1
No matter your desires – whether professional or personal – before you can start on any project, you need a solid grasp of “the basics.” Accordingly, Chapter 1 forms the foundation of your entire MECP training. This chapter introduces some basic principles of electronics, as well as some of the more advanced formulas and laws.
Both the Basic Installer level and the First Class level Electrical section of the MECP certification tests are included here. You should have a thorough understanding of each topic before moving on to the next topic. For the First Class level test you will need to study the complete First Class Study Guide available from MECP.
It’s hard to imagine life without electricity. And while internal combustion engines power our vehicles, it’s electricity that lights the stoplamp when you put your foot on the brakes. And it’s electricity that powers the audio system.
The computer may be fueling today’s technological growth, but it was electricity that started the revolution.
Therefore, before you can move into more specific areas of expertise, you first need a solid foundation in electrical theory and application.
Section 1
Electrical Laws and Formulas for the Mobile Electronics Environment
What do “electrical laws and formulas” have to do with you - an installer?
Good question.
On the surface, it may seem like a plumber studying hydrodynamic physics - sure, they both deal with the motion of fluids, but one is a little overkill.
The same theory does not hold true here.
Today’s installations are increasingly more complex - and the vehicles you are working on are equally sophisticated. It is no longer just about hooking up the components.
Being a mobile electronics installer truly is a profession - it requires skill and training, and there’s always something new to learn. But before you can learn the “new stuff,”
you need to have a solid understanding of the basic electrical theories. That way, when you encounter a particular challenge, you’ll know where to start troubleshooting.
After all, you can easily figure out when you’ve used the wrong size wire gauge or have a bad connection without all that math cluttering your mind. But while hands-on experience is essential; understanding why a wire gauge is too small or what caus-es a bad ground will help you through many practical situations. A firm grasp of electronics knowledge can guide you logically to the source of almost any problem.
Before getting to the mathematical relationships involved in electronics, you need to know about the two types of electrical current you will be working with in the mobile electronics environment - AC and DC.
■ “AC” stands for Alternating Current, which is current that alternates polarity between positive and negative. AC has both an amplitude compo-nent (how much) and a frequency compocompo-nent (how often).
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BASIC AND ADVANCED ELECTRICAL
■ “DC” stands for Direct Current, and it is current which supplies power to electronic components and is EITHER positive OR negative in polarity, but not both. DC has only an amplitude component (called potential) and a fre-quency of zero.
Alternating Current is an electronic current that periodically changes polarity (i.e., it alternates from positive to negative).
■ In an alternating current circuit, the current flow reverses its direction on each alternation. The voltage alternates from positive to negative and back again to positive.
■ The rate of alternation (how often) is called frequency, which is measured in cycles per second, or Hertz (Hz).
■ The number of times the AC signal cycles in one second is its specific fre-quency. Multiple frequencies blended together is how music is sometimes clas-sified as AC.
On an oscilloscope, AC looks like this:
The other form of AC at work in the vehicle is the charging system. A key com-ponent of this system is the alternator. The alternator creates AC that is changed into DC by a process called rectification, which allows the battery to charge.
When it comes to the audio signal, we are concerned with the “AC” that flows from the head unit through the signal processors, which is then amplified to drive the loudspeakers. That audio signal contains many varied frequencies and amplitudes which make up the tempo and pitch of individual sounds in music.
■ Alternating Current and music signals are covered in detail in the AUTOSOUND chapter of the First Class Study Guide.
For now, most of our applications will focus on DC.
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✍
Alternating Current is an electronic current that periodically changes polarity.+
0
— 1 Cycle
Amplitude
Time
✍
The alternator creates AC that is changed into DC by a process called rectification.■ Figure 1. Oscilloscope., AC.
Direct Current is defined as a current that travels in one direction only. One ter-minal is always positive, and the other is always negative.
■ All things that rely on the vehicle battery as their source of power operate with DC. This includes amplifiers, head units, security systems, radar detec-tors, car phones, and other electronic accessories. Sometimes a component, though powered by DC, may output AC. This is the case with car amplifiers.
On an oscilloscope, positive DC looks like this:
When analyzing electronic circuits, you’ll encounter the relationships between these four electronic properties:
1 Voltage (E) 2 Current (I) 3 Resistance (R)
4 Power (P)
Ohm’s Law is the electrical formula that defines the relationship of these proper-ties to each other.
UNDERSTANDING OHM’S LAW
Ohm’s Law is one of the most basic laws of electricity. Using mathematical for-mulas, Ohm’s Law describes a specific and measurable relationship between cur-rent, voltage, resistance and power.
Let’s look at these parameters and see how they apply to mobile electronics:
The properties that you need to understand are Voltage, Current, Resistance, and Power. Power will be discussed later in this chapter.
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✍
Direct Current is defined as current that travels in one direction only.+ ø
✍
Ohm’s Law describes a specific and measurable relationship between current, voltage, resistance and power.■ Figure 2. Oscilloscope., positive DC.
Ohm’s Law mathematically describes the interaction among these parameters.
Understanding the relationship among current, voltage resistance, and power can help you figure out many different installation problems and answer many instal-lation questions (even before the instalinstal-lation begins). For Example, Ohm’s Law will tell you how much power an amplifier really puts out, if the voltage supplied to an amplifier is too low, or if a higher power alternator should be considered. So lets take a look at Ohm’s law and discover how it effects our work environment.
■ Current is the rate of electron flow through a given point, and is mea-sured in Amperes or Amps. If you marked a point on a main road in a city and counted the cars that pass that point in a specific window of time, you could gain an understanding of the traffic flow on that road. A wider road with more lanes would allow more cars to pass in a given window of time, while a narrower road with fewer lanes would allow a smaller number of cars to pass in that window of time. This illustrates the concept of current flow in a wire or circuit.
■ Voltage is the electrical pressure that moves charged particles in a circuit, and is measured in Volts. Voltage can be considered as the force of electrici-ty. Voltage is also sometimes called difference of potential (potential differ-ence) and, like the force of electricity, can be thought of as electrical pressure that moves the current.
Just as the width of the road and number of lanes would effect traffic as we described with current, a vehicle’s natural ability or potential to movement would also affect traffic flow. A vehicle moving downhill could start and move much more quickly than the same vehicle moving uphill. The natural force of gravity assists that. Electrically, the natural force, (determined by potential) that moves the charged particles through the circuit is much the same concept. More electri-cal pressure means more potential for electronic traffic flow.
■ What is the pressure exactly? How does it move the charged particles?
Let’s start to answer these questions by first defining some terms that relate
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✍
Current is the rate of elec-tron flow through a given point.✍
Voltage is the electrical pressure that moves charged particles in a circuit.I Current Amps or Amperes
E Voltage Volts
R Resistance Ohms
P Power Watts
SYMBOL PARAMETER UNIT OF MEASURE
■ Charge — or electrical charge is the fundamental unit for an amount of electricity. Symbolized (Q).
■ Polarity — in an electrical circuit there are two different polarities: elec-trons posses a negative charge while protons posses a positive charge. It can also be said that an electron has a negative polarity and a proton has a pos-itive polarity.
■ Potential - refers to the ability to do work.
Now with these definitions let’s discuss some actions.
■ Like charges repel - two negatively charged particles held together will repel or want to move away from one another. Likewise, two positively charged particles held together will repel or want to move away from one another.
■ Unlike charges attract - when two unlike charges are brought close together they will attract or try to move toward each other.
These two reactions are proof of an electric field. Since potential is the ability of the charges to do work, it’s the difference of potential (using the natural ability to attract and repel) that allows the current to move and do work.
■ Resistance is the opposition to current flow. To understand Resistance think of anything that limits or blocks the flow of electrical traffic. Electrical Resistance describes the property that various materials possess to restrict or inhibit the flow of electricity. Electrical resistance is measured in Ohms (Ω).
Electrical resistance is relatively low in most metals and relatively high in most non-metallic substances.
The basic formulas used by Ohm’s Law to find current, voltage, or resistance are as follows: restrict or inhibit the flow of electricity.
According to Ohm’s Law:
If you want to find… and you know… then the math is…
Current (I) Resistance (R) and Voltage (E) E ÷ R = I Current (I) Power (P) and Voltage (E) P ÷ E = I
Current (I) Power (P) and Resistance (R) (Sq.Rt.) √P ÷ R = I Voltage (E) Power (P) and Resistance (R) (Sq.Rt.) √P x R = E Voltage (E) Current (I) and Resistance (R) I x R = E
Voltage (E) Current (I) and Power (P) P ÷ I = E Resistance (R) Current (I) and Voltage (E) E ÷ I = R Resistance (R) Current (I) and Power (P) P ÷ I2= R Resistance (R) Voltage (E) and Power (P) E2÷ P = R Power (P) Current (I) and Voltage (E) E x I = P Power (P) Current (I) and Resistance (R) R x I2 = P Power (P) Resistance (R) and Voltage (E) E2÷ R = P
Let’s take a less scientific approach to understanding the relationship between cur-rent, voltage, and resistance by comparing electrical characteristics to hydraulics.
Suppose you have a container of water. The pressure at the bottom of the contain-er caused by the volume of watcontain-er above it is similar to voltage. The more watcontain-er, the more pressure, the more voltage, the higher the difference of potential (voltage).
When the valve is opened, pressure forces the water through the pipe.
■ Voltage is like that “pressure” - only it is electrical pressure that is forcing charged particles through a circuit.
■ If you were to open the valve wider, more water would flow through the pipe.
If you were to make the valve opening smaller, less water would flow
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Container of Water
Valve
On Tube
■ Figure 4. Water Tank.
This increase and decrease in the rate of water flow is comparable to the idea of current, but remember that current is the rate of electrons that flow through a conductor.
In addition, if you were to decrease the size of the pipe or bend it slightly, the rate of water flow would decrease because you would be increasing the resistance.
■ This limitation in flow volume is similar to electrical resistance, which restricts the flow of electrons.
The relationship between current, voltage, and resistance is similar to the con-tainer of water - change one parameter while leaving another alone and the third has to change. It will always change according to Ohm’s law, which is the real beauty in knowing this concept.
Understanding the relationship between current, voltage, and resistance can help you figure out many different installation problems.
■ Ohm’s Law will tell you things such as:
■ How much power an amplifier really puts out.
■ If the voltage supplied to an amplifier is too low.
■ If a higher output alternator should be considered.
Let’s say, for example, that you’re powering up a high wattage audio system, but you choose a wire that’s too small to supply the current required by the system.
The resistance in the wire will develop an unwanted voltage drop across it (E = I x R) when the amplifiers draw power. Amplifiers operating with low volt-age may overheat, motorboat, or fail.
An easy way to memorize Ohm’s Law is to use the Ohm’s Law Pie Chart. Simply “cover up” the letter you wish to find the value of and carry out the remaining formula.
Here’s another example of how useful Ohm’s Law can be in every day installations:
■ Suppose you have a resistor with a known value of 8 Ohms (R = 8), and you know the current value that flows through the resistor is 1 Amp (I = 2).
What is the voltage across the resistor?
Simply apply Ohm’s Law:
R = 8 I = 2 E = I x R E = 2 x 8
E = 16 Volts
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E I R
■ Figure 5. OHM’s Law Pie Chart.
The following circuits show examples of how these formulas can be applied to installations. Use what you’ve learned so far about Ohm’s Law to calculate the cur-rent, resistance, and voltage.
1 How much CURRENT will flow through this circuit?
2 What is the RESISTANCE of an alarm siren when 12 Volts causes 11/2
Amperes to flow?
3 How much VOLTAGE is supplying this circuit?
The answers are:
1 2 Amperes 2 8 Ohms 3 4.8 Volts
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■ Figure 7. Resistance Circuit.
■ Figure 8. Voltage Circuit.
■ Figure 6. Current Circuit.
Ohm’s Law is also very practical to know when you’re trying to calculate effective resistance.
■ Effective resistance is the “calculated” resistance that a device presents to a circuit while it is operating.
Knowing how to apply Ohm’s Law to determine resistance is practical because it’s fairly easy to use a VOM (Volt Ohm Meter) to measure current and voltage, but you cannot directly measure resistance in a live circuit.
For example, if you have an amplifier that draws 50 Amps, with an applied volt-age of 12 Volts, for full power output with both channels driven into a 4 Ohm load. How would you determine the effective resistance of the amplifier by apply-ing Ohm’s Law?
Since we know that I = 50 Amps, and E = 12 Volts, we can manipulate Ohm’s Law so that R is the isolated variable.
■ Simply divide both sides of the equation by I:
E = I x R
E = I x R I I R = E
I
Now, insert the known values into the formula:
R eff = 12V 50A
R eff = 0.24 Ohms
ELECTRICAL POWER
Ohm’s Law relates a fourth circuit parameter - Power.
■ Electrical POWER is the conversion of energy into work over a certain period of time, and a watt represents the rate over time that the energy is converted. It’s the result of the collective work of current, voltage, and resis-tance. The last parameter, “P”, allows you to determine how much a system can produce, how many amps it will draw, and therefore what gauge wire
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Power is the conversion of energy into work over a certain period of time.✍
Effective resistance is the“calculated” resistance that a device presents to a circuit while it is operating.
✍
A watt represents the rate over time that the energy isThere are four basic forms of power:
■ Mechanical power, usually measured in horsepower.
■ Heat, measured in BTU’s (British Thermal Units).
■ Nuclear power, measured in Roentgens.
■ Electrical power, which is measured in Watts.
The law of conservation of energy states that energy cannot be created or destroyed, only changed into some other form of energy. The same law is valid in audio cir-cuits, where electrical energy is being converted into heat and sound.
In more advanced studies of electronics, you’ll come across the terms coulomb and joule.
■ A coulomb (pronounced koo-loam) is an electrical charge which contains 6.24 x 1018of electrons.
■ A joule (pronounced jew-el) is the energy required to move 6.24 x 1018 electrons (one coulomb of charge) past a point in a circuit.
■ If one coulomb of charge moves past the point every second, the flow rate (current) is one ampere.
■ Since a watt represents the rate over time that energy (joules) is converted into work (heat, sound, light, etc.), then a watt represents the conversion of one joule per second into light, heat, sound, or some other form of work.
These definitions are not really necessary to know in every day installations; how-ever, they help define the relationship between energy, power, and time.
Getting back to Ohm’s Law, electrical power is equal to volts times amperes, or P = E x I.
■ One volt will move one amp through one ohm of resistance at a work rate of one watt.
■ Resistors convert electrical energy into heat.
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Coulomb is an electrical charge which contains 6.24 x 1018of electrons.P Power Watts
SYMBOL PARAMETER UNIT OF MEASURE
✍
Joule is the energy required to move 6.24 x 1018electrons (one coulomb of charge) past a point in a circuit.✍
A watt represents the conversion of one joule per second into light, heat, sound, or some other form of work.✍
The law of Conservation of energy states that energy cannot be created or destroyed, only changed into some other form of energy.Remember that amperage is current flow per second, and therefore, a watt is rated in seconds.
■ Since power equals: E x I, we know from Ohm’s Law that E = I x R, P = I x R x I.
■ This is the formula we will use to figure out the power (wattage) for most of our DC applications.
Here are some more ways Ohm’s Law can help you figure out different situations (in addition, see the full Ohm’s Law pie chart in the back of this book):
■ How would you find the total current (I) of an amplifier at the electrical system’s idle voltage?
■ Simply divide the amplifier’s total root mean square (rms) wattage (P) by the vehicle’s idle voltage (E).
■ In a system with 250 Watts rms total audio output power, (125 Watts rms/channel into 4 Ohms) and an electrical system with a 12.6VDC, the equation would look like this:
250 = 19.84 Amps 12.6
This can appear to be complicated - but if you focus on each element in the equa-tion, then it’s easy to understand.
Here’s why it is important that you understand this equation:
■ It “tells” you what size wire to run from the battery to the amplifiers.
■ If the amplifiers are in the trunk.
■ You have a 15-foot cable run.
■ According to Figure 9, a #10 American Wire Gauge (AWG) cable is necessary to adequately power up this system.
Ohm’s Law is indeed a very helpful tool to have in the bay.
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Using the same example and applying it to IASCA rules - an international sanc-tioning body for sound-off events - you derive a much different answer.
■ The first formula is perfectly adequate for a system to operate safely.
■ In an effort to compensate for the power wasted by the amplifier,
Margin Notes
The above chart shows wire gauges to be used, if no less than a .5 volt drop is accepted. If aluminum wire or tinned wire is used, the gauges should be of an even larger size to com-pensate. Cable gauge size calculation takes into account terminal connection resistance.
■ Figure 9. Electrical Wire Chart.
■ IASCA’s Recommended Minimum Wire Gauge Size.