Automobile Electric Circuits

Necessity of Electricity in Motor Vehicles The motor vehicles of 19th and early 20th centuries required a handle for starting the engine. It was not easy to start an engine because it required inuch manual labour and time. But, now a days, automobile engines are started by merely pushing a switch placed in front of the seat. It is the electricity which has made it possible. A motor vehicle requires electricity for producing spark at the spark-plug, for head lights during night, for operating radio, tape recorder, fan, petrol gauge, horn etc. A storage battery is used alongwith charging arrangement to meet the electricity requirement of a motor vehicle. The principal circuits used in motor vehicles are as follows: 

1. Starting circuit 
2. Ignition circuit 
3. Lighting circuit 
4. Charging circuit 
5. Instrument circuit 

• Electric petrol gauge circuit 
• Wiper circuit 
• Horn circuit 
• Side indicator 
• Fan, radio, tape recorder circuit.

Starting Circuit The old type of engines were started by a handle, which was rotated many times by hand. Sometimes, the work used to take a lot of time and labour. The rotation of the handle used to produce a spark in the mixture of petrol and air, and as a result petrol gas was formed in the cylinder which operated the piston. But, now a days the driver has to push a button to start the engine and it does not require any labour or time. A d.c. series motor is used for the purpose because it gives more starting torque. Of course, the motor draws a heavy current in the start from the battery which can be of the order of 200 amperes. The field of a d.c. series motor is made of a few turns of thick copper wire, and it is connected in series with the armature. The d.c. supply is supplied to the armature through the brushes and commutator. The motor must not be operated withour Joad because in such condition, it will run at a terrible speed and it will be able to break. Therefore, a system of gears is arranged in such a way that on starting the motor the engine will start. As the engine is started, the motor is switched off and its gear system gets separated from that of the engine. The starting switch may be a foot pedal switch which starts the motor in first push and stops the motor in the second push, or it may be a key operated switch. The following two methods are used for starting the engine:

1. Pedomax system

2. Bendix drive starter switch system.

1. Pedomax System- In this system a foot pedal type push switch is used. The series motor runs only for the time for which the switch is kept pressed. The circuit connections are shown in Fig. 25.1. The switch consists of a plunger, spring and contact points. On pressing the plunger, the contact points touch each other and the series motor operates. The geared fly wheel attached to the series motor will then start the engine. The starting current is nearly 200 amperes in this system, hence sufficiently thick cables are used between the battery and the series motor.

Pedomax Starting System

2. Bendix Drive Starter Switch System- It consists of electromagnet associated with a relay switch, as shown in Fig. 25.2. One end of the coil of electromagnet is earthed while the other is connected to the battery through a push button. A contact leaf is mounted on the plunger of the relay which moves alongwith the plunger. The series motor has two series field coils and four brushes, out of which two are earthed while the rest two are connected to the series field. When the push button is pressed, then the relay is magnetised and it attracts the plunger, so that its contact-leaf touches the two contact terminals. The operation of the relay switch completes the motor circuit and the motor is started. As the pressure is released from the push button, the relay is de-magnetised, the plunger returns back to its original position and the motor is stopped.

Bendix Drive Starter Switch System

Lighting Circuits 

A motor vehicle has the following types of lighting circuits : 

1. Head lamp 
2. Side lamp 
3. Tail lamp 
4. Pannel board lamp 
5. Stop lamp 
6. Door lamp 
7. Internal lamp 
8. Park lamp

1. Head Lamp- A motor vehicle has two front head lamps each having a double filament. One filament is of 30 watts and the other of 5 watts. The reflector of the lamp is capable to radiate the light rays upto a distance of 200-300 metres. Both the lamps are connected to the dimmer and dipper switch; which is a sort of two-way switch. In one position it completes the circuit of 5 watt filaments while in the other that of 30 watt filaments,

2. Side Lamp-These are two small lamps placed near head lamps on their outward sides. The lamps are switched on during night journey so as to indicate the width of the vehicle. They are of 3 to 6 watts having a single filament. 

3. Tail Lamp- This lamp is placed just above the number plate of the vehicle's back. It is covered with a red coloured shade. It is connected in series with the pannel lamp so that the driver may rest assure that the tail lamp of his vehicle is glowing. This lamp indicates the presence of a vehicle on the road to the other vehicles following it; also it enlights the back number plate of the vehicle.

4. Pannel Board Lamp- It is 3 watt lamp which is connected in series with the tail lamp. It enlights the pannel board which houses many types of switches and meters. 

5. Stop Lamp- This lamp is placed on the other side of the tail lamp on the vehicle's back. It is also mounted with a red coloured shade, and it is a 3 watt lamp. It is attached to the brake system. When the brakes are applied, the stop lamp glows, and when the brakes are released, the stop lamp is also switched to OFF. It glows on reducing the speed or stopping the vehicle or on turns. See Fig. 25.4.

6. Door Lamp- The door lamp is mounted on the pannel board but its switches are mounted on each door of the vehicle. All the switches are connected in parallel, whereas each switch is of opposite type i.e., on pressing the switch the circuit is switched off. Now, if any one of the doors is open, then the door lamp will glow and its glow will indicate that any door Is left open. See Fig. 25.5, When all the doors of the vehicles are closed then only the lamp will get off.

7. Internal Lamp- The lamps placed inside the vehicle are called internal lamps. They are of 6 watts. Their switches are mounted on the pannel board. 6 watts lamps are used for dim lighting and 20 watts lamps are used for bright lighting.

8. Park Lamp- Four 3 watts lamps are mounted on the vehicle which are put to ON while parking. Two lamps are mounted in the front side near side lamps and the rest two in the rear side near stop lamps.

Ignition Circuit

Generally, all motor vehicles employ battery for their ignition system. Ignition means firing in the mixture of petrol and air. The ignition is produced through spark- plugs, and it causes an explosion in the petrol-air mixture which generates mechanical power to operate the engine. 12 or 24 volts batteries are used in motor vehicles for ignition and other uses. The system consists of the following parts 

1. Battery 

2. Ignition coil 

3. Distributor 

4. Ignition switch 

5. Spark plug

1. Battery- A 6-volts battery is used for car or jeep while a 12-volts or 24-volts battery is used for bus or truck. The battery used is of lead-acid type.

2. Ignition Coil- A high voltage of the order of 6,000 to 20,000 volts is required for producing the spark. An ignition coil, which is actually a step up transformer, is used for the purpose. Since the battery supplies D.C., therefore, contact-breaker points are used to convert the D.C. into A.C. The ignition coil consists of primary and a secondary winding placed on laminated iron cores. The primary consists of a few turns of thick enamelled copper wire while the secondary consists of a large number o! turns of fine enamelled copper wire. Each layer of the winding is separated by insulating paper, then it is coated with varnish and then dried. One common terminal of the primary and secondary windings is connected to the battery. The other terminal of primary is connected to the contact breaker system, and the other terminal of secondary goes to distributor cap.

3. Distributor- It supplies the high voltage developed by ignition coil to the spark-plugs in rotation. The spark explodes the mixture so as to start the engine. It consists of the following two main parts : 

(1) Contact-breaker

(ii) Distributing cap

(1) Contact-breaker- It makes and breaks the direct current and thus converts in into A.C. It consists of the following : 

(a) Distributing shaft 

(b) Contact breaker points 

(c) Spring 

(d) Condenser 

The distributing shaft has 4 or 6 comers. When it rotates through a motor or gear-system, then the breaker-arm gets disconnected on each corner of the shaft. The operation of the breaker-arm is controlled by a spring. The contact points are mounted at about 3 to 6 mm apart from each other. A capacitor is connected in parallel to these points as  As the engine is started with the self-starter, the distributing shaft or the cam-shaft also starts to rotate with the engine. The helical gears made on the shaft operate the breaker-arm. The breaker- arm géts disconnected on each comer of the shaft

and the C.B. points get opened; while on the flat part of the shaft the arm returns back to its original position and the C.B. points get closed. The repetition of opening and closing of the C.B. points converts the D.C. into A.C., and a high voltage is induced in the secondary winding of the ignition coil (step-up transformer). The capacitor eliminates the sparking at C.B. points while the spring brings back the breaker arm.

(2) Distributing Cap- It is made of bakelite and is mounted on the top of C.B. assembly. It consists of 4 or 6 terminals which are connected to different spark-plugs, as shown in Fig. 25.10. The construction of the cap is  A moving leaf is mounted on the cam-shaft which makes contact with each distributing terminal in rotation and produces spark at each spark-plug at a definite time interval.

4. Ignition Switch- A simple S.P. switch placed in the primary side of the ignition coil is called the ignition switch. It makes on and off the primary circuit.

5. Spark- plug-It is a steel shell mounted on a porcelain insulating cylinder. It has a central electrode and a ground electrode made of nickel alloy, as shown in Fig. 25.11. The ground electrode is bent towards the central electrode, but they both remain at a distance of 0.5 mm to 1.0 mm. The shell is threaded so that it can easily be screwed over the combustion chamber. The central electrode is connected to the high voltage terminal of the ignition coil. A spark is product in the air gap between the central and ground electrode which causes on explosion in the petrol-air mixture.

Working - The assembly of all the above five parts is  As the engine is rotated by the. nself starter and the ignition switch is put on ON, the cam-shaft of the contact-breaker starts to rotate. It produces a make and break in the ignition coil's primary current, which results in the production of a high voltage across the secondary winding. This H.T. current reaches the moving leaf of the cam-shaft and from there it reaches to the spark plugs one by one. Finally, it completes its circuit through the earth.

Charging Circuit 

The battery of a motor vehicle gets discharging in supplying current to the lighting, ignition, starting and other circuits. Therefore, it becomes necessary to charge the battery regularly. The circuit used for battery charging is known as charging circuit. A d.c. shunt wound generator is used for battery charging, because it gives a stable voltage output. When the engine's speed decreases, then the generator will produce a low voltage. In such case, the generator will draw current from the battery, which will increase the field strength of the generator. Now, the generator will produce more voltage. In this way, the voltage generated by the generator will remain stable irrespective of the engine's speed variations. Generally, the following two types of dynamos are used :

1. Dynamo with two brushes.

2. Dynamo with three brushes.

1. Dynamo with two Brushes- A shunt dynamo employs two brushes, out of which one is grounded and the other is used as positive terminal, which charges the battery. A current regulator is connected in series with the battery and the dynamo. The dynamo is coupled mechanically to the engine. The voltage generated by the dynamo will be in accordance with the engine's speed. The voltage is controlled by a voltage regulator. The voltage and current regulators and the cut-out are housed in a separates box. The circuit of all the three electromagnets is shown in Fig. 25.13. The function of the above three units is as follows

• Current Regulator- It consists of an electromagnet mounted with a pair of contact points. If connects and disconnects a resistor in series with the dynamo field, in accordance with the magnitude of current. The electromagnet consists of a few turns of thick enamelled-copper wire wound on a soft iron core. On high speed of the engine, the dynamo will produce more current which will flow towards the battery. This high current will energise the magnet and the contact points will become open, now, the current will flow towards the resistor which will act as a load and will reduce the dynamo current. On low speed of the engine, the dynamo will produce less current which will weaken the electromagnet. The contact points will become close and the dynamo current will again flow towards the battery.

• Voltage Regulator- It also consists of an electromagnet mounted with a pair of contact points. The electromagnet consists of a large number of turns of fine enamelled-copper wire wound on a soft iron core. The winding is connected across the dynamo. The voltage generated by the dynamo varies in accordance with the engine's speed. When the voltage rises above a definite value, the electromagnet energises and the contact points become open. A resistor comes in series with the dynamo field and the voltage is reduced. When the voltage falls below the definite value, the electromagnet de-energises and the contact points become close and again the battery charging is started.

• Cut Out- It consists of two windings wound on a soft iron core. The series winding consists of a fe number of turns of thick enamelled copper wire while the shunt winding consists of a large number of turns of fine enamelled copper wire. The series coil is connected in series with the battery while the shunt coil across the dynamo. On low speed of the engine, the dynamo will produce less voltage and the battery voltage will be greater than the dynamo voltage. The current will flow from the battery to the dynamo. It will reduce the strength of electromagnet because it will act in an opposite direction to that of the voltage present across the shunt coil. The contact points will become open and will not allow the battery voltage to go towards the dynamo. On high speed and thus high voltage, the contact points will be closed again.

2. Dynamo with Three Brushes-This type of dynamo employs three brushes, out of which two are fixed while third is adjustable. The magnitude of voltage and current can be adjusted by changing the position of third brush. It is adjusted before charging the battery. If the battery voltage is lesser than the dynamo voltage, then the battery will store charge, but contrary to it, the battery will start to discharging through the dynamo. In order to avoid such condition, an automatic cut-out is used.

Automatic Cut-out- It consists of two windings wound on a soft iron core. One winding is called the pressure coil which consists of sufficient number of turns of fine enamelled-copper wire, and it is connected across the dynamo. The second winding is called the current coil which consists of a few turns of thick enamelled-copper wire, and it is connected in series with the battery. A pair of contact-breaker points is also connected in series with the battery as  The yoltage regulator hasa 4 terminals labelled as F, I, B and S. F is connected to the alternator field, I to the charge indicator, B to the battery and S to the neutral line. 

Three resistors R, R, and R, of 10, 16 and 50 ohms respectively are connected in the circuit. The neutral line operates the field relay, thus allows the charging current to flow through the contact points. When the alternator voltage falls below the battery voltage, then the alternator field strength will rise and it will automatically compensate the fall in voltage. The charge indicator shows that the charging is in continuation. When the vehicle is at rest, the field circuit is disconnected from the battery by means of an automatic or a manual ignition switch. When the dynamo voltage is higher than the battery voltage, then the current flowing through the pressure coil will magnetise the soft iron bar. The C.B. points will get closed and allow the generated current to charge the battery. In the contrary case, when dynamo voltage falls below a definite value, the direction of current passing through the current coil will be reversed. The reversed current will reduce the strength of electromagnet, thus the C.B. points will get opened and the battery will be disconnected from the dynamo.

Battery Charging System by an Alternator An alternator is smaller in size, lighter in weight and requires lesser maintenance in comparison to a dynamo. It produces sufficient voltage and current even at low speed, therefore alternators are becoming more and more popular now a days. It consists of a single phase field winding on the rotor and a 3-phase armature winding on the stator which is connected in star. When the rotor is rotated by the engine, then a 3-phase e.m.f. is induced in the stator winding. This 3-phase e.m.f. is rectified by a bridge rectifier consisting of silicon rectifiers; and the D.C. is supplied to the voltage regulator, as shown in Fig. 25.15. This system does not require any current regulator and cut-out.

Electric Petrol Gauge An electric petrol gauge indicates the quantity of petrol present in the vehicle's tank. It has F and E indications marked on a scale. F indicates 'full' while E 'empty'. The gauge consists of a pressure coil and a current coil. A variable resistor is connected to the junction of the coils. The resistance is controlled by the float through a gear. A pointer is placed in the field which moves by the joint effect of the two coils and indicates the quantity of petrol, as When the petrol gauge switch is put to ON, then it magnetises the pressure coil. If the tank is full, then the float goes upwards, which will cut most of the resistance out of circuit. Now, the current coil will be magnetised and the pointer will move towards F. If the tank is empty, then the float goes downwards and most of the resistance will be in the current. The magnetisation of current coil will fall below that of pressure coil and the pointer will move towards E.

Electric Petrol Pump The electric petrol pump supplies the petrol to the carburettor in case when the level of the petrol in the tank falls below the carburetter level. It consists of three main parts- body, magnet-assembly and contact-breaker. There are two electromagnets, one on each side of a centrally placed armature, as shown in Fig. 25.17. Both the electromagnets are connected in series. One terminal of the electromagnet coil is joined to one contact-point and the other to the battery through a switch. An armature-rod is mounted on the armature and it rests on two springs fitted on the electromagnets.

The armature-rod has a projected arm for playing on the Ion diaphragm. There are two valves in the pump chamber,

On putting the pump switch to ON, the electromagnet will get magnetised and it will attract the armature-rod. Thus the diaphragm attached to the armature will spread and the pump chamber will be filled with petrol through the inlet valve. At the same time, the C.B. points will get opened and the magnetism of the electromagnet will be finished. The armature rod will now return to its original position due to action of the springs. The armature rod will press the diaphragm and the diaphragm will press the petrol to pass through the outlet into the carburettor. Again the C.B. points will meet and again the electromagnet will be magnetised. The whole action will repeat many times per minute and it will operate the pump. The pump is capable to extract the petrol at the rate of 35 litres per hour from the tank.

Screen Wiper Electric operated screen wipers are used in motor vehicles for the cleaning of glass screen fitted infront of the driver's seat. It consists of a d.c. motor and a gear-box, as shown in Fig. 25.18. The d.c. motor is of shunt or compound wound type. The gear mounted on the motor shaft reduces the motor speed. There are some more gears connected to the shaft-gear which operates the wiper to and fro. The woolen cloth brush mounted on the wiper cleans the glass. One more wiper may also be coupled mechanically to the main wiper so as to clean the glass of the other side also.

Electric Horn Electric operated horns are used for producing a load sound as and when required. They are of following two types: 

1. Magnet type horn 

2. Motor type horn 

1. Magnet Type Horn- It consists of an electromagnet, moving armature, diaphragm, C.B. points and an ádjusting screw. The assembly of the horn is On pushing the horn switch, the current will flow through the electromagnet's coil and will magnetise it. The armature attracts the C.B. points leaf and thus breaks the circuit. The electromagnet will then get de- magnetised and the armature will return to its original position, thereby short circuiting the C.B. points once again. Thus, the circuit will make and break at a frequency of 75 to 125 H, and will cause the diaphragm to vibrate which will produce sound. Magnet type horn assembly

2. Motor Type Horn- This type of horm consists of a 6 or 12 volts d.c. motor whose one terminal is earthed and the other is connected to the battery through the motor field and the commutator. A toothed wheel is mounted on the motor shaft. Another fixed toothed wheel is mounted near the first one and it is attached to a diaphragm, Diaphragm, 

Motor type horn assembly On pushing the horn switch, the motor is started and the toothed wheel mounted on the motor shaft will rotate, and its teeth will strike the teeth of the fixed toothed wheel. Thus, the diaphragm will vibrate and will produce sound. The intensity of the sound produced can be varied in accordance with the motor speed.

Side Indicator Two side indicator lamps one on cach side of the vehicle are used to indicate the direction of taming of the vehicle. They are controlled by a three way switch. The pole point of the switch is connected to the battery and.the two throw points are connected to the left and right side- lamps, while the third throw point is left for OFF. The circuit connections are  Sometimes, iron arrow indicators are also used in association with the side lamps. They are operated by electromagnets and their circuit connections