What Circuits For Relay?

A relay is just an electrically controlled switch. It's like a light switch in your house but instead of you flipping the switch with your hand, you use electrical current to flip the switch.

It doesn't matter if what you're working with is high current or not. If you didn't have relays, you'd have like 30 switches on your dash that you'd need to remember to turn on and off all the time. They are used when you want something to turn on automatically when you turn something else on.

With the headlight relay kits, a relay is used because without a relay, you could potentially only have 10v getting to your headlights with all the losses in the bulkhead connector, splices, headlight switch etc. So instead of connecting that wire with only 10v to your headlights, you connect it to a relay on what I call the "trigger" side. Which is basically what flips the switch in the relay. It's no different than using your hand to flip a switch in your house.

The other side of the relay is wired to the battery so the relay is getting a full 12v from the battery, but it's not allowing that 12v to pass through the relay and on to the headlights until your 10v from your headlight switch gets applied to the relay. Once that 10v is applied, it flips the switch in the relay and allows the 12v to pass from the battery, through the relay and on to the headlights.

If you didn't have the headlight relay but still wanted a full 12v to your headlights, you'd have to walk to the front of the car and flip a switch under the hood or in the grille. That would be annoying, so a relay is used. In this case, the relay is used so you don't have a separate switch but it's also being used to give the headlights a full 12v. In most cases I'd say relays are used to avoid having to flip multiple switches. The voltage drop fix is more of a two birds with one stone kind of a deal.