From an energy perspective, a resistor is essentially an energy-consuming component. It creates an obstacle to hinder the movement of electrons and generates heat through constant friction between them.
In Ohm's law, we define resistance as the amount of current that can flow under a constant voltage, which is R = U / I.
If we use Joule's law to understand this, it can be described as the amount of heat generated per unit time when a current flows through a resistor.
Resistance (usually represented by "R") is a physical quantity in physics that represents the degree to which a conductor impedes the flow of electric current. The greater the resistance of a conductor, the greater its ability to impede the flow of current. Different conductors generally have different resistances; resistance is an intrinsic property of the conductor. The resistance of a conductor is usually represented by the letter R, and the unit of resistance is the ohm, abbreviated as Ω.
I. Basic Principles of Resistance
Resistance, along with capacitance, is known as one of the three fundamental "particles" in the field of electronics. Like the magical gems in the Avengers, they form a rich and magical electronic world.
From an energy perspective, a resistor is essentially an energy-consuming component. It creates an obstacle to hinder the movement of electrons and generates heat through constant friction between them.
In Ohm's law, we define resistance as the amount of current that can flow under a constant voltage, which is R = U / I.
If we use Joule's law to understand this, it can be described as the amount of heat generated per unit time when a current flows through a resistor.
The figure below shows the equivalent model of an actual resistive element.
Real resistors are not ideal; they have lead inductance and inter-electrode capacitance. When used in high-frequency applications, these factors cannot be ignored.
Frequency characteristics of thin film resistors
The above figure shows the frequency response curve of a thin-film resistor. As you can see, this resistor has excellent high-frequency characteristics. Its inter-electrode capacitance is only 0.0262pF and its lead inductance is only 0.00189nH. The 75Ω resistor is still stable at a frequency of 30GHz.
Of course, this kind of thin film resistor must be made by a special process. The chip resistors we usually use are mainly thick film resistors, and their performance is far from the high-frequency characteristics shown in the figure above. The lead inductance of thick film resistors is usually a few nH, and the inter-electrode capacitance between the two leads is a few pF. Therefore, most thick film resistors can only be used in the range of a few hundred MHz to GHz.
II. Standards for Resistance Values
The resistors we select in circuit design follow certain standards, mainly for the convenience of production and design. We cannot just choose any resistor, such as 5.28Ω.
It's not that we can't produce it, but doing so would lead to scattered market demand and a trend towards customization, making it impossible to manufacture and supply in large quantities.
Therefore, the international IEC has established a standard for resistance values and tolerances.
Please note the following three points:
Resistors with different precision correspond to different precision series. Generally, 10% precision is the E12 series, 2% and 5% are the E24 series, 1% is the E96 series, and 0.1%, 0.25% and 0.5% are the E192 series.
The numbers in the series name indicate how many standard resistance values the series has, and are usually multiples of 6. For example, the E12 series has 12 different resistance values, and the E192 series has 192 different resistance values.
The resistance values of each series are approximately a geometric series with a common ratio of 10 to the power of a certain number and a base of 10Ω. For example, the common ratio of the E12 series is 10 to the power of 12, and the common ratio of the E96 series is 10 to the power of 96.
Those who are interested can count according to the table above and see if it follows the pattern described above.
III. Resistance Marking
In our design, we mostly choose surface mount resistors with a precision of 5% and 1%. Generally, resistor packages of 0603 and above will have markings for the corresponding resistance values. Let's first understand the meaning of these markings.
E24 series (5% accuracy)
For resistance values greater than 10Ω, the resistance value is usually represented by 3 digits. The first two digits represent the base resistance value, and the last digit represents the power of 10.
For example, 100 represents 10Ω, not 100Ω, and 472 represents 4.7kΩ. Amounts less than 10Ω are usually represented by the decimal point R, such as 2R0, which represents 2Ω.
E96 series (1% accuracy)
The E96 series is usually represented by two digits and one letter. The two digits represent which resistance value in the E96 series, and the letter represents the power of 10. Y represents -1, X represents 0, A represents 1, B represents 2, C represents 3, and so on.
The above describes the method of marking resistance values for surface-mount resistors. For resistors with axial leads, also known as through-hole resistors, the resistance value is indicated by the color of the color rings on the resistor; therefore, they are also called color-coded resistors. Currently, except for some ultra-high power applications that require color-coded resistors, they are rarely seen on the market due to the continuous increase in circuit density.
The resistance value of a color-coded resistor is marked by a series of colored rings.
From left to right, the first two or three rings represent numbers, the next ring represents the multiplier, and multiplying it by the previous number gives the resistance value. The next ring represents the resistance tolerance, and the last one is the temperature coefficient of the resistor.
IV. Resistor Manufacturing Process and Structure
Here we will only talk about fixed resistors, which are the fixed resistance values that we use most often in our designs. These resistors are currently divided into two categories in design selection: one is the axial lead type, which is the through-hole resistor with color ring markings that we mentioned, and the other is the chip resistor.
1 Axial lead resistance
The axis of this type of axial lead resistor is usually cylindrical, and the two external electrodes are axial wires at both ends of the cylinder.
Further differentiation based on materials and manufacturing processes allows for a more detailed classification into wire-wound resistors.
Wire-wound resistor
Wire-wound resistors are made by winding nickel-chromium alloy wires around an alumina ceramic substrate, and the resistance is controlled by the number of turns.
The wire-wound resistors have high precision and a tolerance of 0.005%.
The low temperature coefficient of resistance in wire winding is mainly due to the nickel-chromium alloy material.
The wire-wound resistance has a large parasitic inductance, which can be seen from the winding method, so it cannot be used in high-frequency scenarios.
Wire-wound resistors can be used to make high-power resistors.
Carbon synthesized resistor
Carbon-based resistors are mainly made by sintering carbon powder and binder together into a cylindrical resistor body. The concentration of carbon powder determines the resistance value. Tin-plated copper leads are added to both ends, and finally, the resistor is packaged.
Carbon-based resistors are simple to manufacture and the raw materials are readily available, making them the cheapest option and the color-coded resistors we commonly use.
1. Carbon-synthetic resistors have poor performance, with an accuracy of only 5% and 1%, which is sufficient for most applications.
2. Carbon-synthesized resistors have poor temperature characteristics, resulting in relatively high noise levels, making them suitable for low-end consumer products. They can be used by children for experiments.
3. Carbon-synthetic resistors have high voltage withstand capability, so since they contain a carbon rod, they are unlikely to burn out.
carbon film resistor
Carbon film resistors are mainly formed by forming a carbon mixture film on a ceramic rod.
For example, a layer can be directly coated on a ceramic rod, where the thickness of the carbon film and the carbon concentration can control the resistance value. This method is exactly the same as that used in many catalytic combustion gas sensors.
