The basic principle of resistance
Resistance, along with inductors and capacitors, is the three basic passive devices of electronics; from the perspective of energy, resistance is an energy-consuming element that converts electrical energy into thermal energy.
A few years ago, a fourth fundamental passive device appeared, called the Memristor, which represents the relationship between magnetic flux and charge. There is also a lot of information in the XX library, and you can learn about it if you are interested.
Usually, the resistance is defined according to Ohm's law, and how much current is generated by adding a constant voltage to the resistor; it can also be defined by Joule's law as how much heat is generated per unit of time when the resistor flows through a current.
Equivalent model of the actual resistance
Similarly, the actual resistance is not ideal, there is a certain lead inductance and inter-pole capacitance, when the application frequency is high, these factors can not be ignored.
The frequency characteristics of a thin film resistor
The high-frequency characteristics of the resistors in the figure above are very good, and it can be seen that the interpolar capacitance is only 0.03pF, and the lead inductance is only 0.002nH, where the 75Ω resistance can reach 30GHz. Most of the chip resistors we usually use are thick film resistors, and the performance is far from this, and the lead inductance has several nHs, and the inter-pole capacitors also have several pFs, most of which can only be used for a few hundred MHz or a few GHz.
Standard resistance table
Source Vishay documentation
Usually the resistance values are standard, the figure above shows the standard resistance of resistors with different accuracies (tolerances), usually multiplied by a multiple of 10 or divided by a multiple of 10 to get all the resistance values.
How do I remember the above resistance table? In fact, it is also very simple, pay attention to the following three points:
Resistors of different accuracy correspond to series with different accuracy. Typically 10% accuracy is the E12 series, 2% and 5% are the E24 series, 1% are the E96 series, and 0.1%, 0.25% and 0.5% are the E192 series.
The number in the series name represents that the series has several standard resistances, 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 of each series is approximately an equal ratio series, with a common ratio of 10 how many powers and a cardinality of 10 Ω. For example, the common ratio of the E12 series is 10 to the 12th power, and the common ratio of the E96 series is 10 to the 96th power.
Those who are interested can count according to the above table, and calculate whether it is the above law. In addition, according to the iec regulations, the 2% accuracy corresponds to the E48 series has 48 resistance values, interested can calculate which values. In the table above, Vishay may not be producing the series anymore.
Marking
Usually we use the most is 5% and 1% chip resistance, generally more than 0603 resistance packages have marks on the resistance value.
E24 Series(5%)
For values greater than 10Ω, there are usually 3 digits representing the resistance, the first two representing the resistance base, and the last digit representing several squares multiplied by 10. For example, the marker 100 represents 10Ω instead of 100Ω, and 472 represents 4.7kΩ. Less than 10Ω is usually used as R to represent the decimal point, such as 2R2, which means 2.2Ω.
E96 Series(1%)
It is usually represented by 2 digits plus a letter, 2 digits representing the first few resistance values of the E96 series, and the letters representing several squares multiplied by 10, where Y stands for -1, X stands for 0, A stands for 1, B stands for 2, C stands for 3, and so on. For example, 47C, from the number of the table to 47 resistance values, is 30.1, and C represents the 3th power multiplied by 10, which is 30.1kΩ.
In addition, for the resistors of the axial lead package, the resistance marks are all circle-by-circle color rings, the specific meaning is shown in the following figure:
Resistance color ring code
From left to right, the first two or three rings represent the number, the next ring represents the multiplier, and multiplying by the previous number is the resistance. The next ring represents the tolerance of the resistor, and finally the temperature coefficient of the resistor.
The process and structure of the resistor
There are many types of resistor processes, which can be divided into two categories according to whether the resistance value can be changed:
Fixed resistor
Variable resistance
2.1 Fixed resistor
Fixed resistance, as the name suggests, is that the resistance value is fixed and immutable. Most of the time, the resistors we use are fixed. It can be roughly reclassified according to different packages
2.1.1 Axial Leaded Resistors
The axial lead resistance is usually cylindrical, and the two outer electrodes are the axial guide lines at both ends of the cylinder, which can be divided into a variety according to the different materials and processes.
Wire Wound Resistor
The wire resistance is to wind the nickel-chrome alloy wire on the alumina ceramic substrate and control the size of the resistance in a circle. The wirewound resistance can be made into a precision resistor, the tolerance can be up to 0.005%, and the temperature coefficient is very low, the disadvantage is that the parasitic inductance of the winding resistance is relatively large and cannot be used for high frequencies. The volume of the wirewound resistor can be made very large, and then an external heat sink is added, which can be used as a high-power resistor.
Carbon Composition Resistor
Carbon synthesis resistance is mainly composed of carbon powder and adhesive together sintered into a cylindrical type of resistor, in which the concentration of carbon powder determines the size of the resistance value, tinned copper leads are added at both ends, and finally encapsulated and formed. The carbon synthesis resistance process is simple, and raw materials are easily available, so the price is the cheapest. However, the performance of carbon synthesis resistors is not very good, the tolerance is relatively large (that is, precision resistance cannot be done), the temperature characteristics are not good, and the noise is usually relatively large. Carbon synthesis resistance has better withstand voltage performance, because the interior can be regarded as a carbon rod, basically will not be broken down and burned.
Carbon Film Resistor
Carbon film resistance is mainly in the ceramic rod to form a layer of carbon mixture film, such as direct coating of a layer, the thickness of the carbon film and the carbon concentration in which can control the size of the resistance; in order to control the resistance more accurately, the spiral groove can be processed on the carbon film, the more spiral the greater the resistance; finally add metal leads, resin encapsulation molding. The process of carbon film resistance is more complicated, and it can be used as a precision resistance, but due to the carbon quality, the temperature characteristics are not very good.
Metal Film Resistor
Similar to the carbon film resistance structure, metal film resistance mainly uses vacuum deposition technology to form a nickel-chromium alloy coating on the ceramic rod, and then process a spiral groove on the coating to accurately control the resistance. Metal film resistance can be said to be a relatively good performance of the resistance, high accuracy, can do E192 series, and then good temperature characteristics, low noise, more stable.
Metal film resistor
Metal Oxide Film Resistor
Source Metal oxide film resistor
Similar to the metal film resistance structure, the metal oxide film is mainly in the ceramic rod to form a layer of tin oxide film, in order to increase the resistance, you can add a layer of antimony oxide film to the tin oxide film, and then process a spiral groove on the oxide film to precisely control the resistance. The biggest advantage of metal oxide film resistance is high temperature resistance.
2.1.2 Chip resistors
Metal Foil Resistor
Metal foil resistance is formed by vacuum smelting to form a nickel-chromium alloy, and then made into metal foil by rolling, and then the metal foil is glued to the alumina ceramic substrate, and then the shape of the metal foil is controlled by lithography process, thereby controlling the resistance. Metal foil resistance is the best resistance whose current performance can be controlled.
Thick Film Resistor
The screen printing method used for thick film resistors is to attach a layer of silver palladium electrode to the ceramic substrate, and then print a layer of ruthenium dioxide between the electrodes as a resistor. The resistance film of a thick film resistor is usually relatively thick, about 100 microns. The specific process flow is shown in the following figure.
Thick film resistors are currently the most used resistors, inexpensive, tolerances of 5% and 1%, the vast majority of products use 5% and 1% of sheet thick film resistors.
Thin Film Resistor
Thin film resistance is the formation of a nickel chromium nickel film on the alumina ceramic substrate by vacuum deposition, usually only 0.1um thick, only one thousandth of the thick film resistance, and then etched into a certain shape by lithography process. The Thin Film process has been mentioned many times in previous articles on capacitance and inductance, and the lithography process is very precise and can form complex shapes, so the performance of thin film capacitors can be well controlled.
2.2 Variable resistance
Variable resistance is that the resistance value can be changed, there can be two kinds: one is a resistance that can be manually adjusted; the other is that the resistance value can change according to other physical conditions.
2.2.1 Adjustable resistors
When I was in middle school, I should have used a sliding varistor to do experiments, and when I moved the sliding varistor, the small bulb could be brightened or darkened. Sliding varistors are adjustable resistors, and the principle is the same.
Adjustable resistors, usually divided into three types:
Potentiometer
Potentiometer or voltage divider, which is a three-port device. The potentiometer is divided into two resistors by the intermediate tap, and the value of the two resistors can be changed through the intermediate tap, and the voltage can be changed.
Rheostat
The varistatic device is actually a potentiometer, the only difference is that the varistor only needs to use two ports, purely a resistor that can accurately adjust the resistance.
Trimmer
The trimmer, in fact, is also a potentiometer, but it does not need to be adjusted frequently, such as when the equipment leaves the factory, it can be adjusted, and usually needs to be adjusted with special tools such as screwdrivers.
2.2.2 Sensitive resistors
Sensitive resistors are a type of sensitive element, most of which are particularly sensitive to a certain physical condition, the physical condition changes, the resistance value will change, usually can be used as a sensor, such as photoresistor, moisture sensitive resistor, magnetoresistor and so on. In circuit design, there should be more thermistors and varistors, commonly used as protection devices.
Thermistor
PTC is a positive temperature coefficient resistor, usually there are two kinds: one is a ceramic material, called CPTC, suitable for high voltage and high current occasions; the other is a polymer material, called PPTC, suitable for low voltage and small current occasions.
Ceramic PTC, whose resistance material is a polycrystalline ceramic, is a mixture of barium carbonate, titanium dioxide and other materials sintered. The PTC temperature coefficient has a strong nonlinearity, when the temperature exceeds a certain threshold, the resistance will become very large, equivalent to a circuit break, so that it can play a role in short circuit and overcurrent protection.
At the same time, there is a negative temperature coefficient resistor, that is, NTC will not be described in detail.
Varistors
Varistors are usually metal oxide variable resistors, or Metal Oxide Varistor (MOV), whose resistance material is a mixture of zinc oxide particles and ceramic particles and sintered together. The characteristic of MOV is that when the voltage exceeds a certain threshold, the resistance drops rapidly and can pass a large current, so it can be used for surge protection and overvoltage protection.
Zinc oxide ceramics are made into multilayer varistors, or MLV, using a process similar to MLCC. The MLV package is small, usually chipy, and has a much smaller voltage rating and current capability than MOV, making it suitable for low-voltage DC applications.
Resistors are used in the selection of applications
Resistor manufacturers mainly include Guoju, Panasonic, Roma, Vista, as well as domestic Fenghua Hi-Tech and so on.
3.1 Application of resistors
Basically, there is no circuit board without resistors, and the most used devices on any board are capacitors and resistors. Various pull-up resistors, feedback resistors, etc. The level is limited, so let's talk about it briefly.
Heating
According to Joule's law, a current flows through a resistor and heats up. There are also many applications of the thermal effect of resistors, electric blankets, electric fire barrels, electric kettles.
For some outdoor applications of electronic equipment, especially for some integrated high-performance CPU SOCs, the working temperature requirements are very demanding, most of them can only meet commercial-grade applications, the winter in the northeast, minus thirty degrees, the temperature is too low, it is likely to not be able to turn on. Usually a high-power resistor is added to do the pre-heating function, when the temperature rises, the device is started and then turned off. This is related because the power consumption of the device itself will also heat up and can maintain the temperature.
As a hardware engineer, you often have to go to the environmental lab to locate problems. In order to reproduce a high temperature problem, you need to run to the environmental laboratory to build a test environment, there are only a few key incubators, and you have to make an appointment, and it is often too troublesome to queue. So I made a very simple positioning artifact myself, that is, to weld a DC power supply holder to the cement resistance, and then plug in various power adapters to adjust the temperature. Then put a few minutes on a certain chip, no problem, change another one, the problem reappears, the problem focuses on a chip, and completes the positioning of the high temperature problem at its own workstation.
Zero ohm resistance
Zero ohm resistance is also called jumper resistor. In circuit design, it is often used for ease of debugging or for compatible designs. For example, in the pre-research design, in order to test the operating current of each set of power supplies of the chip during debugging, it is usually necessary to use zero ohm resistance to divide the power supply into multiple channels.
When using zero ohm resistors, the most common problem encountered is how to calculate the power dissipation, and how to judge whether the selected resistor meets the requirements?
At this time, it is necessary to obtain the relevant parameters from the specification of the resistor, and it can be seen from the following figure that the zero ohm resistance of RC0402 will not exceed 50mΩ, and the rated current will not exceed 1A, so it can be judged whether the resistor meets the design requirements. Typically the zero ohm resistance of 0402 can meet the current requirements below 1A.
Current limiting
Sometimes a set of tens of milliampere power supplies are required in the circuit, but its voltage is not used elsewhere in the circuit, and it is not suitable to get a single set of DCDCs or LDOs at this time, because the current is too small. At this point, a voltage regulator circuit can be used.
Partial pressure
Voltage divisions such as ADC sampling circuits, DCDC output voltage feedback, level shifting, etc.
Matching resistor
For high-speed signals, pcB traces need to consider the transmission line model, to ensure impedance matching, to prevent signal reflections from affecting signal integrity. Impedance matching is to ensure that the load impedance is equal to the characteristic impedance of the transmission line to eliminate reflections. The most commonly used and simplest is the source-side series matching, that is, a resistor is connected in series at the signal source, and the sum of the resistor and the internal resistance of the source is equal to the characteristic impedance of the transmission line, so that even if the load side is not matched, the signal will be reflected back by the source signal and will not be reflected again.
In addition, there are various nonlinear sensitive resistors that can be used as sensors, protection circuits, and so on.
3.2 Selection of resistors
The selection is simply to extract the key parameters according to the specification of the device to determine whether the requirements of the application are met.
3.2.1 Fixed value resistors
The comparison of the main parameters of common types of resistors is shown in the following figure, and the largest shipment should be thick film resistance and metal film resistance.
3.2.2 Thermistors
The main role of PTC in the circuit is similar to the fuse, that is, overcurrent protection, the difference is that the fuse is disposable, while PTC is recoverable, and many times changing the fuse is unacceptable, affecting the customer experience. PTC is also a safety device and is usually required to pass UL1439 certification.
The above figure is the impedance temperature characteristics of PTC, when the overcurrent of PTC heats, the temperature rises rapidly, the impedance of PTC quickly becomes larger, forming a circuit break, the current drops after the circuit is broken, the heat decreases, the temperature drops, and the PTC restores the low impedance. Therefore, PTC is ideal for short-term overcurrents.
Hold the current
When choosing PTC, the first thing to consider is to design the working current, which cannot exceed the PTC holding current, at which time the PTC can maintain a low impedance state. The holding current of the PTC decreases as the operating temperature increases, so there are important factors to consider when operating the temperature.
Operating current
The operating current, i.e. the current at which the PTC enters a high impedance state and is protected against a circuit break.
Rated voltage
That is, the maximum voltage that PTC can withstand, beyond the rated voltage, PTC may be broken through the short circuit, which will cause burnout. Therefore, the design should consider that the operating voltage of PTC cannot exceed its rated voltage under various circumstances.
When PTC circuit break protection, it will withstand the entire supply voltage, and when PTC is selected, the rated voltage is greater than the supply voltage. Usually consider derating to 80%, that is, the supply voltage is 12V, and the PTC with a withstand voltage of more than 15V should be selected.
In the power input port, you need to consider surge protection, at this time to consider the maximum surge current, multiply the PTC's resistance, that is, the surge voltage that PTC bears, can not exceed the PTC rated voltage.
Rated current
That is, at the rated voltage, the maximum short-circuit current that PTC can withstand, and if the short-circuit current exceeds the rated current, the PTC will be damaged.
DC resistance
The presence of PTC DC resistance will make PTC have a certain DC voltage drop, and the design should pay attention to the power supply voltage after the voltage drop to meet the requirements.
Compared to fuses, PTC's rated voltage and rated current are much smaller, and the DC impedance of PTC is usually about two parts of the fuse. When PTC is protected, it is actually a high resistance state, so there will be a leakage current in the milliampere range, and the fuse is a fuse mechanism that cuts off the current path, and there is basically no leakage current.
3.2.3 Varistors
Varistors have similar characteristics to voltage regulator diodes (Zener diodes) and TVS, and are clamped devices, which are mainly used to protect circuit transient overvoltages, such as surges.
Ideal voltammetry characteristics of MOV
The selection of protective devices mainly considers two aspects: one is that the protective devices cannot operate or be damaged under normal working conditions, and the other is that they can play a role in protecting the circuit under abnormal circumstances within the design range, that is, the protection ability.
Rated operating voltage
The rated operating voltage can be considered as the highest continuous operating voltage at which the MOV can maintain a high impedance state. According to the application, MOV can be divided into AC and DC, and the specifications of the devices used in the two occasions are different. MOV for DC applications is generally not available for AC applications.
MOV's rated operating voltage, AC rated voltage considered in AC occasions, that is, Vrms or Vm(ac), the device in the figure above can work normally in the RMS of 130V. Above this voltage, the MOV may move or be damaged, causing the circuit to fail.
It is mainly used to protect against transient high voltages, and continuous excessive voltages can cause MOV damage.
Clamp voltage
MOV is a clamp-type device, when encountering transient high voltage, the impedance will drop, through a large current, the transient high voltage will be suppressed, but will not drop to zero, but still maintain a relative high voltage, usually 2 to 3 times the rated operating voltage. When selecting MOV, it should be noted that the clamping voltage cannot exceed the maximum withstand voltage of the protected device, and when it exceeds, it is necessary to use multi-level protection, such as adding a high-power resistor to decouple the rear stage, and then adding a TVS, using the low clamping voltage of TVS to further reduce the residual voltage.
Maximum pulse current
Lightning strikes or inductive load switching, etc., will produce a large inrush current, MOV in addition to clamping the high voltage, but also need to discharge the inrush current.
Whether MOV can withstand the inrush current is mainly related to the amount of energy that MOV bears over a period of time, the energy is too large, and the MOV is overheated and burned. The size of the energy is related to the waveform and number of surges, and in general, the surge capacity of the device is tested according to the 8/20us waveform energy. The MOV in the figure above, a single 3500A 8/20us surge pulse, two consecutive 3000A 8/20us surge pulses, 20 continuous 750A 8/20us surge pulses.
In addition, the parasitic capacitance of MOV is relatively large and cannot be used on higher-speed signal lines. MOV's response time is slower than that of TVS, and for some fast pulses, like ESD may not work. These are also factors that we need to consider.