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Transcript
Electrical Power Supplies Unit 5 Background • You already learned in Unit 2 that conductors are materials that allow electricity to flow through them. It is important to remember that conductors are not perfect. All materials block, or resist, the flow of current to some degree. • Resistance is the measure used to tell how much a material blocks the flow of current. Resistance is measured in units called ohms. • Electrons flow only when there is a difference in potential (voltage) across the ends of a conductor. • Voltage drop is the difference between the two ends of a conductor through which electrons are flowing. Voltage drop is due to the resistance of a circuit. • In a circuit, the flow of electrons will continue until both ends of the conductor reach a common potential. To maintain a flow of electrons in a conductor, some arrangement must be provided to maintain a difference in potential while electrons flow from one end to the other. • A sustained electric current requires some sort of “electrical pump” to maintain this difference in potential. A device that provides this potential difference is known as a voltage source. Direct Current (DC) • Electrical current can flow in either of two directions through a conductor. If the current flow, either steadily or in pulses, in only one direction, it is called direct current (DC). • There a number of ways to produce direct current. One way is to use chemical generators. For example, if you dissolve table salt in water, the salt will break down into positive sodium ions and negative chloride ions. If two dissimilar metal plates (such as zinc and copper) are immersed in the salt solution, the positive ions will migrate toward one plate which is positive (copper) and the negative ions will migrate toward the other plate which is negative (zinc). If the two plates are connected by a conductor, a current will flow through the solution (as ions) and through the conductor (as electrons). The solution is called the electrolyte. This kind of chemical generator is called a wet cell. Cells in which the electrolyte is absorbed by paper or formed into a paste are called dry cells. You can connect two or more cells together to form a battery. A battery produces direct current in a circuit. Electrons always move from the repelling negative terminal toward the attracting positive terminal. However, the conventional method for current flow is from positive terminal to negative terminal, opposite to the electron flow. The MB100 Kit contains four 1.5 volt (AA) batteries. These batteries are composed of an electrolyte paste and a carbon rod in a zinc container. Batteries will produce an electric current only as long as chemical reactions within the battery take place. When the chemical reactants are used up, the battery will no longer produce a current and we commonly say the battery is “dead”. Alternating Current (AC) • Alternating current (AC) acts as it name indicates – it alternates in direction or polarity. Electrons in the circuit are moved first in one direction and then in the opposite direction, alternating back and forth within relatively fixed positions. This movement of electrons is accomplished by alternating the direction of voltage at the power source. Alternating current is usually produced by rotating a coil in a magnetic field. Alternating current is better suited than direct current for transmission through power lines because AC can be transmitted long distances at high voltages and low currents, lowering heat loss in the wires. Nearly all commercial AC circuits in North America have voltages and currents that alternate back and forth at a frequency of 60 cycles per second. Generators • Electric generators are devices which convert mechanical energy into electrical energy. In 1831, Michael Faraday made one of the first DC generators. He took a copper disk, mounted it on a shaft, then rotated it between the poles of a horseshow magnet. He found that he had a direct, continuous voltage between a contact on the shaft and one on the outer edge of the disk. Faraday’s generator, however, had a low output voltage and was very inefficient. • Thomas Edison’s first direct current generator was displayed at the Paris World Exhibition in 1881. A year later Edison set up a commercial electric generating station in New Your City to provide power for the first electric street lights. Two Main Types of Generators • DC Generator • DC generator rotates the conductor in a stationary magnetic field. • AC Generator • AC generator rotates a magnetic field that is cut by the stationary conductors. The current induced in the conductors of all generators is alternating current. However, the current taken from the generator may be AC or DC , depending on how the generator is constructed. DC GENERATOR • DC generators have a commutator mounted on one end of an armature shaft. The commutator changes (or rectifies) the alternating current in the conductors into direct current. Brushes mounted in holders ride on the rotating commutator bars and carry the direct current from the commutator to the external circuit. The DC generator in an automobile consists of an armature mounted on a shaft. The armature is composed of wires coiled around slots in a soft iron core and is rotated between magnetic field poles and connected to a commutator. It delivers direct current to external terminals. AC Generator/DC Generator An AC generator is the same as a DC generator except instead of a commutator it uses continuous rings that are mounted side by side and insulated from the shaft. One end of the coil is attached to either of the two rings. The voltage is picked up between them. DIODES • A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode. • Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valve allows fluid flow through it in only one direction as in Figure Diode Summary • A diode is an electrical component acting as a one-way valve for current. • In the Figure here (b) Is the schematic representation of a diode and (c) is the real component appearance. Rectifiers • A rectifier is a circuit that converts alternating current (AC) to direct current (DC) This means that the component permits passage of only the positive or negative portion of an alternating current. All rectifiers perform the same basic function of rectifying an electric current, that is changing an alternating current containing both positive and negative components into a direct current that has only a positive or negative component. • This conversion is critical for all sorts of household electronics. AC signals come out of your house’s wall outlets, but DC is what powers most computers and other microelectronics. • Current in AC circuits literally alternates – quickly switches between running in the positive and negative directions – but current in a DC signal only runs in one direction. So to convert from AC to DC you just need to make sure current can’t run in the negative direction. Sounds like a job for DIODES! Half-Wave Rectifer • A half-wave rectifier can be made out of just a single diode. If an AC signal, like a sine wave for example, is sent through a diode any negative component to the signal is clipped out. Input (red/left) and output (blue/right) voltage waveforms, after passing through the half-wave rectifier circuit (middle). Full Wave Rectifier • A full-wave bridge rectifier uses four diodes to convert those negative humps in the AC signal into positive humps. • These circuits are a critical component in AC-to-DC power supplies, which turn the wall outlet’s 120/240VAC signal into 3.3V, 5V, 12V, etc. DC signals. If you tore apart a wall adapter, you’d most likely see a handful of diodes in there, rectifying it up. The bridge rectifier circuit (middle), and the output wave form it creates (blue/right). Can you spot the four diodes making a bridge rectifier in this? Impedance • Impedance is the apparent resistance to current flow in AC circuits that corresponds to true (ohmic) resistance in DC circuits. Impedance (Z) is measured in ohms and is related to the voltage (V) and the current (I) by the equation. • Impedance = Voltage / Current or Z=V/I Many electronic components, such as loudspeakers, radio transmitters, and generators are rated according to their impedances. This enables users to match impedances on components easily. For example, in a home television installation the best reception is obtained when the antenna and the cable are selected to match the impedance of the receiver. Voltage Regulator A voltage regulator is a device used to control the voltage in a DC circuit. The regulator establishes the maximum voltage level for the circuit. However, the voltage in the circuit can never exceed the voltage of the power supply (unless some sort of amplifier is used). For example, a 5 VDC voltage regulator connected to a 12 VDC power supply will limit the voltage in the circuit to 5 VDC. Motor vehicles use a voltage regulator to control the charging rate of the battery by the vehicles' generator. Without the use of a voltage regulator, the battery could be damaged by overcharging.