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If relay is used as switch, why another switch is used to control it?

EFI Wiring Fundamentals

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Relay is a switch. I control high current circuit by applying very small current. Hope the current it control cannot be controlled by any switch. Isn't it? So, why another switch is needed to control the relay in wiring harness of any unit like headlight?

The reason for the relay, is the dash switch cannot handle the current requirements of the load. So, you use low currents from the switch to the relay trigger, then the relay turns on the high current load.

Now, you can avoid relays by using high-current switches, but you fill find these are often very large, low-current switches are much more common.