All electrical circuits have resistance. There is resistance to “resist” current in conductors, loads, and other components. There is an electronic component called a resistor, which can be used to control the current in a circuit or as a voltage divider. There are two types of resistors: fixed resistors and variable resistors.
A resistor is a component that can control current or be used as a voltage divider. The unit of measurement of resistance is ohms. The three most important parameters of resistors are resistance value, rated power and allowable deviation. Divided from the perspective of design and manufacturing, the resistor with a fixed resistance is a fixed resistor, and the resistor with an adjustable resistance is a variable resistor.
① Resistance and allowable deviation
The resistor can have different resistance values, such as 10Ω, 100Ω, 10100Ω, 68000Ω and 1000000Ω. The ohmic value of a small micro fixed resistor can usually be read through the color ring. Figure 3 shows a color ring resistor. The color rings of different colors represent values from 0 to 9. The table in Figure 4 lists each color and its value. The fixed resistor has 4 to 5 color circles.
For a 5-ring resistor, the first ring, the ring closest to the end, represents the first number. The second ring represents the second number. The third ring represents the third number, the fourth ring represents the multiple (that is, the number of zeros after the third digit), and the fifth ring is the error ring, which represents the allowable deviation value of the resistor.
For a 4-ring resistor, the first ring, the ring closest to the end, represents the first number. The second ring represents the second number. The third ring represents the multiple (that is, the number of zeros after the second digit), and the fourth ring represents the allowable deviation value of the resistor.
The resistor in Figure 5 has five color circles from left to right: red, purple, blue, red, and brown. What is its resistance? (Refer to the table in Figure 4.)
First ring: red=2
Second ring: Purple=7
Third ring: blue=6
The fourth ring: Red=2 (2 zeros after the third digit)
The fifth ring: brown (allowable deviation) = ± 1%
The resistance value is 276 plus 2 zeros, that is, R=27600Ω, and the allowable deviation is ±1%.
Therefore, the resistance range of this resistor is 27876-27324Ω, and the nominal value is 27600Ω.
The resistor in Figure 6 has four color circles from left to right, namely: orange, brown, red, and silver. What is its resistance? (Refer to the table in Figure 4.)
The first ring: orange=3
Second ring: brown=1
The third ring: Red=2 (2 zeros after the second digit)
Fourth ring: Silver (allowable deviation) = ±10%
The resistance value is 31 plus 2 zeros, that is, R=3100Ω, and the allowable deviation is ±10%.
Therefore, the resistance value ranges from 3410 to 2790Ω, and the nominal value is 3100Ω.
The rated power of the resistor is the maximum allowable power, and this parameter indicates the maximum heat that the resistor can safely withstand. The heat will be lost to the air flowing around the resistor, and the heat dissipation condition is related to the surface area of the resistor. In operation, the actual power dissipation of the resistor can be calculated using any of the following formulas:
If the actual working power of the resistor calculated by the above formula is less than its rated power, then the resistor will not be burned out.
You can see its rated power from the specifications of the resistor, and you can also estimate the rated power of a carbon composite resistor by its dimensions. The approximate size is:
1/4W resistor: diameter=3/32in, length=1/4in
1/2W resistor: diameter=1/8in, length=3/8in
1W resistor: diameter=1/4in, length=1/2in
2W resistor: diameter=3/8in, length=5/8in
The rated power of the resistor in Figure 7 is 0.25w, the resistance value is 2000Ω, and the current through the resistor is 0.015A. Will the resistor be burned out?
Use the formula P=I2×R to calculate:
P=(0.015 x0.015) x 2000W =0.45W
Because the rated power is only 0.25W, this resistor will be burned out.
(Note: The rated power is the maximum power that the resistor can work safely, not the power that the resistor must reach.)
Fixed resistors are widely used in the electrical and electronic industries. Figure 8 shows the structure of a fixed resistor, and its resistance value is fixed. The fixed resistor can be made of a variety of chemical materials, and the material used depends on the specification requirements such as allowable deviation, power and reliability. The most common type of resistor is a carbon composite resistor, which is made of carbon graphite material and adhesive resin. The error range of carbon composite resistors is usually 5% to 20%, and this value will drift with component aging and temperature changes.
Fixed resistors are also made using metal film and metal glaze processes. This type of resistor has some advantages over carbon composite resistors. Its resistance is less affected by component aging and temperature changes, and the error is lower, usually 1% to 2%.
The variable resistor is designed so that the resistance of the resistor can be adjusted and changed. This kind of resistor is composed of a brush arm and a resistance circuit in contact with it. When the brush arm rotates, the resistance value between the corresponding contacts will change.
Resistors are usually used to limit current or as voltage dividers. In Figure 11, the voltage E is 12V and the resistance R is 2Ω. According to Ohm’s law, I=6A (E/R=122). If E remains constant and R increases to 10Ω, the current in I decreases to 1.2A (E/R=12/10). The current of the circuit will decrease as the resistance increases, or increase as the resistance decreases.
Figure 12 shows a voltage divider. Early radios used this circuit to obtain different voltages from the output of a main battery, and the new voltage could be used to power electronic tubes and other circuits.
In Figure 12, assuming that the power supply voltage E=100V, if the voltmeter is connected between points a, b, c, d, the following readings will be obtained. The calculation formula is as follows:
In the formula, VX is the measured voltage; RT is the sum of resistances; ES is the power supply voltage.
Va-b=(Ra/RT)×ES=(100/600)×100V = 16.67V
These voltages can be “taken out” from the different terminals of the voltage divider and supplied to other circuits. Please note that the sum of the total voltage drop (the sum of the voltages across each resistor) is equal to the power supply voltage.