Self Reflection

During the 4 week electrical course I think I have come to have a sound knowledge of the electrical systems used in the automotive trade. I feel that I have been more interested in the practical part of the course as it helps me understand the theory side a lot better. My workbooks from the past 4 weeks have shown that I understand the concepts and theories behind Charging Alternator systems, Charging Battery systems, Starting systems (starter motors) and Electrical Components (relays, diodes, resistors, capacitors).
With knowledge taken from my practical classes and Dad (who is an Auto Electrician) I have be able to develop in-depth blogs that explore and discuss the backgrounds of the Electrical systems.

From my timetable you can see that a lot of my time was taken up by my job, however I didn't let myself fall behind too much with my self directed study which I made time for when I could. though it may not show on the timetable but during my breaks at work I usually spent time on reading over notes and workbooks. The task where we got to make our own logic probes was by far the highlight of the course, as it let us use the knowledge we had learnt to apply in a practical construction of a working circuit.

Admittedly my theory test results could of been better but thats just something that you can't help past the fact. I found that my problem solving and practical ability were much better than I originally expected them to be. Though my Dad is an Auto Electrician I've never really taken an interest until now.

Overall I have enjoyed myself during this course and have taken a lot of practical knowledge and understanding away from it.  

Capacitors

the capacitor is an electrical device which has the capacity to hold and store an electrical charge. it is made of metallised plates layered with an insulating substance, rolled up tightly, and fitted inside a metal cylinder. A terminal at one end of the capacitor is connected to the moving contact, and the metal cylinder, acting as the other terminal of the capacitor, is connected to earth. The capacitor holds an electrical charge and releases its stored energy when connected in a circuit. The capacitor is connected across contacts of the contact breaker, and provides an alternative path for the current and accepts and stores and electrical charge from the primary coil until conditions are suitable for releasing it back into the circuit. This stored energy would otherwise cause destructive arcing across the contacts. The current thus ceases to flow, and the capacitor immediately discharges itself through the primary coil in the opposite direction to the flow of induced current. This reverses the polarity of the coil and increases the rate at which the field collapses. this in turn increases the voltage induced in the secondary winding. A capacitor takes time to charge, they charge faster when first connected to a power source and start to slow down as their charge builds up. The size of a capacitors capacitance is measured in Farads (F).

To calculate how fast a capacitor should charge you use the formula
                                           Resistance X Capacitance X 5 = Time required to charge
                                                       (R x C x 5= T)

In one of our practical lessons we had to build a circuit with
                                  1 x resistor
                                  1 x capacitor
                                  1 x bridging wire
                                  1 x voltage source (battery or power supply)

After building the circuit we had to time the capacitor to see how long high it would charge over the course of 3minutes (180secs). Recording the charge every 10seconds on a table (which can be found in my 'capacitor circuits workbook') We then had to graph the results. The results showed that during the first 90seconds the charging was going quite fast, shooting up from a starting voltage of 3.20v to 10.8v, the second half (90seconds) the charging slowed down because the electrical store was getting taken up. Kind of like how when you blow up a balloon and if you blow it up too much it gets harder to blow up, harder to store air. This made the second 90seconds only rise 0.8v as opposed to the 7v in the first 90 seconds. This graph can also be found in my capacitor circuits workbook.  

Relays

A relay is n electromagnetically-operated remote-control switch in which a small current operates a switch which controls a much larger current. examples of relays in a vehicle are the horn relay, starter relay, headlight relay and so on. the relay is usually mounted in a location that forms a short path between the battery and the operating unit. A relay consists of a soft iron core wound with many turns of thin insulated wire. the coil is insulated from the core and one end of the coil is connected to the battery and the other is earthed through, for example, the horn button. There is a soft iron frame around the outside and the core has a springloaded armature across the top. One contact is carried on the movable armature which is connected to the frame and then underneath the battery terminal. the other contact is mounted on a fixed support which is connected to the "horn" terminal. when the horn button is pressed, current is fed to the fine winding around the central soft-iron core to the earthed base of the relay and so back to the earthed terminal of the battery. the armature is attracted towards the core and closes the contacts. therefore when the contacts close, the horn is connected directly to the battery. An advantage of the relay system is that the horn button wire has to carry only sufficient current to energize the relay winding (usually around 0.5-1amp), and therefore the horn button contacts remain in good condition almost indefinitely. another advantage is that the voltage drop in the main horn circuit is reduced because of the shorter length of cable required.
Relays usually have a wiring diagram printed on their cover to show how the relay can be used. Most relays usually have pin numbers on them to designate what they are usually connected to

86- positive side of control circuit

85- negative side of control circuit

30- battery supply for switched circuit

87a- normally closed switch circuit

87- other switch circuit

During one of our practical lessons we had to wire up a circuit and test our relay, we wired up a parallel 3-bulb circuit, the use of the relay was to see if the switching circuit of the relay would turn on all three light bulbs. After wiring up the circuit we had to measure available voltage at the following terminal points when the circuit is off and then when the circuit is on. 

CIRCUIT OFF

86- 12.5V

85- 12.5V

30- 12.5V

87a- 12.5V

87- 0.0V

CIRCUIT ON

86- 12.4V

85- 0.01V

30- 12.4V

87a- 0.0V

87- 12.4V

There are three terminals where the most voltage change is seen, these are terminals 85, 87 and 87a. This is because when the circuit is off it is a closed circuit so voltage flows through it, because no components are using an current, so when you turn the circuit on there is current being used by the components so there is voltage changes in the relay terminals.

Testing Diodes

Diode testing.

To test a diode you need to use a multimeter set to Ohms range. A working diode should only allow current to flow in one direction; Anode to Cathode. The most reliable way to test a diode is with the ‘Diode test’ position on your multimeter. In this position a higher voltage is used and the meter shows the voltage drop required to push through the boundary layer.

When testing diodes in the ohms range, a measurement of more than 2K needs to be set, because 2K is not enough to measure the diode, you will only get a reading of infinity. Using the diode test range again only work in Anode-> Cathode direction, because Cathode -> Anode is blocked.

we built a circuit with a 1K resistor and use a 12v battery supply and measured the voltage drops across the resistor and the Diode, the voltage drop across the resistor was around 12.7v, the VD across the diode was 0.65v, the amp flow through the diode was 0.01amps. we then took a voltage reading from the battery which gave us13.39v, lastly we added the voltage drop across the resistor and the diode- VDr + VDd= 13.35V. The rules of electricity apply here because the available voltage from the supply was equal to the voltage drop across the resistor and the diode, which are the components of the circuit, and in a series circuit like this was, the voltage is maintained through-out the circuit.

If you swap the diode for an LED you get a bigger voltage drop across the LED because the LED uses more voltage then the diode because it has to light up, whereas the diode doesn’t have to. Apart from that everything else in the circuit is the same.

Identifying Resistors

Identifying, testing and combining resistors

Resistors are a piece of apparatus used mainly to add resistance to a circuit. Resistors are identified by a code using the colour and position of the bands on the resistor itself. To calculate the total resistance a resistor can hold, you use the colour coded bands to figure it out. The first two or three bands are the numbers to write down. The next band is the multiplier (how many zeros to add to the number) a gold multiplier makes one decimal place smaller, silver makes two decimal places smaller. The last band is the tolerance values. So if I had a resistor with the colour coding of green, blue, brown and gold, by using my table I can figure out this resistors low and high tolerance values.

·         GREEN=5

·         BLUE= 6

·         BROWN= 1 ZERO

The gold band is the indicator of which end to read from. So my value to start off with for figuring out this resistor is 560, now to calculate the low tolerance value you must minus 5% of that 560 from 560, so 5% of 560 is 28, so I minus 28 from 560, which gives me the low tolerance value of 532. To find out the high tolerance level, I add that 28 (5%) back onto the original 560 which gives me a high tolerance value of 588. To test that the resistor is working connect it to a multimeter set on Ohms range and if the value falls between that 532 and 588, you have a working resistor.

Charging Systems- Alternator Testing

Alternator output on car testing

Before working on any vehicle to do an alternator test you must do a check for safety and general preliminary checks before starting to test the charging circuit. These checks include

·         Is the park brake applied?

·         Is the vehicle transmission in park or neutral?

·         Does the charge warning light operate?

·         Are the ignition and all accessories turned off?

·         Have you carried out a visual inspection of the connections?

·         Have you checked the alternator mounting?

·         Check the condition of fuses and the fusible links

After inspecting these parameters you can start to carry out the circuit tests on the alternator. Firstly you must determine the condition of the battery by doing an OVC check on it. To check the OVC you place your voltmeter Positive and Negative to the batteries positive and negative terminals. The results you should be looking for are between 12.4v and 12.6v this is equal to 50% and 100% charge. Next, you have to determine the condition of the voltage regulator, the specifications for this is anything between 13.8v and 14.5v. to check the regulating voltage you attach he voltmeter to the battery again, turn the car on and rev to 3000RPM, what you are looking for in this check is a change in the voltmeters reading, as it should go up, if the regulating voltage is too high is would affect the charging system by blowing bulbs in the dashboard. Next you have to determine the ‘No load current’ the specs for this test is anything between 10amps-18amps for a fuel injected engine. To check the no load current you have to turn the car on and rev the engine to approximately 1500-2000RPM, clamp the ammeter to the B wire terminal coming off the alternator.

The next test is determining the voltage and output load; to perform a load test on the alternator you have to have the car running, connect the leads of the load tester to the positive and negative terminals, clamp the ammeter on the lead running from the battery to the alternator. Also the voltmeter must be connected to the battery as well. Turn on all accessories such as headlights, fans etc. What you are expecting to happen is the readings on the meters to increase from the no load test, as there is now load be introduced to the circuit. Lastly you need to perform a charging system voltage drop test. The voltage drops you need to carry out are

·         Positive side volt drop, the spec for this is <0.2v

·         Negative side volt drop, the spec for this is again <0.2v

To carry out the positive side volt drop, attach the positive of the voltmeter to the positive of the battery and the negative of the voltmeter to the alternator output (B+) while the engine is running. For the negative side attach voltmeter negative lead to the battery negative and the positive lead to the alternator body. The reading you would expect from a good system is less than 0.2v. a high voltage drop could affect the system by not letting it charge properly.

The alternator testing is now complete. 

Charging Systems- Alternators

Alternators

An alternator is needed on a vehicle because they are equipped with power consuming accessories such as radios, heaters, horns, lights etc. Because of these components there needs to be a generating system that can produce a considerable output of electrical energy. The construction of a alternator is made up of the following component parts

·         A stationary winding assembly, the stator

·         A rotating electromagnet, the rotor

·         A slip ring and brush assembly

·         A rectifier assembly

·         Two end frames

·         A cooling fan

The stator is composed of a cylindrical, laminated iron core with three sets of windings inserted in slots on the inside. These windings are arranged in such a way that a separate alternating waveform is included in each winding as the rotating magnetic field cuts it. The rotor is an electromagnet consisting of a coil wound on an iron core, which is pressed on a shaft. When current is passed through the winding, magnetic poles are established at the ends of the iron core and shaft. An iron end-piece is fastened on each side of the coil assembly so that projections on the end-pieces interlace. These projections take on the same polarity as the ends of the shaft on which they are mounted, forming pairs of north and south poles around the periphery of the rotor. The rotor rotates within the stator with a very small air gap between the two parts so that as strong a magnetic field as possible cuts the stator windings and the maximum possible current is induced in the windings. The ends of the rotor coil are connected to insulated slip rings mounted on the shaft. A current supplied from the battery passes through the brushes and slip rings to energise the rotor windings and produce the magnetic field.

Although the alternating current could be used for lighting or heating purposes, direct current is needed to charge the battery, this is why the rectifier is required. The operating temperature of the rectifier must be kept within reasonable limits. To prevent overheating, the rectifier assembly is mounted in a piece of heat-conducting metal known as the “heat sink”. A fan is also mounted on the front end of the shaft which provides a flow of air for cooling the assembly. Alternators are constructed with a three-phase stator winding and six diodes are required to provide full-wave rectification of the current produced.

Alternators are designed to prevent production of a current great enough to cause damage to the alternator parts. The charging current will not rise above the maximum safe output of the alternator irrespective of speed or electrical load, so that no external current regulator is needed.