Quote Originally Posted by kinyo View Post
The "RELAY" is essentially a switching device for loads that require higher current. The switches found on cars are not capable of carrying higher currents for heavy loads such as headlights, horns, starter motor, etc. These low-power switches are simply used to activate relays, which in turn, switches their bigger contacts to carry the bigger loads, with minimal delay.

Inside a relay is basically an electromagnet, than when applied with low current, attracts a metallic member which causes a bigger switch to close and complete a circuit, allowing a bigger current to flow to a load. And so it can be said, the "low curent" applied to the relay coil is multiplied to allow a "higher current" to flow. A typical bosch relay only requires 0.20A on its coil but its contacts are capable of 40A. That would be some 200 times multiplication. The switches on the stalks found on cars for headlights, wipers and horns are only capable of 2A, not enough for the load, but durable enough for the relay coils.

So it can also be said that a relay can be used as a wired remote controlled heavy-duty switch. It makes economical sense to position a relay near the load, to avoid the bigger cost of running long thick wires around.

But take note, those bigger relay contacts are still limited to how much current they can safely and reliably (long-lasting) switch. There are some applications where you would need two or three relays for reliable operation. Forty amperes on a bosh relay means 480 watts of power. Since the starter requires some 800 watts of power, it has its own big-ass relay built-in. In fact, driving the starter relay alone requires another relay so that the keyswitch contacts will not be damaged and operate reliably over the car's lifetime.

Those car audio amplifiers rated hundreds of watts, although wired with thick wires for main power supply, have their relays built-in so that they can be turned on/off by simply using the "remote" terminals with smaller wires.

A little off-topic ... in the extreme end, our power system, delivering megawatts of power, naturally use relays too for remote control. It is not difficult to imagine that those relays, switching up to 500,000 volts, and thousands of amperes would we be sized bigger than a human.



kinyo,
very well done! danke schon!

tholitz,
the 200 zone is the area underneath the windshield within the dashboard after the 100 zone (engine compartment from the radiator grille to the firewall).
jick