Basic concepts of electronic technology

Current: The directional movement of electric charge is called current. In the circuit, current is often represented by I. Current is divided into 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 choose a range where the ammeter pointer is close to full deflection. This can prevent the current from being too large and damaging the ammeter. Provided by: Basics of Analog Electronic Technology . Voltage: The reason why river water can flow is because of the water level difference; the reason why electric charge 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) are also commonly used as units. 1V=1000mV, 1mV=1000uV. Voltage can be measured with a voltmeter. When measuring, connect the voltmeter in parallel to the circuit and select the 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 the 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 often represented by R. The unit of resistance is ohm (Ω), and kilo-ohm (kΩ) or mega-ohm (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 with the ohm range of a multimeter. When measuring, select 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 signals is called a signal source. A transistor can amplify the signal sent from the front and transmit the amplified signal to the circuit behind. For the circuit behind, a 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. A motor can convert electrical energy into mechanical energy, a resistor can convert electrical energy into heat energy, a light bulb can convert electrical energy into heat energy and light energy, and a speaker can convert electrical energy into sound energy. Motors, resistors, light bulbs, speakers, etc. are all called loads. A transistor can also be regarded as a load for the signal source in front.
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 at a certain point is called a break or open circuit. A circuit that is directly connected at both ends, so that the voltage of this part becomes zero, is called a short circuit.
Electromotive force: Electromotive force is a physical quantity that reflects the ability of a power supply to convert other forms of energy into electrical energy. Electromotive force generates voltage at both ends of the power supply. In a circuit, electromotive force is often represented by δ. The unit of electromotive force is the same as that of voltage, which is also volts.
The electromotive force of a power supply can be measured with a voltmeter. When measuring, do not connect the power supply to the circuit. Use a voltmeter to measure the voltage at both ends of the power supply. The voltage value obtained can be regarded as equal to the electromotive force of the power supply. If the power supply is connected to the circuit (Figure 2), the voltage at both ends of the power supply measured by the voltmeter will be less than the electromotive force of the power supply. This is because the power supply 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 supply 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 supply is not connected to the circuit, the voltage across the power supply 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 supply 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 measured with a voltmeter. Sometimes it is still relatively high, but after being connected to the circuit, the load (radio, tape recorder, etc.) cannot work properly. 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 load is quite different, which is also caused by the large internal resistance of the power supply.
Cycle: The time required for AC to complete a complete change is called the cycle, which is usually represented by T. The unit of cycle is second (s), and milliseconds (ms) or microseconds (us) are also often used as units. 1s=1000ms, 1s=1000000us.
The frequency is the number of times that the alternating current completes a periodic change within 1s, and is usually represented by f. The unit of frequency is Hz, and kilohertz (kHz) or megahertz (MHz) is also commonly used as a unit. 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 electric charge.

When a certain voltage is applied to them, they will store a certain amount of electricity. One conductor stores positive charge, and the other

Conductors store equal amounts of negative charge. The greater the voltage applied, the more charge is stored.

The amount of electricity is proportional to the applied voltage, and their ratio is called capacitance. If voltage is represented by U and electric charge is represented by Q

The unit of capacitance is Farad (F), and microfarad (uF) or picofarad (pF) is also commonly used as the unit. 1F=10 6 uF, 1F=10 12 pF.
Capacitance can be measured with a capacitance tester, or roughly estimated with the ohmmeter 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 the charging is completed, the pointer returns to zero. When the red and black test leads are swapped, 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 and 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 ohmmeter. However, the ohm range of an ordinary multimeter can only estimate capacitance with a larger value. For capacitance with a smaller value, the ohm range of a transistor multimeter with a large median resistance must be used to estimate it. Capacitances less than a few dozen picofarads can only be measured with a capacitance tester.
Capacitive reactance: Alternating current can pass through a capacitor, but the capacitor still has an obstruction effect on alternating current. The obstruction effect of a capacitor on alternating current is called capacitive reactance. When the capacitance is large, alternating current 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 alternating current is high, alternating current can also easily pass through the capacitor, which means that the frequency is high and the obstruction effect of the capacitor is also 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 alternating current and 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 will be generated around the coil, and magnetic flux will pass 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) is also commonly used as a unit. 1H=1000mH, 1H=1000000uH.
Inductive reactance: Alternating current can also pass through the coil, but the inductance of the coil has an obstruction effect on 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 effect of the inductance is large; when the frequency of the alternating current is high, it is difficult for the alternating current to pass through the coil, which means that the frequency is high and the obstruction effect of the inductance is also large. Experiments have shown that inductive reactance is proportional to inductance and also proportional to frequency. If the inductive reactance is represented by XL, the inductance is represented by L, and the frequency is represented by f, then

the unit of the inductive reactance is ohm. Knowing the frequency f of the alternating current and the inductance L of the coil, the inductive reactance can be calculated using the above formula.
Impedance: In a circuit with resistance, inductance and capacitance, the resistance to the alternating current is called impedance. Impedance is usually represented by Z. Impedance consists of resistance, inductive reactance and capacitive reactance, but it is not a simple addition of the three. If the three are connected in series, and the frequency f of the alternating current, resistance R, inductance L and capacitance C are known, then the impedance of the series circuit The unit of impedance is ohm. For a specific circuit, the impedance is not constant, but changes with the frequency. In a series circuit of resistance, inductance and capacitance, 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 inductance and capacitance, 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 the alternating current at any time. The magnitude and direction of the 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 phase, and ψ is called initial phase, or initial phase. Phase difference: The difference between the phases of two alternating currents with the same frequency is called phase difference, or phase difference. These 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, study the phase difference between the AC voltage applied to the circuit and the AC current passing through the circuit. If the circuit is a pure resistor, the phase difference between the AC voltage and the AC current is zero. That is to say, when the AC voltage is zero, the AC current is also zero. When the AC voltage reaches its maximum value, the AC current also reaches its maximum value. This situation is called in-phase, or in-phase. If the circuit contains inductance and capacitance, the phase difference between the AC voltage and the AC 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 AC voltage applied to the base of the transistor amplifier and the AC voltage output from the collector have a phase difference of exactly 180°. This situation is called anti-phase, or anti-phase.