What is a relay?
                Basically, a relay is a remote control switch in which an electromagnetic ally produced force operates the contacts
            through which the main power current flows.

          Why use a relay?
                Leads are resistance's. Resistance's cause voltage drop, which means that consumers performance is reduced.

                To provide the minimum possible resistance, leads between power sources (i.e.. battery, alternator) and consumers
            (i.e.. headlamps, fog lamps)should be as short as possible and have a large enough wire (gauge).

                When you use a relay, it allows the power current to be routed directly from power source to consumer and the
            cable section can be correspondingly large. The weak control current between the relay and dashboard switch means
            that even small cable sections produce negligible voltage drop.
 
 

           Important:                                             Not Good:                                   The Best Way:
            The importance of optimum operating        Power current must travel from       The power current is routed over the
            current and the negative effect voltage       the battery to the headlamps via      shortest distance between the battery
            drop can have (i.e.. on the luminous           the dashboard switch.                     and the headlamp (via the relay). The
            intensity) of a bulb, can be seen from                                                              current consumption between the relay
            the table below.                                                                                             and the switch is less than 150mA.
                                                                                                                                 This means that almost all the power
                                                                                                                                 reaches the headlamps.

          What different types of relays are there?

           The normally-open relay:
                A normally-open relay is used to close the electrical circuit between the power source and the electrical consumer
            (ie. the lights are turned on). Relays are operated by means of switches, impulse generators or control devices. These
            relays can be used for headlamps, auxiliary driving lamps, auxiliary fog lamps, horns, heaters, fans, window lift motors,
            air conditioning and ignition assemblies, etc.

           The normally-closed relay:
                In practice, the normally-closed function is performed by the change-over relay. A normally-closed relay is used to
            break the circuit between power supply and the electrical consumer (i.e.. the consumer is switched off). One
            application of this type of relay is for starting up to reduce load on the battery. The relay temporarily switches off the
            electrical consumers such as headlamps or heated rear windows.

           The change-over relay:
                The change-over relay changes the current path from one electrical consumer to another. The relay is operated by a
            dashboard switch. These relays can be used for switching over from a townhorn to an airhorn, for 2 speed applications
            such as heated rear windows or fan motors etc. Change-over relays can be used as change-over, normally-open, and
            normally-closed relays.

           Eliminating voltage spikes:
              Some relays are available with resistor's and/or diode's. Voltage spikes from 300v to 500v can occur momentarily
            when a relay is switched off. Sensitive electronic equipment can be damaged or malfunctions can occur if these spikes
            reach the vehicle electrical network without suppression. A relay with a resistor reduces these voltage spikes to less
            than 100v. A relay with a diode eliminates them completely. The correct polarity of the connections is marked on the
            relay.