I. Basic Principles of Resistance
Resistors, along with inductors and capacitors, are the three basic passive components in electronics. From an energy perspective, a resistor is an energy-consuming component that converts electrical energy into heat energy.
Several years ago, a fourth type of fundamental passive device emerged, called a memristor, which represents the relationship between magnetic flux and electric charge. There's a lot of information about it in XX Library; you can look into it if you're interested.
Image from Wikipedia page Memristor
Resistance is usually defined according to Ohm's law, which measures how much current is generated when a constant voltage is applied to a resistor; alternatively, it can be defined according to Joule's law, which measures how much heat is generated per unit time when a current flows through a resistor.
Equivalent model of actual resistance
Similarly, real resistors are not ideal, and have a certain lead inductance and inter-electrode capacitance. When the application frequency is high, these factors cannot be ignored.
Frequency characteristics of a thin film resistor
The resistor in the image above exhibits excellent high-frequency characteristics. The inter-electrode capacitance is only 0.03pF, and the lead inductance is only 0.002nH. The 75Ω resistor can operate up to 30GHz. Most surface-mount resistors we commonly use are thick-film resistors, whose performance falls far short of this. Their lead inductance is several nH, and their inter-electrode capacitance is several pF, limiting their usability to only a few hundred MHz or a few GHz.
Standard resistance table
The image above is from the Vishay documentation.
Resistor values are usually standardized. The image above shows the standard resistance values for resistors with different tolerances. You can usually get all the resistance values by multiplying or dividing by a multiple of 10.
How do you memorize the resistance table above? It's actually quite simple; just pay attention to 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 interested can count and calculate according to the table above to see if the pattern described above applies. Additionally, according to IEC standards, 2% accuracy corresponds to 48 resistance values in the E48 series; those interested can calculate which values these are. Vishay may no longer produce this series, as shown in the table above.
Resistance marking
The most commonly used types are 5% and 1% chip resistors. Generally, resistors with a resistance value of 0603 or higher have markings on the package indicating the resistance value.
E24 series (5%)
For resistances greater than 10Ω, a three-digit number is typically used to represent the resistance value. The first two digits indicate the base resistance, and the last digit indicates the power of 10. For example, 100 represents 10Ω, not 100Ω, and 472 represents 4.7kΩ. For resistances less than 10Ω, the decimal point is usually indicated by 'R', such as 2R2, which represents 2.2Ω.
E96 series (1%)
Typically, resistance values are represented by two digits and one letter. The two digits indicate which resistance value in the E96 series it is, and the letter indicates the power of 10. Y represents -1, X represents 0, A represents 1, B represents 2, C represents 3, and so on. For example, 47C, counting to the 47th resistance value in the table, is 30.1. C represents multiplying by 10 to the power of 3, which is 30.1kΩ.
Additionally, for resistors in axial lead packages, the resistance value is marked with a series of color rings, the specific meaning of which is shown in the following figure:
Resistance value color ring code
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.
II. Resistor Manufacturing Process and Structure
There are many types of resistors, which can be divided into two main categories based on whether their resistance value can be varied:
Fixed resistor
Variable resistor
2.1 Fixed resistor
Fixed resistors, as the name suggests, have a resistance value that is constant and cannot be changed. Most of the time, the resistors we use have fixed values. They can be further classified according to their package type.
2.1.1 Axial Lead Resistance
Axial lead resistors are usually cylindrical, with two external electrodes being axial wires at both ends of the cylinder. They can be further divided into various types depending on the materials and processes used.
Wirewound resistor
Wire-wound resistors are made by winding nickel-chromium alloy wire around an alumina ceramic substrate, controlling the resistance with each turn. Wire-wound resistors can be made into precision resistors with tolerances down to 0.005%, and have a very low temperature coefficient. The disadvantage is that they have relatively high parasitic inductance, making them unsuitable for high-frequency applications. Wire-wound resistors can be made quite large, and with the addition of an external heatsink, they can be used as high-power resistors.
Carbon Composition Resistor
Carbon-synthetic resistors are primarily made by sintering carbon powder and binder together into a cylindrical resistive element. The concentration of carbon powder determines the resistance value. Tinned copper leads are added to both ends, and finally, the resistor is encapsulated. Carbon-synthetic resistors have a simple manufacturing process and readily available raw materials, making them the cheapest option. However, their performance is not very good; they have a relatively large tolerance (meaning they cannot be made into precision resistors), poor temperature characteristics, and typically high noise levels. Carbon-synthetic resistors have good voltage withstand performance, and because their internal structure can be considered as a carbon rod, they are unlikely to be broken down and burn out.
Carbon film resistor
Carbon film resistors are primarily made by forming a carbon mixture film on a ceramic rod, such as by directly coating a layer. The thickness and carbon concentration of the carbon film can control the resistance. For more precise resistance control, spiral grooves can be machined into the carbon film; the more spirals, the higher the resistance. Finally, metal leads are added, and the film is encapsulated in resin. The manufacturing process of carbon film resistors is more complex and can be used to make precision resistors, but due to the nature of carbon, their temperature characteristics are not ideal.
Metal film resistor
Similar to carbon film resistors, metal film resistors primarily utilize vacuum deposition technology to form a nickel-chromium alloy coating on a ceramic rod. Spiral grooves are then machined into this coating to precisely control the resistance. Metal film resistors are considered high-performance resistors, offering high precision (e.g., the E192 series), good temperature characteristics, low noise, and greater stability.
Metalfilm resistor
Metal oxide film resistor
The image above is from Metal Oxide Film Resistor.
Similar to metal film resistors, metal oxide film resistors primarily consist of a tin oxide film formed on a ceramic rod. To increase resistance, an antimony oxide film can be added on top of the tin oxide film, and then spiral grooves are machined into the oxide film to precisely control the resistance. The biggest advantage of metal oxide film resistors is their high-temperature resistance.
The image above is from Metal Oxide Film Resistor.
2.1.2 Chip Resistors
Metal Foil Resistor
Metal foil resistors are made by vacuum melting a nickel-chromium alloy, then rolling it into a metal foil, bonding the foil to an alumina ceramic substrate, and finally using photolithography to control the shape of the foil, thereby controlling the resistance. Metal foil resistors currently offer the best controllable resistance performance.
Thick film resistor
Thick-film resistors utilize screen printing, which involves attaching a layer of silver palladium electrode to a ceramic substrate and then printing a layer of ruthenium dioxide between the electrodes as the resistive element. The resistive film of a thick-film resistor is typically quite thick, approximately 100 micrometers. The specific process flow is shown in the figure below.
Thick film resistors are currently the most widely used type of resistor. They are inexpensive and have tolerances of 5% and 1%. The vast majority of products use 5% and 1% chip thick film resistors.
http://www.vishay.com/docs/28893/usinglasertrimmableresistors.pdf
Thin Film Resistor
Thin-film resistors are formed by vacuum deposition of a chromium nickelate film on an alumina ceramic substrate. They are typically only 0.1µm thick, just one-thousandth the thickness of thick-film resistors. The film is then etched into a specific shape using photolithography. Thin-film technology has been mentioned many times in previous articles on capacitors and inductors. Photolithography is a highly precise process that can create complex shapes, thus allowing for excellent control over the performance of thin-film capacitors.
The image above is from Panasonic Chip Resistors.
2.2 Variable Resistor
A variable resistor is one whose resistance value can change. There are two types: one is a resistor whose resistance value can be manually adjusted; the other is a resistor whose resistance value can change according to other physical conditions.
2.2.1 Adjustable resistor
In middle school, most of us probably used sliding rheostats for experiments. By moving the rheostat, a light bulb could be made brighter or dimmer. A sliding rheostat is simply an adjustable resistor; the principle is the same.
Adjustable resistors are generally divided into three types:
Potentiometer
A potentiometer, or voltage divider, is a three-port device. A potentiometer is divided into two resistors by a center tap; by changing the resistance of these two resistors through the center tap, the voltage across the potentiometer can be changed.
Rheostat
A rheostat is essentially a potentiometer. The only difference is that a rheostat only requires two ports, essentially a resistor whose resistance value can be precisely adjusted.
Trimmer
A fine-tuner is actually a potentiometer, but it doesn't need to be adjusted frequently. For example, it only needs to be adjusted at the factory. Usually, it requires special tools such as a screwdriver to adjust.
2.2.2 Sensitive Resistor
A sensitive resistor is a type of sensitive element. These resistors are mostly particularly sensitive to certain physical conditions; when the physical condition changes, the resistance value changes accordingly. They are commonly used as sensors, such as photoresistors, humidity sensors, and magnetoresistors. Thermistors and varistors are the most commonly used in circuit design and are often used as protective devices.
Thermistor
The above image is from Murata Application Manual-PTC
PTC stands for Positive Temperature Coefficient resistor. There are generally two types: one is made of ceramic material, called CPTC, which is suitable for high voltage and high current applications; the other is made of polymer material, called PPTC, which is suitable for low voltage and low current applications.
Ceramic PTCs use a polycrystalline ceramic as their resistive material, which is a mixture of barium carbonate, titanium dioxide, and other materials sintered together. PTCs exhibit a highly non-linear temperature coefficient; when the temperature exceeds a certain threshold, the resistance becomes very high, effectively breaking the circuit and thus providing short-circuit and overcurrent protection.
There are also negative temperature coefficient resistors, i.e., NTC, which will not be discussed in detail.
Varistor
The image above is from Varistor and the Metal Oxide Varistor Tutorial
Varistors are typically metal oxide variable resistors (MOVs), whose resistive material is a mixture of zinc oxide particles and ceramic particles sintered together. The characteristic of MOVs is that their resistance drops rapidly when the voltage exceeds a certain threshold, allowing them to handle large currents. Therefore, they can be used for surge protection and overvoltage protection.
Zinc oxide ceramic is fabricated into multilayer varistors, or MLVs, using a process similar to that used for MLCCs. MLVs have smaller packages, typically on a chip, and their rated voltage and current carrying capacity are much lower than those of MOVs, making them suitable for low-voltage DC applications.
III. Application and Selection of Resistors
The main manufacturers of resistors include Yageo, Panasonic, Rohm, Vishay, and domestic companies such as Fenghua Advanced Technology.
3.1 Application of Resistors
Virtually no circuit board operates without resistors; capacitors and resistors are the most frequently used components on any circuit board. Various pull-up/pull-down resistors, feedback resistors, and so on. My knowledge is limited, so I'll just give a brief overview.
thermal effect
According to Joule's law, an electric current flowing through a resistor will generate heat. The heating effect of resistance has many applications, including electric blankets, electric heaters, and electric kettles.
For some electronic devices used in outdoor applications, especially SOCs with integrated high-performance CPUs, the operating temperature requirements are very stringent. Most can only meet commercial-grade application needs. In the dead of winter in Northeast China, where temperatures can drop to below -30 degrees Celsius, the devices may not even be able to power on. Typically, a high-power resistor is added for preheating. Once the temperature rises, the device starts up and is then turned off. This is because the device's own power consumption generates heat, which helps maintain the desired temperature.
As a hardware engineer, I often have to go to the environmental lab to locate problems. To reproduce a high-temperature issue, I need to go to the environmental lab to set up the test environment. The key is that there are only a few temperature chambers, and you have to make appointments, which is often inconvenient due to long queues. So I made a very simple problem-solving tool: I soldered a DC power connector to a cement resistor, plugged in various power adapters, and adjusted the temperature. Then I placed it on a certain chip for a few minutes. If there was no problem, I tried another chip, and the problem was reproduced. The issue was focused on a specific chip, and I was able to locate the high-temperature problem right at my workstation.
Zero Ohm Resistance
A zero-ohm resistor, also called a jumper resistor, is frequently used in circuit design for ease of debugging or compatibility purposes. For example, during preliminary design work, to test the operating current of each power supply group of the chip during debugging, a zero-ohm resistor is typically used to split the power supply into multiple paths.
When using zero-ohm resistors, the most common problem is how to calculate power consumption and how to determine whether the selected resistor meets the requirements.
At this point, it's necessary to obtain the relevant parameters from the resistor's datasheet. As shown in the diagram below, the resistance value of a zero-ohm RC0402 resistor will not exceed 50mΩ, and the rated current will not exceed 1A. This allows us to determine whether the resistor meets the design requirements. Typically, a zero-ohm RC0402 resistor can meet current requirements below 1A.
Original image from GENERALPURPOSECHIPRESISTORS-Yageo
Traffic limiting
Sometimes a circuit needs a power supply of tens of milliamps, but its voltage is not used elsewhere in the circuit. In this case, using a separate DC-DC converter or LDO is not suitable because the current is too small. In such cases, a Zener diode voltage regulator circuit can be used.
partial pressure
Voltage dividers are used in applications such as ADC sampling circuits, DC-DC output voltage feedback, and level conversion.
Matching resistor
For high-speed signals, PCB routing needs to consider the transmission line model and ensure impedance matching to prevent signal reflection from affecting signal integrity. Impedance matching ensures that the load impedance is equal to the characteristic impedance of the transmission line to eliminate reflection. The most common and simplest method is source-end series matching, which involves connecting a resistor in series at the signal source. The sum of this resistor and the source's internal resistance equals the characteristic impedance of the transmission line. This way, even if there is a mismatch at the load end, the reflected signal will be received at the source and will not be reflected again.
In addition, there are various nonlinear resistors that can be used as sensors, protection circuits, and so on.
3.2 Resistor Selection
Simply put, selection involves extracting key parameters from the device's datasheet and determining whether they meet the application requirements.
3.2.1 Fixed value resistor
The following figure shows a comparison of the main parameters of common types of resistors. Thick film resistors and metal film resistors are the most shipped types.
3.2.2 Thermistor
The main function of a PTC in a circuit is similar to that of a fuse: overcurrent protection. The difference is that a fuse is disposable, while a PTC is resettable. Replacing a fuse is often unacceptable and negatively impacts customer experience. PTCs are also classified as safety devices and typically require UL1439 certification.
The above diagram shows the impedance-temperature characteristic of a PTC. When an overcurrent occurs, the PTC heats up, its temperature rises rapidly, and its impedance increases rapidly, forming an open circuit. After the circuit is broken, the current decreases, the heat generation decreases, the temperature drops, and the PTC returns to its low impedance. Therefore, PTCs are very suitable for short-time overcurrent.
Maintaining current
When selecting a PTC, the first consideration should be the design operating current, which must not exceed the PTC's holding current. Under these conditions, the PTC can maintain a low impedance state. The PTC's holding current decreases as the operating temperature increases; therefore, operating temperature is an important factor to consider.
Operating current
Operating current is the current that the PTC turns into a high-impedance state and provides circuit breaking protection.
Rated voltage
This refers to the maximum voltage a PTC can withstand. Exceeding the rated voltage may cause the PTC to break down and short-circuit, leading to burnout. Therefore, during the design phase, it is essential to ensure that the PTC's operating voltage does not exceed its rated voltage under various conditions.
When the PTC circuit breaker trips, it will withstand the entire power supply voltage. Therefore, when selecting a PTC, the rated voltage must be greater than the power supply voltage. Typically, derating to 80% is considered, meaning for a power supply voltage of 12V, a PTC with a withstand voltage of 15V or higher should be selected.
At the power input port, surge protection needs to be considered. In this case, the maximum surge current should be multiplied by the resistance of the PTC, i.e., the surge voltage that the PTC can withstand should not exceed the rated voltage of the PTC.
Rated current
This refers to the maximum short-circuit current that a PTC can withstand under rated voltage. If the short-circuit current exceeds the rated current, the PTC will be damaged.
DC resistance
The presence of the PTC's DC resistance will cause a certain DC voltage drop in the PTC. When designing, it is important to ensure that the power supply voltage after the voltage drop meets the requirements.
Compared to fuses, PTCs have much lower rated voltage and current, and their DC impedance is typically about two-thirds that of fuses. When a PTC is in operation, it operates under high resistance, resulting in milliampere-level leakage current. In contrast, a fuse, by melting and cutting off the current path, essentially eliminates leakage current.
3.2.3 Varistors
Varistors have similar characteristics to Zener diodes and TVS diodes; they are all clamping devices mainly used to protect circuits from transient overvoltages, such as surges.
Ideal I-V characteristic of MOV
When selecting protective devices, two main aspects need to be considered: first, the protective device must not operate or be damaged under normal working conditions; second, it must be able to protect the circuit under abnormal conditions within the design scope, i.e., protection capability.
Rated operating voltage
The rated operating voltage can be considered as the highest continuous operating voltage at which an MOV can maintain a high impedance state. Depending on the application, MOVs can be divided into AC and DC types, and the specifications used in these two types are different. MOVs designed for DC applications are generally not suitable for AC applications.
For MOV rated operating voltage, AC applications should consider the AC rated voltage, i.e., Vrms or Vm(ac). The device in the diagram above can operate normally in AC with an effective value of 130V. Exceeding this voltage may cause the MOV to activate or be damaged, resulting in the circuit malfunctioning.
It is mainly used to protect against transient high voltage; continuous excessive voltage can damage the MOV.
Clamping voltage
MOVs are clamping devices. When encountering transient high voltage, their impedance decreases, allowing a large current to flow through, suppressing the transient high voltage. However, the voltage does not drop to zero; it remains relatively high, typically 2 to 3 times the rated operating voltage. When selecting an MOV, it is crucial that the clamping voltage does not exceed the maximum withstand voltage of the protected device. If it does, multi-stage protection is required. For example, a high-power resistor can be added for decoupling, followed by a TVS diode to further reduce the residual voltage using the TVS's low clamping voltage.
Maximum pulse current
Lightning strikes or inductive load switching can generate large inrush currents. In addition to clamping the high voltage, MOVs also need to discharge the inrush currents.
Whether an MOV can withstand surge current depends primarily on the amount of energy it absorbs over a given period. Excessive energy can cause the MOV to overheat and burn out. The energy level is related to the waveform and number of surge pulses. Typically, the surge capability of devices is tested using an 8/20µs waveform. The MOV in the diagram above can withstand a single 3500A 8/20µs surge pulse, two consecutive 3000A 8/20µs surge pulses, and twenty consecutive 750A 8/20µs surge pulses.
Furthermore, MOVs have relatively large parasitic capacitance, making them unsuitable for high-speed signal lines. MOVs also have a slower response time than TVS diodes, potentially rendering them ineffective against fast pulses like ESD. These are also factors we need to consider.
