thinking, if I wired a relay into the hot wire running to the headlights, then I could control the ROPS-mounted lights with the headlight switch. But wait. Why not just splice into the headlight wire and run my lights directly off that? Assuming, that is, that the headlight wire could handle the current draw.
Real life example - I have a '96 Dodge Dakota. From the factory, power to the headlights goes through the switch in the passenger compartment, no relay involved. It's a common upgrade to add a relay and a short power wire, with fuse, from the battery to the headlights. You still use the same light bulb. There's less of a voltage drop in this arrangement so the bulb gets more power. I haven't done it yet (it's on the list), but it's supposed to be a big improvement in the amount of light the bulbs put out. Could you do this without a relay? Yes, but it would involve using bigger gauge wire, even bigger than the short power wire you'd use in the relay arrangement, from the battery to the switch to the lights.
Also, some switches are not easily upgraded to handle the current needed by the devices they control (ie, ignition switch).
Keith