Electrical Components

Resistors    
A resistor is an electronic component which has one simple job to do, which is to restrict current flow (cause resistance). The resistance of a resistor is measured in ohms. A resistor has no positive end or negative end and therefore can be wired up either way. It doesn't matter how it is wired up, it will still restrict current flow. But the amount of resistance to current flow is dependent on the resistor itself. There are a range of resistors. They are all similar in size but have different resister ratings. Resistors have colored bands drawn on them. Resistors can be identified by a code using the color and the position of the bands. 
How to identify
The first two or three bands may be the numbers to write down.
Next band is the multiplier
The last band is the tolerance value

Here's the resistor color code chart

E.G: Say your resistor has these bands on it in this order. Brown, Red Orange, Gold.. This will give you a reading of 1,2,1000,5% which means 12,000ohms 5%
The 5% is the tolerance value. It means that the resistance of the resistor can vary plus or minus 5% of the actual value. In this case the tolerance will be 11,400ohms (lowest) and 12,600ohms (highest).  




Diodes    
A diode is an electronic component which conducts and lets current flow through in only one direction. From Anode (positive) to Cathode (negative). 

 

In our practical class we did a few tests on a diode using a multimeter. The first test we did was we measured the diode's resistance in both directions. The result came out infinity (no circuit). This meant that there was no circuit either way of the diode. But why? A diode requires a small amount of voltage (theoretically 0.6V) to turn on, which our meter did not supply (only supplied 0.245V). Hence the diode didn't let current through and the result was an open circuit. We then set the meter on diode test mode and measured the diode in both directions. The readings we got was 0.808V (anode to cathode) and infinity (cathode to anode). These results meant that our diode only lets current through one way which is from positive to negative (anode to cathode). And that it requires 0.808V to turn on. We then did the same tests with another type of diode. A LED (light emitting diode). In the diode test mode the result for anode to cathode was 1.617V. This means that this diode requires 1.617V to turn on. But why? This is because compared with the other diode the LED produces an output as well as allow current flow. That output is light. Hence the LED requires more voltage to start up. These were the two diodes that we tested in class but we were briefly exposed to three other types of diodes. A SCR diode, Photo diode and a Zender diode.

Capacitor  

A capacitor is an electronic component that stores electrical charge. It does this by providing ground when there is an open circuit (switch is open). This storage of electrical charge prevents voltage spikes from happening. A capacitor consists of two metal plates very close together. They are separated by an insulator. When connected to a battery or a power source electrons flow into the negative plate and charge up the capacitor. This charge still remains when the battery or the power source is removed. The amount of charge a capacitor can store depends on the capacitance of the capacitor (measured in Farads F).

In our practical class we wired up a capacitor in a circuit and then charged it. Every 10seconds voltage readings were taken from the capacitor. The capacitor was supplied with 16.81V for 210seconds. The results were then graphed. (see in worksheet). The graph showed us that the capacitor when first started charging gained voltage quite fast and then it gradually reduced its rate of charge. Why? This was due to the potential difference of the capacitor equalizing with the power source. As it equalizes the pressure voltage decreases hence the capacitor starts to slow down its rate of charge.

   

Relays   

A relay is an electronic component that uses a low amperage circuit to switch on a higher amperage circuit. This low amperage circuit is called a control circuit. The control circuit will have a coil of wire that creates a magnetic field around it when the circuit is powered and earthed. The switching circuit (higher amperage circuit) will have a set point of contacts that are switched on and off by having the magnetic field pull (attract) the points over to connect with another set of points. 

The control circuit of the relay usually gets its power from the battery. It will also have a switch that will turn on and off the circuit. This switch can either be on the positive side of the circuit or the negative side of the circuit. The circuit can be switched by either a switch, a sensor with a switch inside it, or an ECU (electronic control unit) that does the switching based on a logic circuit.

The switching circuit (high amp circuit) also gets its power from the battery and this circuit is connected to the component. 

 

 

 

 

Starter Motor

In this section of the course we looked at how the starter motor works and what its meant to do in a car. The only function the starter motor performs in the car is turn over the engine to get it started. That's it. The starter motor we looked at was the Pre-engaged Starter motor.

How does it work
How this works is that the pinion gear is spinning very slowly and engages with the ring gear on the flywheel before it starts to turn over the engine. Hence the name pre-engaged starter motor. This is good because the pinion gear suffers less wear as it is engaging with the ring gear. Behind the pinion gear is a one way clutch. This clutch ensures that the engine in turned over by the starter motor and not the engine turning the starter motor. This unit does this by locking up whenever the pinion gear is spins the other way. The pinion gear and the one way clutch are mounted on the armature shaft. The armature is a group of conductors that are mounted around a iron core in a cylindrical manner and are supported on a central pivot. Attached to this is the commutator. A commutator is a rotating switch that supplied the armature conductors with their own positive and negative. (Brushes supply the commutator with the positive and the negative). This creates a magnetic field around the conductors. Around the commutator attached to the inner body of the starter motor is the field coils. The field coils consist of soft iron pole shoes with heavy conductors wound around them. Current flows through this winding to produce a magnetic field. the pole shoes then intensifies this magnetic field making it stronger. This magnetic field then reacts with the magnetic field produced around the conductors causing the conductors to move. This then creates a motor. But the current through the battery first flows through the solenoid. 

Here's a diagram to help explain how the solenoid works.

  
When the starter switch is off, voltage is supplied to the battery side of the starter switch and the B terminal of the solenoid. When the starter switch in on the current flows through the hold in wind and to the earth creating a magnetic field. At the same time current flows through the pull in winding, the M terminal, then via  starter motor through the field coils, through the brushes, through the armature, through the negative brushes to earth. This produces a strong magnetic field around the pull in winding and turns the starter motor slowly. (this is the time when the pinion gear slowly attaches to the ring gear on the flywheel). The plunger moves up connecting the B terminal to the M terminal. At this point the pull in winding is turned off because of the equal pressure (voltage) at each end of the winding. The heavy current can now flow directly from the battery to B via M to the starter motor. (The starter motor then turns over the engine).

This is how the starter motor works.

Starter Motor Circuit Testing (on car)  
Before we test the circuit the first thing we have to do is to check the OCV of the battery. The OCV of the battery has to be above 12.4V (50%charge). After the OCV is recorded the EFI fuse (electronic fuel injection) is taken out. This is because all the testing is done when the starter motor is cranking the engine. By taking the fuse out we ensure that the engine doesn't start and we can keep cranking the engine and perform the tests. (when the engine turns on the starter motor stops working). The second thing you do is record the cranking voltage of the battery. The is the reading of how much voltage the battery is actually supplying while cranking. The figure should not be less than 9.5V. If is's lower than 9.5V it means that the starter motor is not receiving the voltage it requires. Therefore by receiving less voltage the starter motor will under perform (not work properly). You then need to check the voltage drops on the starter circuit. Three readings are taken.
1st from the battery positive post and solenoid starter stud (while cranking)
2nd accross the solenoid main input and output terminal stud (while cranking)
3rd between the battery negative and starter motor body (while cranking)
The maximum voltage drop allowed is 0.5V. Any higher and the starter motor wont receive the proper amount of voltage it requires to operate. 

We also check the current being drawn by the solenoid. The spec for this is between 120 to 150Amps. It is not a bad thing if the starter motor draws less current, all it means is that it is easier to get the starter motor working. There's nothing wrong with a lower reading. But if the solenoid is drawing too much current it means that it is harder to get the starter motor to work and this means that there is an internal fault in the motor. This internal fault is causing resistance within the motor. This resistance will weaken the magnetic field inside the starter motor and therefore resulting in more current (amps) to be drawn.    

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Alternators

In this section of the course we looked at the different components of the alternator and how they work. An alternator provides electrical charge to electrical components of the car and charges the battery when the car is on. We dismantled the alternator and performed tests on the different components as we went along.

Rotor winding to ground test
This test is done to see if there is a circuit between the rotor shaft and the the slip rings. There should be no circuit between the rotor shaft and the slip rings. The slip rings supply the electro multi pole magnet with a positive and a earth. If there was a circuit between the rotor shaft and the slip rings, it would mean that the rotor winding has shorted and would result in no magnetic field from the rotor. No magnetic field would mean no current is induced and as a result no current is produced or supplied by the alternator. You use a multimeter to perform this test. You set your meter on 2K. Place the black lead on the center of the rotor shaft and the red lead on either one of the slip rings. The meter should read infinity.

Rotor winding internal resistance test
This test is done to see how much internal resistance the rotor winding has. The rotor winding has to resist current to produce a magnetic field and thus the resistance has to be within the spec of 2-6ohms. If the internal resistance is lower than the spec (which can be caused by an internal short circuit), the magnetic field produced by the rotor will be weaker and as a result less current will be induced by the stator windings. This will mean that the output of the alternator is low. If the internal resistance is higher than spec (which can be caused by corrosion), the magnetic field produced by the rotor will also be weaker. This is because the winding will be restricting too much current. This will also result in lower output from the alternator. This test is done by setting your meter on 200ohms. You then place one end of each lead on the slip rings to obtain a reading. (Note: the internal resistance of the meter must be subtracted out of the reading).

Testing Stator winding resistance
This test is done to see how much resistance the stator winding has. The winding resistance should be no more than 0.2ohms. If the winding has a resistance higher than that, it would then mean that current induced buy the winding will be lower as  has a higher resistance to current flow. This will result in less current being induced and therefore the alternator's output will be lower. To do this test set the meter on 200ohms. Connect the black lead to the common terminal (one with the most wires) and the red lead to the other terminals one after another. Record each different result.

Testing Stator winding to ground test
This test is done too see if there is a circuit between the stator windings and ground. There should be no circuit between the two.  If there is a circuit it would mean that the winding has shorted to ground. This is not good as it would then result in no current withheld by the stator. Resulting in no output created by the alternator. This test is done by setting your meter on 2K. Then placing the red lead on the common terminal and placing the black lead of the meter on the body of the alternator. 

Testing the rectifier positive diodes
This test is done to see how much voltage is being used up (voltage drop) by the diodes, and to ensure that the diodes are letting the current through the right way. A rectifier converts AC in DC, so it is important that the diodes pass this test. To test the voltage drop you put the black lead on the B terminal and then touch the positive lead on each of the P terminals and record the readings. The meter has to be set on diode test mode. The resistance of the diodes should be between 0.5VD to 0.7VD. If the VD of a diode is higher, it will mean that the diode will be using up more voltage and this will result in a less affective rectifier. A higher voltage drop means less voltage can get through resulting in an loss of output from the alternator. There is nothing wrong if the VD reading is less than the spec. All it means is that the diode needs less voltage to operate. To test which way the current flows through a diode is done by placing the red lead on the B terminal and then the black lead on each of the P terminals. The result should be infinity. This result means that there is no circuit through the opposite way of the diode. This is good as it means that the diode will let the current go through the right way.

Testing the voltage regulator
The voltage regulator controls the alternators output by increasing and decreasing the strength of the rotors magnetic field. It regulates the voltage. Different regulators have different specified voltage spec. This test is done to see if the voltage regulator (when performing) regulates the correct voltage. If the voltage reading is lower than the spec, it will mean that the magnetic field provided by the rotor is weaker. Thus less current is induced by the stator windings and therefore results in lower output by the alternator. If the voltage reading is higher than the spec, it will mean that the voltage regulator is allowing more voltage and therefore is overcharging the battery. This can cause the hydrogen levels in the battery to rise which potentially be dangerous for the car and its occupants.

Checking the brush protrusion lenght
It is important to check the length of the brushes as they supply electricity to the slip rings and if they are too short the brush springs can't apply enough pressure to maintain constant contact. This causes excess sparking that damages the slip rings and reduces the output of the alternator. The minimum length of a brush should be no less than 4.0mm.


On car testing
The on car testing of the alternator is simple. It is done in stages. The first thing we do is we perform a visual inspection of the alternator, drive belt and tension, leads. What we are looking for here is to see that all the components are in good order. The alternator should be mounter firmly (no movement). The leads should be attached properly and firmly (to prevent them from shorting out or grounding anywhere). The belts should have no cracks on it (cracks are sigh of wear and belts with cracks can snap). The belts should be of the right tension (should only flex 5mm). After you perform the visual check you record the OCV of the battery (should be higher than 12.4V).
After this all the tests performed should be done when the car is running and at 1500 to 2000 rev's. The reason for doing this is that the alternator is actually supplying current to the engine electrical components and charging the battery like it would if the car was running.

No Load Test
The no load test is done to check how much amperes is produced by the alternator when all the accessories are turned off in the car (no load). All the ampere drawn out of the alternator during no load is supplied to components such as your ignition system (spark plugs), your EFI (electronic fuel injection), your ecu (on board car computer) etc. In a fuel injected car the no load amp output should be between 10 - 18amps. Any more can mean that you have a fault in one of your electronic component, or that there is a wire that in causing resistance. Any less can mean that your alternator is not working as well as it should be. Reasons can be faulty alternator, The rotor winding can have a internal short circuit causing the magnetic field to be weaker. The brushes inside the alternator can be of a low length (lower than 4.0mm) therefore causing spark and reducing alternator's output.
An alternator also bneeds to charge the battery. So during the no load test the regulating voltage of the battery needs to be between 13.5V - 14.8V. Any lower and that means that the alternator is not supplying enough charge to the battery, resulting in the battery to take longer to charge or never be fully charged. This can be caused by corrosion on the terminals causing resistance and therefore a voltage drop. It can also be caused by the voltage regulator of the alternator (not regulating enough voltage to charge the battery). If the regulating voltage is higher than the spec, it means that the voltage regulator on the alternator is regulating too much voltage. This is dangerous because it will end up overcharging the battery and increasing the hydrogen levels inside the battery (danger to the car and its occupants).

Load Test
Before you do a load test you first need to turn off the engine, turn on all accessories other than the radio and the wipers and then record the current being drawn from the battery. This is a good indication of how much amps the alternator needs to be supplying the engine with. You then turn on your engine (engine speed 1500 - 2000 revs) with your accessories on, take down your amp output from the alternator. (Note: this reading will be higher than the reading taken from the battery as current will be drawn to supply your basic engine functions such as your ignition and EFI as well). The current draw should roughly be the sum of the the current draw recorded from the battery and the no load current draw recorded from the alternator. If the current draw is lower than that it means that the alternator is not able to supply all the current needed and therefore the battery has to compensate the alternator by supplying amps. This is not good for the battery as it means that it still has to supply charge even with the alternator on. This in the long run can reduce the battery life as the battery is in constant use and is not being able to get charged under load. The charging voltage under load should be your OCV + 0.5V (E.G: if your OCV is 12.5V then your charging voltage under load should be 13V). This is just an indication of what the figure needs to be and can be more. If the charging voltage reading under load is lower than OCV of the battery, it will mean that the battery will not be fully charged. This will lower the battery's life as it never reaches full charge. The low figure will also mean that the battery has to compensate for some of the voltage needed, and as a result you will end up with a drained battery.

Voltage Drop Test
This test shows you the amount of voltage being lost from the alternator to the battery. To obtain a valid voltage drop reading a reasonable amount of load needs to be applied to the circuit. (Voltage drop in a circuit is the highest when the circuit is under load). You check the voltage drop between the battery positive post and the alternator output (B terminal), and the voltage drop between the battery negative post and the alternator body. (The engine is still on with a speed of 1500 to 2000 rev's). The voltage drop on each side of the circuit should be no more than 0.2V. If the reading is higher than that would mean that your battery is getting less voltage supplied to it. Thus reducing its regulating voltage. This drop in voltage can be caused by corrosion on the battery terminals. It can be caused by faulty wiring (high resistance in the wire). It can be caused by black marks caused by spikes on the battery terminal and it can also be caused by dirt building up between the terminals and the wires.

Those were the tests performed on the alternator by me both on and off the car.        
             

     

 

Automotive Batteries

In this section of the course we learned about the automotive batteries. What they are, How they work and how to check them for faults.
There are two types of batteries:
Primary cells - such as a torch battery once used throw away.
Secondary cells - such as a car battery (can be recharged but do wear out over time).

The purpose of an automotive battery is to convert chemical energy into electrical energy. A battery is an important component of the car. It supplies electrical energy to the components in the car when the car is turned off. When the engine is starting the battery provides current to the starter motor, alternator, ignition (spark plugs), fuel systems of the car etc. So without a battery a car would never work. But batteries do need to be maintained.

A battery has to be regularly inspected visually. To make sure that there is no corrosion building up on the terminals and to ensure that the terminals are attached firmly. Corrosion can build up around the battery terminals due to oxidation. This corrosion can restrict the contact surface of the wires to the battery terminals, which can result into a small voltage drop. This is bad because the components of the car wont get the full amount of voltage that they require to operate. This corrosion can then be cleaned of using a solution mixture of baking soda and warm water. This solution will neutralize the acid and allow it to be cleaned off. Loosely attached terminals can also cause voltage drop as the surface contact area to the battery is reduced. Therefore the terminals should always be attached firmly to the battery. Another thing to check for is battery swelling. Battery swelling is caused by overcharging. When a battery is being overcharged it produces hydrogen which can be explosive.
Another check that can be done is to check the electrolyte levels over each cell of the battery. In theory the electrolyte levels in a battery should only be about 1 to 2mm above the cells. Its easy to perform this check. All you have to do is remove the battery cell covers (caps on top of the battery) and check the levels visually. Low electrolyte levels can mean that the battery is leaking. If no evidence of the leaking is found it can indicate a high charging rate, or faulty cells.

Practical Class
We performed three tests on the battery of our own cars to check and see if they were in good working order. These tests were Checking electrolyte levels (explained above), Testing electrolyte specific gravity and a High rate discharge test.

Testing electrolyte specific gravity
This test shows us the strength of the acid in each individual cells of the battery. Electrolyte specific gravity test should be performed on a battery before it is topped up. If the test is done after it has been topped up the specific gravity off the acid will be low. This is because by topping it up you will be diluting the acid which means the acid is now weaker, hence the specific gravity is now lower. This test is done using a tool called the hydrometer. A hydrometer is a open ended test tube with a rubber bulb at one end and a point at the other end. It also has a float with readings inside it. To perform this test you undo the cell cap, introduce the hydrometer into the electrolyte and slowly squeeze the rubber bulb. Release the bulb slowly and watch the electrolyte rise up in the tube until the float is floating. (Note: if you release the bulb too quickly then you will cause air bubbles into the electrolyte in the tube. These air bubbles will then cause you to get a false reading of the float). You now check the color of the acid (it should be clear, if its murky then that means that the battery plates are disintegrating. 'Signs of wear') and then take the reading of the float at the top of the electrolyte level. Record your results. You then continue this processes with each individual cell. The variation between the specific gravity readings should not be more then 50 points. If its more then that means that your battery has a fault or is worn out. Here's what the hydrometer readings mean 1265 = 100% charge ... 1250 = 95% charge ... 1230 = 75% charge ... 1200 = 50% charge and 1175 = 25% charge.. If the readings for your battery are lower than the spec here or below 25% charge, then that means that your battery is flat and needs to be charged. The best way to charge a battery is to slow charge it. Using 4amps for about 10 to 12hours and then letting the battery sit for 24hours. When you let the battery sit, it lets the acid settle down and rise up to its state (charge level). Perform the test again. If your readings are higher than the spec, that means that the battery is carrying extra charge (surface charge) or that your battery acid strength is stronger. This is not a bad thing.

High rate discharge test
The high rate discharge test is a load test which shows you your battery's ability to supply cranking voltage. The battery has to be at least 50% charged, any lower will mean that the readings you'll get will be unreliable. If battery charge is lower than 50%, then the battery should be recharged (as explained above). The load applied to the battery is going to be half the battery's CCA. (E.G: if the CCa is 400 then the load applied is going to be 200Amps). The load should be applied for 10secounds and the battery must hold a voltage above 9.5V when the load is being applied. The load is applied using a load tester. Before you connect up the load tester make sure that load controller is off. If its not off and you connect it to the battery, it could cause a spark. Which can result in the battery exploding. After that you connect the positive lead to the positive terminal of the battery and then the negative lead to the negative terminal of the battery. Apply the specified load by turning the load control knob. Wait for the specified time and take the voltage held and load current readings. Turn of the load tester and disconnect the load tester's leads in the reverse order (negative first then the positive). If the voltage held reading was above 9.5V it means that the battery is in good working order. But if the reading was below 9.5V. That would mean that the battery is not capable of supplying the voltage required. This is bad in your car as it means that the electronic components in the car wont get the voltage they need to operate properly. It is advisable then to discard the battery.

               

Electricity Circuits

In this section of the course we looked at simple electrical circuits that would be found in the automotive electrical industry. Within this topic there were a few new terms and tests to do. These tests were done on four different types of electrical circuits: Individual, Series, Parallel and compound circuits. The results were then analyzed and compared with eachother to get an unerstanding of how each circuit works. A few terms to remember: Voltage, Ampere, Ohms, Wattage, Voltage drop, Available voltage.

What do these terms mean?
Voltage: Voltage is the force or pressure that is required to move the electrons (current) in a circuit.
Ampere (Amps): Ampere is the measure of current. The more current in a circuit the larger the reading and vise
verse.
Ohms: Ohms is the unit of resistance. This is the resistance to the flow of electrons (current).
Wattage: This is a measure of the power that is being used by a consumer to produce an output.
Voltage drop: This is a reading that shows how much voltage is being used up (consumed) by a component in the circuit.
Available Voltage: This is a reading that shows how much voltage is available to use at different points in the circuit.

Individual Circuits   
An individual circuit is a very simple circuit. It has only one consumer. Only one path for the electricity to flow through. This circuit was used to understand simple laws of electricity. The power supply was set to 12.73V. One law of electricity sates that the voltage supplied must be used within the circuit. In this case the voltage drop over the bulb was 12.71V. This means that the bulb was using this voltage to produce an output (work done), and in this case the output was light. The remaining voltage (0.02V) is being used up by the wires in the circuit to push the current through. This is caused by the internal resistance of the wires. In this circuit most of the resistance was caused by the bulb we used. This then affected the amperes (current flow) in the circuit (0.36A). By using a bigger bulb like we did in the following circuit the ampere reading changed to 0.77A. the reason being is the bigger bulb offers less resistance as it allows more current to flow through it. Since its the only consumer and a majority of the resistance holder, the overall resistance of the circuit is reduced. Therefore the current flow increases.

Series Circuits       
A series circuit is the same a individual circuit except it has more than one consumer. Both the consumers are wired one after another. Meaning there is still only one path for the electricity to flow through. This creates a flaw because if one consumer doesn't work or shorts out then the entire circuit wont work as the circuit is now open and electricity doesn't have a clear path to flow. There are two rules we follow when it comes to series circuits. The first rule is that the overall resistance of the circuit is the sum of each individual resistance the circuit has. E.G: If a series circuit has two light bulbs each with the resistance of 5ohms. The overall resistance of the circuit will be 10ohms (5ohms + 5ohms). The second rule is that the voltage is shared equally depending on the resistance of each individual component in the circuit. E.G: If there are two bulbs both with the resistance of 5ohms, the voltage used by each bulb will be 6V (12V supply). But if there are two bulbs with different resistance 5ohms and 7ohms the voltage used by each bulb will be different. (5ohms uses 4V and the 7ohms uses 8V). The ampere reading (current flow) in a series circuit will be lower to that of the individual circuit. This is because the overall resistance in a series circuit will be higher than the overall resistance of the individual circuit as the series circuit has more consumers (more load on the circuit). The more number of consumers in a series circuit the lower the current flow (ampere reading) as each extra consumer adds to the total resistance of the circuit. 

Parallel Circuits     
In a parallel circuit each consumer has its own power supply (positive) and its own earth (negative). This is good as it means that the circuit will still work even though one consumer shorts out. This is because electricity has more than one path to flow through and if one path is closed (short circuit) electricity will just flow through the other components, as there still is a circuit. There are two rules we follow when it comes to parallel circuits. The first rule is that the voltage over each individual consumer (each link of the circuit) is the same as the voltage supplied. E.G: If three bulbs are in a parallel with a power supply of 12V, the voltage drop over each bulb will be 12V. This is because each bulb will have its own power (positive) and earth (negative). The second rule is that the overall resistance in a parallel circuit is lower than the lowest resistance. E.G: If there were three light bulbs in parallel with different resistance (5ohms, 3ohms and 9ohms). The overall resistance of that circuit will be lower than 3ohms. Lower resistance means more current flow. That's why the current flow  in a parallel circuit is higher than the current flow in the series circuit. The overall current flow is higher but the current flow over each individual component is determined by its resistance. Electricity is lazy and will always try and take the easiest path. That's why you'll find that the current flow through a bigger bulb will be greater than the current flow through a smaller bulb. The bigger bulb offers less resistance than the smaller bulb and therefore more current flows through the bigger bulb. The more number of consumers in the circuit the greater the overall current flow, as the overall resistance of the circuit decreases. 

Compound Circuits    
A compound circuit is made up of part series circuit and part parallel circuit. Both the rules from the series circuits and the parallel circuits apply here. The parallel part of the compound circuit acts as one unit or one consumer. Since its in parallel the overall resistance of that part of the circuit will be low. Most of the resistance of the circuit will come from the components that are wired up in series (resistance = sum of all individual resistance). Since the parallel side of the circuit has lower resistance, the voltage drop (volts used) over that part of the circuit will be low and voltage drop over the components in series will be greater. The current flow in a compound circuit will be higher compared to the current flow in a series circuit. But the current flow will be lower compared to the parallel circuit. This is because the parallel side of the circuit acts as one consumer with a low resistance to flow. This affects the overall resistance of the circuit as it will be lower than a series but still higher than a parallel circuit.