To add on to what Joe said, there actually is a reason to use a relay.
The starter circuit is an excellent example. That huge battery cable runs over
to a relay and then down to the starter motor. That starter relay (or solenoid
in some cases) is controlled by a little wire that runs up to the ignition
switch. You could actually eliminate the relay and run that huge wire up into
the dashboard, to a massive switch, and down to the starter motor. Pretty
clearly, this wouldn't be a good thing. You'd have huge wires under the
dashboard, bulky switches, lots more weight, and voltage drop problems with the
long wire runs.
Other circuits take advantage of this, just not as dramatically. Things that
take a lot of current benefit from the use of relays. Headlights, fuel pumps,
overdrive solenoids, etc. The benefit varies. I kinda doubt the overdrive
relay gains much. But a headlight relay can give dramatic results in
illumination improvement by reducing voltage drop.
>>> "Gary Hutton" <hutton@evansville.net> 02/16 12:12 PM >>>
Can someone explain to an electrically-challenged owner the actual
function of the horn and overdrive relays in our cars. Are they just
electrical switches instead of mechanical ones? And is there a
procedure for testing them with a multi-meter?
|