Specifications

31
INSTALLATION TIPS
In this section we will discuss some of the various tools and devices available to
the installer that will aid him/her in interfacing
the alarm to the vehicle.
RELAYS
What is a relay?
-
In simplistic terms, a relay is merely a
mechanical switch
which
is controlled by the magnetic field from an electromagnetic coil. Once you have an
understanding of how a relay works their applications
are only limited by your
imagination.
A central locking control module incorporates relays, as does a power window
control module, and, without getting too involved with all the other components in
those devices, you could quite easily build one of these
using only relays.
Within a relay there are two primary components; an electromagnet and a switch.
The electromagnet, or coil as it’s commonly known, consists of wire wound around
a core material. By connecting power to one end of the coil and ground to
the other,
the electromagnet will become energised, creating an electromagnetic field
surrounding the coil. This will cause the switch to operate and change the contacts
from the normally closed position to the normally open position. When power is
removed
from the coil the magnetic field collapses and the switch will return to the
normally closed position.
Relays are classified by the way their contacts are arranged. A relay, therefore,
may be designated a SPST (Single Pole Single Throw), or a SPDT (Singl
e Pole
Double Throw) or one of many other configurations.
The coil contacts on a common automotive relay are usually numbered as follows:
* Pin 86
-
Positive coil pin
* Pin 85
-
Negative coil pin
* Pin 30
-
Common contact
* Pin 87A
-
Normally closed conta
ct
* Pin 87
-
Normally open contact
Whilst pins 85 and 86 can be either positive or negative it pays to always be in the
habit of putting positive to pin 86 as some relays have an anti
-
spike diode built into
the relay, so be aware of this. These spikes, w
hich occur when the relay is
switched on or off, can play havoc with some electrical components found in
modern vehicles (such as computers), so it’s necessary to suppress these spikes
when working with sensitive parts of the vehicle’s electrical system. T
he diagram on
the relay housing will indicate the presence of a diode. If the relay you are using
doesn‘t have a diode built in, it may pay to solder one across the coil pins on the
outside of the relay.