Basic concepts of electronics

Current: The directional movement of electric charge is called current. In the circuit, current is often represented by I. There are two types of current: direct current and alternating current. The magnitude and direction of the current that does not change with time is called direct current. The magnitude and direction of the current that changes with time is called alternating current. The unit of current is ampere (A), and milliampere (mA) or microampere (uA) are also commonly used as units. 1A=1000mA, 1mA=1000uA.
Current can be measured with an ammeter. When measuring, connect the ammeter in series in the circuit and select a range where the ammeter pointer is close to full deflection. This can prevent the ammeter from being damaged by excessive current.

Voltage: The reason why river water can flow is because of the water level difference; the reason why electric charges can flow is because of the potential difference. Potential difference is also voltage. Voltage is the reason for the formation of current. In the circuit, voltage is often represented by U. The unit of voltage is volt (V), and millivolt (mV) or microvolt (uV) is also commonly used as a unit. 1V=1000mV, 1mV=1000uV.
Voltage can be measured with a voltmeter. When measuring, connect the voltmeter in parallel to the circuit, and select a range where the voltmeter pointer is close to full deflection. If the voltage on the circuit cannot be estimated, use a large range first, make a rough measurement, and then use an appropriate range. This can prevent the voltmeter from being damaged due to excessive voltage.

Resistance: The part of the circuit that hinders the passage of current and causes energy consumption is called resistance. Resistance is usually represented by R. The unit of resistance is ohm (Ω), and kiloohm (kΩ) or megaohm (MΩ) are also commonly used as units. 1kΩ=1000Ω, 1MΩ=1000000Ω. The resistance of a conductor is determined by the material, cross-sectional area and length of the conductor.
Resistance can be measured using the ohm range of a multimeter. When measuring, choose the ohm range where the meter pointer is close to half deflection. If the resistor is in the circuit, open one end of the resistor before measuring.

Ohm's law: The current I in a conductor is directly proportional to the voltage U across the conductor and inversely proportional to the resistance R of the conductor, that is,

This law is called Ohm's law. If you know two of the three quantities, voltage, current, and resistance, you can use Ohm's law to calculate the third quantity, that is,

In AC circuits, Ohm's law also holds true, but the resistance R should be changed to impedance Z, that is,

Power supply: A device that converts other forms of energy into electrical energy is called a power supply. A generator can convert mechanical energy into electrical energy, and a dry cell can convert chemical energy into electrical energy. Generators, dry cells, etc. are called power supplies. A device that converts AC into DC through a transformer and a rectifier is called a rectifier power supply. An electronic device that can provide a signal is called a signal source. A transistor can amplify the signal sent from the front and transmit the amplified signal to the subsequent circuit. For the subsequent circuit, the transistor can also be regarded as a signal source. Rectifier power supplies and signal sources are sometimes also called power supplies.

Load: A device that converts electrical energy into other forms of energy is called a load. Motors can convert electrical energy into mechanical energy, resistors can convert electrical energy into heat energy, light bulbs can convert electrical energy into heat energy and light energy, and speakers can convert electrical energy into sound energy. Motors, resistors, light bulbs, speakers, etc. are all called loads. For the previous signal source, the transistor can also be regarded as a load.

Circuit: The path through which current flows is called a circuit. The simplest circuit consists of power supply, load, wires, switches and other components, as shown in Figure 1. A circuit that is connected everywhere is called a path. Only when there is a path can current flow through the circuit. A circuit that is disconnected somewhere is called a break or open circuit. When the two ends of a part of a circuit are directly connected, making the voltage of this part zero, it is called a short circuit.

Electromotive force: Electromotive force is a physical quantity that reflects the ability of a power source to convert other forms of energy into electrical energy. Electromotive force generates voltage across the power source. In the circuit, electromotive force is often represented by δ. The unit of electromotive force is the same as the unit of voltage, which is also volt.
The electromotive force of a power source can be measured with a voltmeter. When measuring, the power source should not be connected to the circuit. The voltage across the power source is measured with a voltmeter, and the voltage value obtained can be regarded as equal to the electromotive force of the power source. If the power source is connected to the circuit (Figure 2), the voltage across the power source measured by the voltmeter will be less than the electromotive force of the power source. This is because the power source has internal resistance. In a closed circuit, the current has an internal voltage drop when passing through the internal resistance r, and an external voltage drop when passing through the external resistance R. The electromotive force δ of the power source is equal to the sum of the internal voltage Ur and the external voltage UR, that is, δ=Ur+UR. Strictly speaking, even if the power source is not connected to the circuit, the voltage across the power source is measured with a voltmeter, and the voltmeter becomes an external circuit, and the measured voltage is also less than the electromotive force. However, since the internal resistance of the voltmeter is large and the internal resistance of the power source is small, the internal voltage can be ignored. Therefore, the voltage across the power supply measured by the voltmeter can be regarded as equal to the electromotive force of the power supply.

When dry batteries are worn out, the voltage at both ends of the battery is sometimes still relatively high when measured with a voltmeter, but the load (radio, tape recorder, etc.) cannot work properly after being connected to the circuit. This is because the internal resistance of the battery has increased, even larger than the resistance of the load, but still smaller than the internal resistance of the voltmeter. When the voltage at both ends of the battery is measured with a voltmeter, the internal voltage divided by the internal resistance of the battery is not large, so the voltage measured by the voltmeter is still relatively high. However, after the battery is connected to the circuit, the internal voltage divided by the internal resistance of the battery increases, and the voltage divided by the load resistance decreases, so the load cannot work properly. In order to determine whether the old battery can be used, the voltage at both ends of the battery should be measured when there is a load. For some poor-performance voltage-stabilized power supplies, the voltage at both ends of the power supply measured with and without a load is quite different, which is also caused by the large internal resistance of the power supply. 電+电脑*修+修-知.知_网(w_ww*dnw_xzs*co_m)

Cycle: The time required for AC to complete a complete change is called a cycle, usually represented by T. The unit of cycle is second (s), and milliseconds (ms) or microseconds (us) are also commonly used as units. 1s=1000ms, 1s=1000000us.

Frequency The number of times that alternating current completes a periodic change within 1 second is called frequency, usually represented by f. The unit of frequency is hertz (Hz), and kilohertz (kHz) or megahertz (MHz) are also commonly used as units. 1kHz=1000Hz, 1MHz=1000000Hz. The frequency f of alternating current is the reciprocal of the period T, that is,

Capacitance: Capacitance is a physical quantity that measures the ability of a conductor to store charge. When a certain voltage is applied to two insulated conductors, they will store a certain amount of charge. One conductor stores positive charge, and the other stores an equal amount of negative charge. The greater the applied voltage, the more charge is stored. The amount of stored charge is proportional to the applied voltage, and their ratio is called capacitance. If voltage is represented by U, charge is represented by Q, and capacitance is represented by C, then

The unit of capacitance is Farad (F), and microfarad (uF) or picofarad (pF) are also commonly used as units. 1F=10 6 uF, 1F=10 12 pF.
Capacitance can be measured with a capacitance tester, or roughly estimated with the ohm range of a multimeter. When the red and black test leads of the ohmmeter touch the two legs of the capacitor respectively, the battery in the ohmmeter will charge the capacitor, and the pointer will deflect. When charging is completed, the pointer returns to zero. Swap the red and black test leads, and the capacitor will charge in the reverse direction after discharge. The larger the capacitance, the greater the pointer deflection. By comparing the deflection of the measured capacitance with the known capacitance, the value of the measured capacitance can be roughly estimated. In general electronic circuits, except for the tuning circuit and other capacitors that require relatively accurate capacitance, the most commonly used DC blocking, bypass capacitors, filter capacitors, etc. do not require capacitors with accurate capacitance. Therefore, it is practical to roughly estimate the capacitance value with the ohm range. However, the ohm range of an ordinary multimeter can only estimate capacitance with larger measurement values. Capacitance with smaller measurement values ​​must be estimated using the ohm range of a transistor multimeter with a large median resistance. Capacitances less than a few dozen picofarads have to be measured using a capacitance tester.

Capacitive reactance: AC can pass through capacitors, but capacitors still have an obstruction effect on AC. The obstruction effect of capacitors on AC is called capacitive reactance. When the capacitance is large, AC can easily pass through the capacitor, which means that the capacitance is large and the obstruction effect of the capacitor is small; when the frequency of AC is high, AC can also easily pass through the capacitor, which means that the frequency is high and the obstruction effect of the capacitor is small. Experiments have shown that capacitive reactance is inversely proportional to capacitance and frequency. If capacitive reactance is represented by XC, capacitance is represented by C, and frequency is represented by f, then

The unit of capacitive reactance is ohm. Knowing the frequency f of the alternating current and the capacitance C, the capacitive reactance can be calculated using the above formula.

Inductance: Inductance is a physical quantity that measures the ability of a coil to generate electromagnetic induction. When a current is passed through a coil, a magnetic field is generated around the coil, and magnetic flux passes through the coil. The greater the power supplied to the coil, the stronger the magnetic field and the greater the magnetic flux passing through the coil. Experiments have shown that the magnetic flux passing through the coil is proportional to the current supplied, and their ratio is called the self-inductance coefficient, also called inductance. If the magnetic flux passing through the coil is represented by φ, the current is represented by I, and the inductance is represented by L, then

The unit of inductance is Henry (H), and millihenry (mH) or microhenry (uH) are also commonly used. 1H=1000mH, 1H=1000000uH.

Inductive reactance: Alternating current can also pass through the coil, but the inductance of the coil has an obstruction to the alternating current, and this obstruction is called inductive reactance. When the inductance is large, it is difficult for the alternating current to pass through the coil, which means that the inductance is large and the obstruction of the inductance is large; when the frequency of the alternating current is high, it is also difficult for the alternating current to pass through the coil, which means that the frequency is high and the obstruction of the inductance is also large. Experiments have shown that inductive reactance is proportional to inductance and frequency. If inductive reactance is represented by XL, inductance is represented by L, and frequency is represented by f, then

The unit of inductive reactance is ohm. If you know the frequency f of the alternating current and the inductance L of the coil, you can use the above formula to calculate the inductive reactance.

Impedance: The resistance to AC in a circuit with resistance, inductance and capacitance is called impedance. Impedance is usually represented by Z. Impedance is composed of resistance, inductance and capacitance, but it is not a simple addition of the three. If the three are connected in series, and the frequency f of AC, resistance R, inductance L and capacitance C are known, then the impedance of the series circuit is

The unit of impedance is ohm.
For a specific circuit, impedance is not constant, but changes with frequency. In a series circuit of resistors, inductors and capacitors, the impedance of the circuit is generally greater than the resistance. That is, the impedance is reduced to the minimum value. In a parallel circuit of inductors and capacitors, the impedance increases to the maximum value when resonating, which is the opposite of the series circuit.

Phase: Phase is a physical quantity that reflects the state of alternating current at any time. The magnitude and direction of alternating current change with time. For example, the formula for sinusoidal alternating current is i=Isin2πft. i is the instantaneous value of the alternating current, I is the maximum value of the alternating current, f is the frequency of the alternating current, and t is time. As time goes by, the alternating current can change from zero to the maximum value, from the maximum value to zero, from zero to the negative maximum value, and from the negative maximum value to zero, as shown in Figure 3A. In trigonometric functions, 2πft is equivalent to an angle, which reflects the state of the alternating current at any time, whether it is increasing or decreasing, positive or negative, etc. Therefore, 2πft is called phase, or phase.

Figure 3
If i is not equal to zero when t is equal to zero, the formula should be changed to i=Isin(2πft+ψ), as shown in Figure 3B. Then 2πft+ψ is called the phase, and ψ is called the initial phase, or the initial phase.

Phase difference: The difference between the phases of two alternating currents with the same frequency is called phase difference, or phase difference. The two alternating currents with the same frequency can be two alternating currents, two alternating voltages, two alternating electromotive forces, or any two of these three quantities.
For example, the phase difference between the alternating voltage applied to the circuit and the alternating current passing through the circuit is studied. If the circuit is a pure resistor, the phase difference between the alternating voltage and the alternating current is equal to zero. That is to say, when the alternating voltage is equal to zero, the alternating current is also equal to zero, and when the alternating voltage reaches its maximum value, the alternating current also reaches its maximum value. This situation is called the same phase, or the same phase. If the circuit contains inductance and capacitance, the phase difference between the alternating voltage and the alternating current is generally not equal to zero, that is, they are generally out of phase, or the voltage leads the current, or the current leads the voltage. The phase difference between the
alternating voltage applied to the base of the transistor amplifier and the alternating voltage output from the collector is exactly equal to 180°. This situation is called anti-phase, or anti-phase.