29.2.
Lighting Circuit
Statutory regulations specify the number, position and specification of many of the external lights fitted to a vehicle. In addition to the obligatory lights, manufacturers and owners of the vehicle often install other supplementary lights to meet other requirements. Various lamps are grouped in separate circuits, which include the following :
(i) Side and rear lamps including lamps for the number plate, glove compartment and
instrument panel illumination. (ii) Main driving lamps (headlamps) incorporating a dipped facility to prevent approaching drivers being dazzled. (Hi) Rear fog lamp(s) to guard the rear of the vehicle in conditions of poor visibility.
(iv) Auxiliary driving lamps including spot lamps for distance illumination and fog lamps to reduce the reflected glare from fog.
(v) Reversing lamps to illuminate the road when the vehicle is moving backwards and warn other drivers of the movement.
(vi) Brake lights to warn a following driver that the vehicle is slowing down.
(vii) Interior light and courtesy lights on doors.
(viii) Instrument panel lights to signal the correct operation of a unit or the presence of a fault in a particular system.
(ix) Directional indicators and hazard warning lights.
29.2.1.
Circuit Layout
To obtain maximum illumination the lamps are connected in parallel. With the arrangement an open circuit in any branch causes failure only in that one branch, the other lamps still function normally. The vehicle lighting systems normally use an earth return circuit, because this requires less cable length than an insulated return or two wire system. When the vehicle body is used as an earth, a good clean connection must be provided at suitable earthing points on the main body. This earthing lead is specifically essential if the lamp is fitted in a plastics body panel.
Lighting circuit diagrams are presented in either a locational or compact theoretical from. In the former type each component is located relative to its position on the vehicle. This is useful in indicating the location of the various connectors and components, but the diagram becomes more difficult to trace out a particular circuit path. To avoid this problem, some manufactures make use of extra diagrams to indicate separate parts of the circuit. Figure 29.1 illustrates a simple circuit drawn on above concepts.
Fig. 29.1. Lighting circuit (simplified).
The lamps in this parallel circuit are controlled by three switches. Switch 1 operates the side and rear lamps. It also supplies power. Switch 2 operates the headlamps and supplies power.
Switch 3 distributes the current to either the main beam or the dip-beam head lamp bulbs. 29.2.2. Circuit Protection and Extra Features
If a single fuse is installed along with in the external lighting supply cable, then all lights go out when the fuse fails. This is a dangerous situation when the vehicle is travelling at speed along a dark road. To eliminate such a situation manufacturers either fit separate fuses for each
light system, or refrain from fusing the headlamp circuit altogether (Fig. 29.2). The circuit in Fig. 29.2 also incorporates extra features.
Fig. 29.2. Lighing circuit incorporating fuses and relays.
Headlamp Flash Switch.
This switch enables the driver to signal to other drivers during daylight avoiding the use of the main light switches. This is a spring-loaded switch and hence operates only when the lever is held in the ‘on’ position.
Ignition-controlled Headlamps.
As per the regulations the headlamps should not be used when the engine and vehicle are stationary. To accomplish this ignition switch controls the feed to the headlamps. Additionally a relay is installed to minimize the current load on the switch.
Auxiliary Driving Lamps.
These long range auxiliary lamps (spot lamps) are utilized when the headlamps are set to main beam. But these lamps should not be in operation when other vehicles are approach and to achieve this the auxiliary lamps are connected to the main beam branch of the circuit. Since the power consumed by these lamps is considerable, a relay is used to control these lamps.
Front Fog Lamps.
During fog the main headlamps cause glare. This problem is minimized by using low mounted fog lamps. These twin lamps can be used in place of headlamps and hence feed is taken from the side lamp branch of the circuit.
Rear Fog Guard.
These high intensity fog lamps highly illuminate the rear of the vehicle, and hence they must be used only in conditions of poor visibility to guard the rear. To prevent illegal use of these lights the supply is taken from either the dipped beam or the front fog lamps. A warning light must indicate the operation of the rear fog guard lamps.
Lamp Failure indicator.
Many manufacturers also incorporate a warning system to indicate the driver the nonfunctioning of a light. Often the lamp signal indicator on the instrument panel glows either when the lights are operating normally, or as a signal to warn the driver that a light is out. Most graphical display systems provide full illumination for a few seconds after switching-on the ignition. This feature permits checking of the lights in the graphical display and indicates that they are functioning correctly.
In addition to this graphical display unit, a module (sometimes called a bulb outage module) is installed to monitor when a particular section of the circuit does not consume the rated current. When an open-circuit condition happens the module triggers a light on the instrument panel to indicate the driver the lamp which is out. Each branch of the circuit passes through the module for monitoring the system. However, this feature complicates the circuit and increases the length of cable needed for the lighting system. But, it is still in use on the grounds of safety.
Dim-dip Lighting Devices.
Some regulations compel for fitting dim-dip lighting system in vehicle. This system does not permit the vehicle to be driven on side lights alone, because the side lights can be operated only when the ignition is switched off so they may be regarded as parking lights.
Headlamps can be operated in two dip modes. A dim-dip light is of low power, which is used without dazzling other road users in well-lit streets at night or dull weather at twilight. Whereas the dip beams of standard intensity is used for normal night driving in out-of-town areas. As per the regulation one pair of headlamps should incorporate a dim-dip device. Compared with the normal dipped beam, the dim-dip light intensity should be 10% for halogen lamps or 15% for normal filament lamps.
