Somebody asked a while back "What is the purpose of a relay?"
I haven't seen any correct answers yet, so I feel an explanation is in
order.
A relay is simply a switch that is turned on/off by electric current.
Here's a simplified diagram of how a relay is incorporated into a fog
light setup:
+-----------+
| |------( +12V )
| Fog Light | +------------+
| Switch |--------------------|-+ +-|------------( +12V )
| | | | | |
+-----------+ | | Relay / |
| | / | +-----------+
( GND )-----|-+ +-|------------| |
+------------+ | Fog Light |
( GND )----| |
+-----------+
(Please excuse my lack of ASCII art talent.)
When you flick the switch in the cab to turn on your fog lights, electric
current flows from +12V to ground following the path through the switch
and through the relay. (There's a resistor in there somewhere too.)
When current flows through the relay terminals shown on the left in the
above diagram, the terminals on the right are shorted, thereby connecting
the fog lights to +12V. That is all a relay does.
The most compelling reason to use relays like this is because the wire
connecting the switch to +12V, switch to relay, and relay to ground can
be light gauge wire, i.e. thin, light, and cheap. However, the wire
connecting the fog light to its power sources, +12V and GND, must be
heavy guage wire (i.e. fat, expensive) because that is a relatively high
current circuit.
We could run heavy duty wire from +12V, to a switch in the cab, and directly
back to the fog light. However, we don't want to do this because:
1) Every inch of wire has some resistance, which translates into
a voltage drop over long distances. Notice that the path from
+12V to the fog light in the figure above can be relatively short.
2) If we did this for every electrical system in the car/truck that
normally uses relays, we would add significant weight to the
vehicle and probably double the diameter of the wire bundle
that goes through the firewall.
3) Good quality heavy guage wire is expensive!
4) Relays isolate the passengers (and other electrical components
like your stereo) from high current circuits, thereby providing
a certain level of safety and reducing interference.
Hope that clarifies the purpose of a relay. Sorry about the loooong post!
John - 97 CC 5.2L 3.55SG (2-4 weeks and counting...)
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