This paper introduces the concepts of direct current and alternating current, the symbols of direct current and alternating current, the rectification method of direct current and alternating current, the current whose magnitude and direction do not change with time, and the rectification and half rectification methods of alternating current.
symbols for direct current and alternating current
Put the head (positive pole) of one battery against the tail (negative pole) of the other battery in the flashlight, and the flashlight will light up. If it is reversed, head to head or tail to tail, the flashlight will not light up. This is because the current produced by the battery always flows in one direction, so it is called direct current.
The electricity that is brought into the home through power lines or cables is not direct current, but alternating current. Because this current flows in one direction at one time and the opposite direction at another time.
Although alternating current is “variable”, it has one of the biggest advantages over direct current, which is that a transformer can be used to step up or down the voltage of the alternating current as needed.
Electricity produced by power plants must be transported to long distances for users to use. The higher the voltage, the smaller the loss in transmission. When the voltage rises to 35,000 volts or 220,000 volts, or even as high as 500,000 volts, it is more economical to transmit.
Wherever electricity is used, the voltage has to be lowered to suit its particular purpose. For example, household electricity is only 220 volts, while factories commonly use 380 volts.
Direct current also has its advantages. In the chemical industry, such as electroplating, direct current is necessary. It is also better to use direct current to drive an electric car.
In order to adapt to the specific use of various electrical appliances, alternating current can also be converted into direct current, which is called rectification. Some semi-drawing machine conductor radios or recorders can use external power supply aluminum wire drawing machines. Through a block-shaped device, alternating current is converted into direct current for use. This step-down and rectification device is called a power converter.
A current whose magnitude and direction do not change with time, also known as a constant current. The circuit passed through is called a DC circuit, which is a closed conductive loop composed of a DC power source and a resistor. In this circuit, a constant electric field is formed. Outside the power supply, the positive charge flows from the high-potential copper wire drawing machine to the low-potential place through the resistor. Reach a high potential, and so cycle, forming a closed current line. Therefore, in the DC circuit, the role of the power supply is to provide a constant electromotive force that does not change with time, to supplement the energy for the Joule heat consumed on the resistance.
In a relatively simple DC circuit, the relationship between power supply electromotive force, resistance, current and voltage at any two points can be obtained according to Ohm’s law and the definition of electromotive force. Complex DC networks can be solved according to the GR Kirchhoff equations. It includes two parts: node current equation and loop voltage equation. The former points out that for any node (intersection of 3 or more branches), the algebraic sum of each current flowing into and out of the node is zero, which is a requirement of constant conditions, The latter states that for any closed loop (grid) the algebraic sum of the voltage drops in each part is zero, which is the result of the electrostatic field loop theorem, and the two form a complete system of equations.
Instruments that measure physical quantities such as current, voltage, resistance, and power supply electromotive force in DC circuits are called DC instruments. Commonly used are ammeters, ammeters, voltmeters, bridges, potentiometers and so on.
DC power sources include chemical batteries, fuel cells, thermoelectric cells, solar cells, DC generators, etc. DC power is mainly used in various electronic instruments, electrolysis, electroplating, DC power drag and so on.
In terms of power transmission, after the 1880s, due to the inconvenience of raising the low voltage of direct current to high voltage for long-distance transmission, direct current transmission once gave way to alternating current transmission. Since the 1960s, due to the use of high-voltage, high-power converters to convert direct current into alternating current, the direct current transmission system has received renewed attention and gained new development.
Alternating current, referred to as “AC”, generally refers to a voltage or current whose magnitude and direction change periodically over time. The most basic form is sinusoidal current.
The standard frequency of AC power supply in China is 50 Hz. The form of alternating current that changes with time can be a variety of different forms of alternating current, its application range and effect are also different, sinusoidal alternating current is the most widely used, and other non-sinusoidal alternating currents can generally be converted into sinusoidal after mathematical processing. Superposition of alternating current.
The simple harmonic function i=Imsinωt+φ0 of the time of sinusoidal current (also known as simple harmonic current) When the coil rotates at a constant speed in the magnetic field, alternating current whose magnitude and direction change periodically will be generated in the coil. The alternating current used today generally changes direction and intensity by 50 Hz per second.
Common electric lights, motors, etc. are powered by alternating current. In practice, alternating current is represented by the symbol “~”.
The changing law of the current i with time, it can be seen from this that the sinusoidal alternating current needs to be described by three physical quantities: frequency, peak value and phase. The basic issues to be discussed in alternating current are the relationship between current and voltage in a circuit, and the distribution of power (or energy). Due to the time-varying characteristics of alternating current, a series of characteristics that distinguish it from direct current circuits have arisen. The components used in the AC circuit not only have resistance, but also have capacitive components and inductive components.
rectification of alternating current
The process of converting alternating current into direct current is called “AC rectification”. Rectification can be divided into several forms such as half-wave rectification, full-wave rectification, and bridge rectification. Common rectifier devices use the unidirectional conduction properties of electron tubes and crystal diodes to rectify. For example, rectifiers made of semiconductor materials such as germanium and silicon have been widely used in many aspects.
In order to adapt to higher voltage rectification, many single rectifiers can be connected in series and sealed in a piece of insulating material, called “silicon stack” rectifier, which can remove the waveform of the negative half cycle of the AC, so that the AC becomes a pulsating DC.
Therefore, the output waveform after rectification contains only the positive half-cycle waveform of the sine wave. An ideal rectifier can be regarded as a switch. When the AC input is in the positive half cycle, there is a voltage output, just like the switch is turned on; on the contrary, if the AC input in the negative half cycle is used, there is no voltage output, which is equivalent to the switch being turned off. Therefore, when the AC input is in the positive half cycle, the effective resistance of the switch is zero; and when the AC input is in the negative half cycle, the effective resistance is infinite. The actual rectifier can’t be so ideal, but it’s not far off.
When the tube rectifier is not conducting, its resistance is extremely large, and the resistance at this time is called reverse resistance; when the rectifier is conducting, its resistance is small, and the resistance at this time is forward (forward) resistance. In any case, all rectifiers allow conduction in only one direction. This characteristic is called unidirectional conduction or unidirectional characteristic, and diodes (including transistors) have this unidirectional characteristic. Any electronic component that contains an emitter or cathode and a collector or anode is called a diode (including electron diodes and crystal diodes) because electrons in a diode can only flow in one direction. Therefore, all diodes have rectifying characteristics.
During rectification, only one and a half cycles of the alternating current pass through the rectifier. Half-wave rectifier is the simplest rectifier, but its efficiency is very low. It is also difficult to smooth the current waveform rectified by it, a simple transistor rectifier circuit.
The input and output waveforms of the half-wave rectifier are shown in Figure 362. The output waveform of the half-wave rectifier can be compared with the input AC waveform. When there is a positive half cycle of current flow, the output DC is one positive and one negative does not change! Current always flows from positive to negative! Alternating current means that the positive and negative poles change back and forth. If the positive and negative poles change back and forth after a while, it will become the negative pole. For example, if the alternating current in our country is 50Hz, the positive and negative poles will change 50 times in 1 second.
A current whose magnitude and direction do not change with time, also known as a constant current. The circuit passed through is called a DC circuit, which is a closed conductive loop composed of a DC power source and a resistor. The direction and intensity (magnitude) of the alternating current change periodically.