Direct Current and Alternating Current

Electrons need to flow in a circuit for work or action to be undertaken. For example, the action of a lamp glowing brightly is caused by the flow of electrons through the filament, heating it up and causing it to glow. There are two fundamental types of current flow: direct current (DC) and alternating current (AC) Figure 36-7. DC is produced by a battery. The battery maintains the same positive and negative polarity; therefore, the current flows in one direction only. The characteristics of DC are the fixed polarity of the applied voltage and the flow of charges in only one direction. It is possible to have varying DC; however, the charges always flow in the one direction and the applied voltage polarity remains the same.

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Waveforms. A. Direct current B. Alternating current.

AC is the type of current in your home electricity supply. It continuously changes its direction of current flow, and the alternating voltage repeatedly reverses or alternates its polarity. Thus, the current flow moves back and forth within a circuit. AC is produced in what is called a sine wave. It operates on a cycle, gradually building to a maximum current flow in one direction (positive value), then gradually reducing to zero, then gradually building to a maximum current flow in the other direction (negative value), and finally gradually reducing back to zero current. In most cases, this cycle can occur many times a second. For example, the AC flow in the house supply changes direction 60 times per second. Hertz is the measurement of frequency and indicates the number of cycles per second. So, since the AC supply in the home typically changes polarity 60 times a second, its frequency is 60 hertz, or 60 Hz. Hertz simply means “cycles per second.”

AC is used in vehicles to a lesser extent than DC. Alternators use it to create current flow to charge the battery and run the electrical accessories. The AC is first transformed to DC before it leaves the alternator so it can be effectively put to use in the DC electrical system.

AC is used in the electric motors on most hybrid vehicles. Since those motors generally require high amounts of electrical power, and because AC is more efficient than DC, AC is more advantageous for that application. The AC is created by a sophisticated electronic inverter.

Some manufacturers use sensors that create an AC signal that varies in frequency. This varying signal is sent to the vehicle’s computer as an indication of changes within the system being monitored. The computer can use that signal to make adjustments based on its programmed software.

In general, electrical components are designed to work on either AC or DC, but not both. For example, a DC motor will not work on AC , and vice versa. Devices can be made to convert or change AC to DC and DC to AC . For example, a battery charger that has an input AC of 110 volts at 60 Hz power can change this to 14 volts DC through a transformer and rectifier to charge a battery. Devices are also available to change DC into AC , and are called inverters. For example, an inverter could have an input of 12 volts DC and convert that to 110 volts AC at 60 Hz. AC and DC power sources can be of any voltage and are not limited to the relatively low 12 volts DC found in the batteries on vehicles or the 110 volts AC found in the home power supply. For example, some hybrid vehicles use high-voltage DC invertors drawing 200 to 300 volts DC to power the 500-volt three-phase AC to power the main electric traction motors or accessory motors. Phase refers to the number of separate staggered power windings in the motor or alternator. Generally, three-phase motors or alternators produce more output than single-phase motors or alternators because they can maintain a much more even voltage compared to single phase AC.

AS-58: AC /DC: The technician can explain the difference between direct and alternating current.

Direct current (DC) and alternating current (AC ) are two forms of electricity that are produced differently and have different uses. DC electricity is the simpler form. It starts in one place and then flows in the same direction to its destination. AC electricity flows in one direction for a period of time, then changes direction over and over again continuously.

In modern automotive applications, AC electricity is generated by the alternator using electromagnets. The AC electricity is converted to DC, or rectified, by diodes in the alternator before being supplied to the vehicle’s electrical systems or being stored in the battery.

AS-53: Electricity: The technician can demonstrate an understanding of and explain the properties of electricity as they relate to lighting, engine management, and other electrical systems in the vehicle.

Most automotive electrical systems use 12-volt DC electricity supplied from the vehicle’s battery to power items such as light bulbs, electric motors, and heater elements. Some circuits may use switching with varying degrees of resistance to control the output from the circuit’s load; examples may include interior blower fan motors that run at varying speeds and the vehicle’s fuel gauge, which uses a variable resistor to determine the position of the gauge.

Engine and power train management systems are also powered from the vehicle’s 12-volt battery but use various other voltages to monitor operating parameters via sensors. Some sensors operate using a reference voltage, generally 5 volts, which is modified to determine operating parameters. Examples could be a digital crankshaft position/speed sensor or a digital mass airflow meter. The reference voltage is effectively switched “on” or “off” by the digital sensor, producing a square-wave output with a frequency that changes according to the engine speed or airflow rate.

Some sensors generate their own voltage; examples include zirconia oxygen sensors, analog crankshaft or camshaft position sensors, which generate AC voltage, and piezoelectric knock sensors. Voltages generated from these sensors are monitored by the electronic control unit (ECU) to determine engine operating parameters.

Some computer-controlled devices can simply be turned on and off. A cooling fan may be turned on when the coolant reaches a temperature of 219°F (104°C) for example and turned back off when the coolant temperature falls below 201°F (94°C) for example. Pulse-width modulation provides a means for a computer to control a device with variable operating parameters. Fuel injectors, for example, need to be open for differing amounts of time depending on the fueling requirements of the engine. An ECU can control injector opening time by sending a pulsed signal with shorter or longer duration.

Technician Tip
While the United States has chosen 60 Hz for the electrical grid, many other countries use 220 volts at 50 Hz and have different-shaped plugs. If you travel to these countries, make sure to purchase the proper converter or transformer so your electrical devices will work and not be damaged.
AM-14: Mentally: The technician can determine the proper mathematical operation (addition, subtraction, multiplication, or division) and mentally arrive at the solution.

Determining the operating parameters of electrical circuits can require a selection of different mathematical operations. Some examples may include adding the resistance of individual components in a series circuit to determine the total resistance of the circuit, dividing the voltage by the resistance to determine the current flow in a parallel circuit, or multiplying the amperage of a circuit by its resistance to determine the voltage a device is using.

AM-44: Formulas: Using formulas, the technician can predict the outcome(s) under different variations.

Ohm’s law, a mathematical formula, can be applied to predict the outcome of changes made to electrical systems. Say we have a 12-volt automotive electrical circuit to drive a bulb in an interior light. A customer has complained that the interior light in his car is not bright enough and has requested that a higher wattage bulb be fitted. (Caution: We are only using this as an example. Additional wattage creates additional heat, which can cause the fuse to blow, or in an extreme case, a fire. It is recommended that you never modify a vehicle from the manufacturer’s original condition.) The resistance of bulbs varies according to their wattage. Generally, the higher the wattage, the lower the resistance.

The original 5-watt bulb fitted to the vehicle had a resistance of 30 ohms. Through Ohm’s law, we know that current flow equals voltage divided by resistance, or in this case 0.4 amps. The new 10-watt bulb to be fitted has half the resistance, 15 ohms. Current flow in the circuit is now calculated as 12 volts divided by 15 ohms, which is 0.8 amps, which provides twice the wattage.

AM-46: Equivalent Form: The technician can write or rewrite an algebraic equation to solve for any unknown variables.

Ohm’s law is commonly used to calculate the value of an unknown variable from known values in electrical circuits. The equation is commonly stated as voltage (V) equals current flow (A) multiplied by resistance (R), or V = AR. To calculate current flow or resistance, instead of voltage, the equation must be rewritten. To apply the rules of basic algebra, anything we do to one side of the equation we must also do to the other. To calculate current flow, we need the A by itself on one side of the equation, so we divide both sides by R to arrive at A = V/R. To calculate resistance, the R must stand alone, so we divide both sides of the equation by A to arrive at R = V/A. Ohm’s law is commonly represented in a triangle or circle format, making the correct algebraic equation very simple to find.