SPDT Relay : (
Single
Pole
Double
Throw
Relay) an electromagnetic
switch, consist of a coil (terminals
85 & 86), 1 common terminal (30), 1
normally closed terminal (87a), and
one normally open terminal (87).
In our application:
Relay
87 (Yellow) Power Positive (+)
86 (Black) Ignition Switch
(+)
30 (Blue Wire) Out put to Fan
(+)
85 (White) Ground from
Temperature Sensor (-)
87A Is not being used in our
application.
Fans
Each fan
will get its own relay to eliminate
over heating. From my personal
experience, one relay per fan.
The relays would be wired together
with the exception of Number 30
(Blue Wire). Each blue wire
from the relays should connect to
the fan and other of wire from the
fan will be ground (get negative
from the frame).
When the coil of the relay is at
rest (not energized), the common
terminal (30) and the normally
closed terminal (87a) have
continuity. When the coil is
energized, the common terminal (30)
and the normally open terminal (87)
have continuity.
The diagram above center shows the
relay at rest, with the coil not
energized. The diagram above right
shows the relay with the coil
energized. As you can see the coil
is an electromagnet that causes the
arm that is always connected to the
common (30) to pivot when energized
whereby contact is broken from the
normally closed terminal (87a) and
made with the normally open terminal
(87).
When energizing the coil of a relay,
polarity of the coil does not matter
unless there is a
diode across the coil. If a
diode is not present, you may attach
positive voltage to either terminal
of the coil and negative voltage to
the other, otherwise you must
connect positive to the side of the
coil that the cathode side (side
with stripe) of the diode is
connected and negative to side of
the coil that the anode side of the
diode is connected.
Diodes are most often used
across the coil to provide a path
for current when the current path to
the relay is interrupted (i.e.
switched off, coil no longer
energized). This allows the coil
field to collapse without the
voltage spike that would otherwise
be generated. The diode protects
switch or relay contacts and other
circuits that may be sensitive to
voltage spikes.
Why do I want to use a relay and
do I really need to? Anytime you
want to switch a device which draws
more current than is provided by an
output of a switch or component
you'll need to use a relay. The coil
of an SPDT relay that we most
commonly use draws very little
current (less than 200 milliamps)
and the amount of current that you
can pass through a relay's common,
normally closed, and normally open
contacts will handle up to 30 or 40
amps. This allows you to switch
devices such as headlights, parking
lights, horns, etc., with low
amperage outputs such as those found
on keyless entry and alarm systems,
and other components. In some cases
you may need to switch multiple
things at the same time using one
output. A single output connected to
multiple relays will allow you to
open continuity and/or close
continuity simultaneously on
multiple wires.
The above information is courtesy
of http://www.the12volt.com
