I. Resistors and their principles
A resistor typically consists of a series of resistive elements with equal resistance values, each varying in material, shape, length, and resistivity. In a DC circuit, when a voltage is applied across the resistor, electrons drift within the resistive elements, creating a current. According to Ohm's law, the voltage across the resistor and the current flowing through it can be expressed by the following formulas:
V = I × R
Where V represents voltage, I represents current, and R represents resistance. The larger the resistance R of a resistor, the more it restricts the current flowing through it, and the higher the voltage across it will be. Conversely, when the resistance is small, it allows a larger current to flow, and the voltage across it will be lower.
Besides limiting current, resistors have other applications. For example, connecting a resistor in a circuit can act as a voltage divider, distributing voltage to different parts of the circuit to achieve control.
In addition, resistors can also adjust circuit gain and improve circuit write speed. In some electronic devices, such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), resistors have become indispensable components.
In short, although resistors may seem simple, their role is crucial. Only by understanding how resistors work can they be better applied in electronic circuit design.
II. The function of resistors
1. Voltage divider
When a resistor and another component (such as a light bulb) are connected in series in a circuit, the current flowing through the resistor and the light bulb is the same. When the resistor and the light bulb are considered as a whole, the sum of the voltages across the resistor and the light bulb is equal to the total voltage across them. In this case, the resistor acts as a voltage divider.
2. Diversion
The voltage across the resistor is the same as the voltage across the bulb because when a resistor and another component are connected in parallel in a circuit, the sum of the current flowing through the resistor and the current flowing through the bulb equals the total current flowing through both components. In this case, the resistor acts as a shunt.
3. Impedance matching
Impedance matching refers to the process of adapting the load impedance and the internal impedance of the excitation source to achieve the operating state of maximum power output during signal transmission. One method is to achieve this by changing the impedance force, in which case the resistance acts as the impedance matching function.
4. Filtering
In an RC charging and discharging circuit with a resistor and capacitor connected in series, switch S is initially connected to point B, as shown in the diagram. Capacitor C is not charging, and its voltage is zero. Then, switch S is connected to point a. At this point, the power supply begins charging the capacitor through resistor R. When the charge across the capacitor reaches the circuit's equilibrium, the power supply stops charging the capacitor. Switch S is then returned to point B. As the capacitor begins to discharge, the charge across its terminals gradually decreases until it reaches zero, at which point it stops discharging. Then, switch S is returned to point a, and the charging and discharging cycle begins indefinitely. We call the function of resistor R the filtering function.
5. Special resistors
Specialty resistors are made of semiconductor materials, and their resistance values are also variable. However, unlike variable resistors, whose resistance can be manually adjusted, the resistance of specialty resistors depends on external environmental conditions. For example, the resistance of a thermistor decreases as temperature increases. In circuit applications, its resistance value can be used to determine temperature, serving as a thermal sensor. The resistance of a photoresistor decreases significantly in the presence of light, making it suitable as a light switch in a circuit. The resistance of a variable resistor changes with the voltage across its terminals, providing protection in circuits against voltage fluctuations. Superconducting elements are the most special type of resistor; they do not generate heat when conducting electricity, eliminating the need for cooling systems in computer component manufacturing, thus significantly reducing the size and energy consumption of computers.
