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# How To Select A Capacitor？Purchase Recommendations

Question:

“What capacitor types should I choose?”

This is a question asked by many beginners. I will give you a simple answer to this question without going into all the details. After reading this, my goal is that you should be able to go and find the capacitor you need right away. I realized that it would be helpful to explain why one might pick one capacitor type over another. This information is important because there are a lot of factors (temperature characteristics, package, etc.) which might make a type of capacitor (electrolytic or ceramic) better than others for your project.

Contents

 Article Core Capacitors Purchase Recommendations Capacitors Definition Capacitor Functions Stopping DCBypass (decoupling)CouplingFilteringTemperature CompensationTimingTuningRectifierEnergy Storage Capacitors Types Aluminum Electrolytic CapacitorCeramic CapacitorTantalum CapacitorFilm CapacitorMica Capacitor Polymer Capacitor Volume Marking Method Direct Scaling MethodCharacter-symbol MethodColor-codes MethodMathematical Counting Method Main Characteristic Parameters of Capacitors Volume and ErrorRated Working VoltageTemperature CoefficientInsulation ResistanceLossFrequency Characteristics Capacitors Electricity Symbol Fixed Capacitor Value Selection Requirements Selections Principles Selections Supplements Matters Needing Attention Conclusion

Capacitors

A capacitor is a passive two-terminal electronic component in circuit, Learning the basics of capacitors with a simple explanation and demonstration.

A capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuit, a capacitor is a component designed to add capacitance to a circuit. The capacitor was originally known as a condenser or condensator.[1] The original name is still widely used in many languages, but not commonly in English.

The physical form and construction of practical capacitors vary widely and many capacitor types are in common use. Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium. A conductor may be a foil, thin film, sintered bead of metal, or an electrolyte. The nonconducting dielectric acts to increase the capacitor's charge capacity. Materials commonly used as dielectrics include glass, ceramic, plastic film, paper, mica, and oxide layers. Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, an ideal capacitor does not dissipate energy.

When two conductors experience a potential difference, for example, when a capacitor is attached across a battery, an electric field develops across the dielectric, causing a net positive charge to collect on one plate and net negative charge to collect on the other plate. No current actually flows through the dielectric, however, there is a flow of charge through the source circuit. If the condition is maintained sufficiently long, the current through the source circuit ceases. However, if a time-varying voltage is applied across the leads of the capacitor, the source experiences an ongoing current due to the charging and discharging cycles of the capacitor.

When you know what is capacitor completely, then you must know what functions it has.

Functions of Capacitor

1.Stopping DC.: the function is to prevent the passage of DC and allow the AC to pass through.

2. Bypass (decoupling): it provides a low impedance path for some parallel components in AC circuits.

3. Coupling: as a connection between two circuits, AC signals are allowed to pass and transmitted to the next stage of the circuit.

The purpose of using capacitor as coupling part is to transmit the front stage signal to the next stage, and to separate the influence of the DC of the former stage on the latter stage, so that the circuit is simple to debug and its performance is stable.

The amplification of AC signal without capacitor will not changed, but the work points at all levels need to be redesigned. Because of the influence of the front and back stages, to debug at working points is very difficult and can hardly be realized at multistage.

4. Filtering: this is very important for the circuit, the capacitor behind the CPU is having this function basically.

That is, the larger the frequency f, the smaller the impedance Z of the capacitance. At low frequency, the capacitance C can pass smoothly because of the large impedance Z, and at high frequency, the capacitance C is very small because of the impedance Z, which is equivalent to shorting the high frequency noise to the GND.

5. Temperature compensation: it improves the stability of the circuit by compensating for the influence of other components on the temperature adaptability.

Analysis: because the capacity of the timing capacitor determines the oscillation frequency of the horizontal oscillator, the capacity of the timing capacitor must very stable and does not change with the humidity in the environment. Therefore, the capacitors with positive and negative temperature coefficients are used for temperature complementation.

When the operating temperature increases, the capacity of Cl is increasing, while the capacity of C2 is decreasing, and the total capacity of two capacitors is the sum of the two capacitors after parallel connection. Because one capacity is increasing and the other is decreasing, the total capacity is basically stable.

Similarly, when the temperature decreases, the capacity of one capacitor decreases while the other increases, and the total capacity is basically unchanged, which stabilizes the oscillation frequency and realizes the purpose of temperature compensation.

6. Timing: the use of capacitors in conjunction with resistors to determine the time constant of the circuit.

Inputting signal from low to high, after buffer 1 then input RC circuit. The characteristics of capacitor charging make the B point signal not change immediately with the input signal, but there is a gradual process of increasing. When it becomes larger to a certain extent, the buffer 2 flips over, resulting in a delay jump from low to high at the output end.

7. Tuning: having systematic tuning to circuits which related to frequency, such as cell phones, radios, and televisions.

8. Rectifier: switch on or off a semi-closed conductor component at a predetermined time.

9. Energy storage: storage of electrical energy for release when necessary. For example, camera flashlights, heating devices, etc. (some capacitors now store energy at levels close to lithium batteries; a capacitor can store electricity as one-day power for a mobile phone.

Capacitor Types

There are several different types of capacitors which differ by polarity, performance and cost. Below are some of the common capacitor types: aluminum electrolytic, ceramic, tantalum, film, mica and polymer capacitors, along with their features, applications, package information as well as info on part selection.

Aluminum Electrolytic Capacitor

Aluminum electrolytic capacitor, a kind of capacitors made of water absorbent paper impregnated with a paste electrolyte wrapped between two aluminum foils, thin oxide film as a medium. In addition, due to the oxide film has unilateral conduction properties, aluminum electrolytic capacitors have polarity.

With large capacity, it can withstand large pulsating current.

Advantages: Low frequency bypass, signal coupling, power filter.

Disadvantages: large capacity error, large leakage current, and the common is not suitable for high frequency and low temperature applications, as for the working environment, the frequency shouldn’t excess 25kHz.

Ceramic Capacitor

Ceramic capacitor, with high dielectric constant, using ceramic barium titanate extruded into a circular tube, a wafer or a disk which used as the medium, and silver plating on the ceramic was used as an electrode by silver burning infiltration method. It is also divided into high-frequency porcelain and low-frequency porcelain.

A capacitor with a small positive capacitance temperature coefficient used in high stability oscillating circuits as a loop capacitor and padder capacitor.

Low frequency ceramic capacitors are limited to use as bypass or stopping DC in low frequency circuits, or in cases where the requirements of stability and loss are not high(including high frequency). This kind of capacitors are not suitable for use in pulse circuits because they are easily breakdown by pulse voltages.

Supplement: Ceramic capacitors can be divided into plug-in capacitors and chip capacitors according to the package.

The uses of high-voltage ceramic capacitors are mainly used for power transmission, distribution system power equipment and equipment for processing pulse energy.

Tantalum Capacitor

Use sintered tantalum block as positive electrode, and solid manganese dioxide as the electrolyte.

Advantages: the temperature characteristics, frequency characteristics and reliability are superior to those of conventional electrolytic capacitors, especially the leakage current is minimized, the storage property is good, the life service is long, the capacity error is small. Although the size is small, the maximum capacitance voltage product can be obtained under the unit volume.

Disadvantages: the ability to withstand pulsating current is poor, and it is easy to be short-circuit if damaged.

Other features: ultra-small, high reliability.

Film Capacitor

Structures: the film capacitor is a kind of capacitor with metal foil as the electrode and the plastic film such as polyethyl ester, polypropylene, polystyrene or polycarbonate, which is folded from the two ends and wound into a cylindrical structure.

The types of films are also called polyethyl ester capacitors (also called mylar capacitors), polypropylene capacitors (also called PP capacitors), polystyrene capacitors (also called PS capacitors) and polycarbonate capacitors.

Advantages: no polarity, high insulation impedance, excellent frequency characteristic (wide frequency response) and low dielectric loss.

Application: film capacitors are mainly used in many industries, such as electronics, home appliances, communication, electric power, electrified railway, hybrid electric vehicles, wind power generation, solar power generation and so on. The steady development of these industries has promoted the growth of film capacitor market.

Mica Capacitor

Structure: an electrode plate made of metal foil or coated with silver on a mica sheet. Then the electrode plate is laminated with mica and finally put in bakelite powder or sealed in epoxy resin with diecast technology .

Characteristic: it is suitable for high frequency circuit because of its low dielectric loss, large insulation resistance and low temperature coefficient.

Advantages: mica capacitor is one of the high frequency capacitors, which is widely used in the field of requiring high stability and reliability of capacitors. Small size, light weight, solid structure, easy installation and stable performance.

Application: due to its superior performance, it is widely used in radio receiving equipment, precision electronic instrument, modern communication instrument and equipment, radio, power amplifier, TV set, etc.

Polymer Capacitor

Structure: polymer capacitor is a laminated aluminum electrolytic capacitor with high conductivity as cathode. It has better electrical properties than liquid chip aluminum electrolytic capacitor and solid sheet tantalum electrolytic capacitor.

Polymer capacitors do not need to be used for voltage reduction within rated voltage range.

ESR is extremely low, has strong ability to reduce ripple voltage and allows larger ripple current. At high frequency, the impedance curve of polymer laminated aluminum electrolytic capacitor is similar to that of ideal capacitor. In the case of variable frequency, the capacitance is very stable.

Application: this kind of capacitor is mainly used in main board (notebook computer, flat panel display, digital switch) bypass decoupling, energy storage filter, switched power supply, DC/DC converter, high-frequency noise suppression circuit and portable electronic equipment.

Volume Marking Method

1) Direct Scaling Method

Use letters and numbers to mark models and specifications directly on the shell. For example, 1uF denotes 1 microfarad, some capacitors use letter "R" to denote decimal points, such as R56 for 0. 56 microfarad.

2) Character-symbol Method

A regular combination of numbers and characters is used to express the volume, and the character symbols represent the units of its electric capacity: P, N, u, m, F, etc. For example, p10 means 0.1 pF, 1p0 means 1pF, 6P8 means 6.8pF, 2u2 means 2.2uF. The nominal deviation is allowable, if it less than that of 10pF, it can replaced by letters: B- ±0.1pF, C- ±0.2pF, D- ±0.5pF, F- ±1pF.

3) Color-codes Method

The main parameters of capacitors are represented by color bands or dots, and this method of capacitors is the same as the resistors.

The mark symbol of the deviation of the capacitor: +100%-0-H、+100%-10%-R、+50%-10%-T、+30%-10%-Q、+50%-20%-S、+80%-20%-Z.

4) Mathematical Counting Method

Mathematical counting method is usually three digits, the first and second digits are significant numbers, and the third digits are multiples. The nominal value is 272, the capacity is: 27X10 ^ 2=2700pf, if the value is 473, it is 47X10 ^ 3=47000pf, the value 332. it is 332=33X10 ^ 2=3300pf (the number 2, 3, and so forth are the powers of 10.). The name of capacitors in different countries is different. The models of domestic capacitors are generally composed of four parts: name, material, classification and serial number (but it is not suitable for varistor capacitors, variable-capacitors and vacuum capacitors).

Main Characteristic Parameters of Capacitors

(1) Volume and error: the maximum allowable range of deviation between actual capacitance and nominal capacitance, generally divided into three grades: grade-I: ±5%; grade-II: ±10%; grade-III:  ±20%. In some cases, there is a grade zero, and the error is ±20%. The allowable errors of precision capacitors are smaller than the electrolytic capacitors, the latter adopt different error grades.

The commonly used capacitor has the same accuracy grade expressions as the resistor. In alphabetical terms: D-005-±0.5%, F-01-±1%, G-02-±2%, J-I-±5%, K-II-±10%, M-III-±20%.

(2) Rated working voltage: capacitors can work stably and reliably in the circuit for a long time. For devices with the same structure, medium and capacity, the higher the voltage, the larger the volume.

(3) Temperature coefficient: within a certain temperature range, the relative change value of capacitance is obtained for each change of temperature value, in addition, the smaller the temperature coefficient, the better.

(4) Insulation resistance: used to indicate the size of leakage. In general, for small capacitors, insulation resistance is very large, in the hundreds of ohms or several gigabytes ohms. The insulation resistance of electrolytic capacitor is generally small. Relatively speaking, the larger the insulation resistance, the smaller the leakage.

(5) Loss: the energy consumed by the capacitor during heating per unit time under the action of an electric field. These losses come mainly from dielectric losses and metal losses. It is usually represented by the tangent value of the loss.

(6) Frequency characteristics: the properties of the electrical parameters of capacitors vary with the frequency of the electric field. Because the dielectric constant at high frequency is smaller than that at low frequency, the capacitance of capacitor operating at high frequency is reduced accordingly. And the loss increases with the increase of frequency. In addition, the distribution parameters of the capacitor, such as the resistance of the electrode, the resistance between the lead and the pole, the inductance of the electrode and the inductance of the lead, will affect the performance of the capacitors. In a word, all this limits the frequency of using capacitors.

Different types of capacitors, the highest using frequency is different. The small mica capacitor is within 250MHZ, the circular ceramic dielectric capacitor is 300MHZ, the tubular ceramic dielectric capacitor is 200MHZ; the disk type ceramic capacitor is up to 3000MHZ; the small-size paper dielectric capacitor is 80MHZ; the medium-size paper dielectric capacitor is only 8MHZ.

Electricity Symbol

Here are the schematic symbols for various capacitors:

(1)①: basic capacitor symbols, such as ceramic capacitors, electrolytic capacitors, mica capacitors, thin film capacitors;

(2)②-⑥: polarized capacitor and electrolytic capacitor symbols, bending plate is negative electrode, hollow is positive electrode;

(3)⑦: variable capacitor symbol; ⑧: adjustable(pre-set) capacitor symbol.

Which Capacitor Values Are There?

Capacitors are available in a lot of values. Over time, some standard values have emerged. Here is a table with the most commonly found capacitor values:

 Fixed Capacitor Value pF pF pF pF uF uF uF uF uF uF uF 1.0 10 100 1000 0.01 0.1 1.0 10 100 1000 10000 1.1 11 110 1100 1.2 12 120 1200 1.3 13 130 1300 1.5 15 150 1500 0.015 0.15 1.5 15 150 1500 1.6 16 160 1600 1.8 18 180 1800 2.0 20 200 2000 2.2 22 220 2200 0.022 0.22 2.2 22 220 2200 2.4 24 240 2400 2.7 27 270 2700 3.0 30 300 3000 3.3 33 330 3300 0.033 0.33 3.3 33 330 3300 3.6 36 360 3600 3.9 39 390 3900 4.3 43 430 4300 4.7 47 470 4700 0.047 0.47 4.7 47 470 4700 5.1 51 510 5100 5.6 56 560 5600 6.2 62 620 6200 6.8 68 680 6800 0.068 0.68 6.8 68 680 6800 7.5 75 750 7500 8.2 82 820 8200 9.1 91 910 9100

Selection Requirements

1) Selecting the appropriate model based on your requirement;

2) Determining the accuracy of capacitor reasonably;

3) Determining the rated working voltage of the capacitor: for the general circuit, the working voltage of the circuit should be 10%~20% of the rated voltage of the capacitor. When there is a pulsating voltage, the working voltage shall be the highest voltage of the pulse. When applied to AC, the rated voltage increases with the increase of frequency. When the ambient temperature is relatively high, the rated voltage should be selected larger;

4) Selecting the capacitor with high insulation resistance as far as possible;

5) Considering the temperature coefficient and frequency characteristic;

6) Paying attention to the applications of the environment.

How to choose capacitor correctly?

What is the right choice of capacitors for those who need them?

(1) The type of capacitor shall be selected according to the requirements of the circuit. For low frequency circuit and DC circuit whose operating requirement is not high, we can generally choose paper capacitor, also can choose low-frequency ceramic dielectric capacitor. In high frequency circuit, mica capacitor, high-frequency ceramic capacitor or core ceramic capacitor can be used when the requirements of electrical performance is high. In the mid-frequency and low- frequency circuits with high requirement, plastic film capacitor can be selected.

In the electric shock filter and decoupling circuit, aluminum electrolytic capacitor is generally used. For the circuits with high stability and reliability, mica capacitor, paint film capacitor or tantalum electrolytic capacitor should be selected. For high-voltage circuits, high-voltage ceramic dielectric capacitors or other types of high-voltage capacitors should be selected. For tuning circuits, variable capacitors and adjustable(pre-set) capacitors should be selected.

(2)Determine the capacitance and allowable deviation of the capacitor reasonably.

In low frequency aluminum alloy and destin circuit, the capacitance of capacitor is not very strict, only a larger capacity can be selected according to the calculated value. In timing circuit, oscillation circuit and tone control circuit, the capacitance of capacitor is very strict, so the nominal value of capacitance should be as close as possible to the calculated capacitance, and the capacitor with high accuracy should be chosen as far as possible. In some special sister circuits, the capacitance of capacitors is often required to be very accurate. In this case, the high precision capacitors with allowable deviations in the range of ±0.1% ~ ±0.5% should be selected.

The working voltage of the selected capacitor should conform to the circuit requirements.

In general, the rated voltage of the selected capacitor should be 1.2-1.3 times of the actual working voltage.

For circuits with high ambient temperature or poor stability in working environment, the rated voltage of capacitor should be reduced and leave a larger margin. If the working voltage in the circuit of capacitor is higher than the rated voltage of capacitor, it is easy for the capacitor to break down, which makes the whole circuit unable to work normally. In addition, the rated voltage of capacitor generally refers to DC voltage, if it is used in AC circuit, it should be selected according to the characteristics and specifications of capacitors.

Supplements

1) Selection based on application frequency

Circuit working frequency is over MHZ, circuit signal strength is weak, choosing laminated ceramic capacitor. When the operating frequency is the same, the frequency characteristics of different capacitors and the frequency of the working circuit should be considered.

If the working frequency is below the middle-frequency range, the consistency of the capacitor parameters at different temperatures is high, and the solid tantalum capacitor is more suitable.

2) Selection according to environmental temperature change requirements

The circuit of different products requires different temperature characteristics of capacitors. Solid tantalum capacitors are currently the best capacitors with temperature characteristics. The capacity changes of some high voltage solid tantalum capacitors in the range of -55±125 degrees can be controlled within -3%±5%. This feature makes it a golden partner in things like aeronautical and aerospace circuits.

General ranking of capacitance temperature characteristics:

solid tantalum capacitor ≥ NPO type ceramic capacitor ≥ solid aluminum capacitor ≥ liquid tantalum capacitor≥ laminated ceramic capacitor (MLCC) ≥ liquid aluminum capacitor

3) Selection based on input / output power size

Different capacitors have different capacities, so the capacitors with sufficient capacity and withstand voltages should be selected according to the output power requirements. Even if the leakage current is high and the ESR is low, it is difficult for the products to have quality problems (unless the capacitor itself is a waste product) in the mobile phone electronic products with minimal power consumption and extremely high working frequency. If it is used in the circuit with high input and output frequency, such as power supply filter and discharge circuit, it is requiring lower ESR and smaller leakage current, otherwise, the breakdown probability will increase and the output power waveform can’t meet the requirements. Ripple-current resistance, it is also a index of capacitance stability.

For example, in a filter circuit, you have to consider whether the capacitor can withstand the surge of high DC voltage and large current generated at the switch moment, if you do not consider this problem,the heat shock caused by AC voltage and current will be generated.

The withstand voltage characteristic ranking of capacitors:

MLCC ≥ rolled polyester capacitor ≥ chip niobium oxide capacitor ≥ polymer chip tantalum capacitor ≥ polymer solid chip Aluminum capacitor ≥ (MNO2 as the negative pole)chip tantalum capacitor ≥ liquid aluminum capacitor ≥ liquid tantalum electricity capacitor

4) Selection according to voltage level

In high voltage circuits, once the breakdown occurs, the damage can not be avoided. Under this condition, the safety of high-voltage ceramic capacitors is the best, followed by polyester capacitors and high voltage aluminum capacitors, and the lowest voltage resistance is polymer capacitors and niobium oxide capacitors.

5) Selection based on comprehensive factors

There are differences in the structure and electrical characteristics of various capacitors. However, because of various types, designers face difficulties in choosing suitable types according to specific applications. Factors to be considered include stability, size, loss, rated working voltage, tolerance, cost and structure.

When selecting, determine the type of dielectric used by the capacitor. Dielectric materials include air, glass, ceramics, mica, plastic film, aluminum, tantalum, etc. In addition, a suitable mechanical structure must be selected: small size, thin dielectric and effective packaging to avoid performance degradation.

Matters Needing Atteention

Five things to keep in mind to avoid selection problems

Many buyers of passive components tend to think of these devices as very interchangeable or even “generic.” Unlike ICs, it’s relatively easy to find multiple manufacturers of components with roughly similar specs for most passives. However, there are often significant differences in how these devices will perform in different applications and how they continue to perform over time. This is especially true of capacitors. Here are five things to keep in mind to avoid performance problems:

1. Looks aren’t everything.

Don’t be fooled by capacitors that physically look the same. Over the years, capacitors have continued to evolve and improve, utilizing newer materials and better manufacturing technologies. However, that doesn’t mean that they outwardly look different than the components they replaced, as not every manufacturer may keep up with these improvements. The major brands do stay current with technology, which can result in longer life and better performance for your end products. A generic manufacturer may offer a product that physically looks similar and even claim similar specs, but might actually have a shorter life and not even meet all specs.

2. Know from whom you are buying.

Buy capacitors from authorized distributors or directly from manufacturers. By their physical nature, all aluminum electrolytic capacitors dry out over time, losing capacitance and weakening other performance specs. When you buy from a non-franchise source, you may be buying products that are near or past their date code. As seen with grocery products, the “sell-by date” assures you of getting a fresh product. These date codes are not always easy to decode and, as a result, you may not be able to determine the quality of the product you are buying. A franchise source will also provide assurance that the capacitors you are purchasing are not counterfeit.

3. Don’t stay stuck in an old buying pattern.

Higher-performance capacitors have become less expensive in recent years. If you are still using a general purpose device because you remember that higher-temperature or lower-ESR devices are too expensive, it’s time for a fresh look. Some of the newer dielectric capacitor types have also dropped in price significantly. For example, aluminum polymer capacitors, while still higher priced than aluminum electrolytics, can be a better value. One polymer capacitor can often replace two or three aluminum electrolytics in a circuit, potentially improving performance, saving space, and even lowering end-product cost.

4. Make the right connection.

A much greater choice of standard terminations are now available for film capacitors. The right termination can simplify assembly and mounting, increase current capacity, and improve mechanical stability and product reliability. Some terminations are designed to directly mount to specific types of power semiconductor devices, while others use multiple leads for highest reliability in PCB mounting.

5. Don’t miss out on the power of EDLC/supercapacitors.

Supercaps, also called EDLC and ultra capacitors, provide incredible amounts of capacitance in small packages. They are much more cost-effective than they were just a few years ago, and are leading to exciting applications in energy management, storage, and much more. While available voltage ratings are lower than with aluminum electrolytics, they can be networked for higher ratings. To simplify this task, supercapacitor modules are available in both standard and custom values. For example, modules that back up automotive batteries are available, while others capture energy in green power applications. Combining supercapacitors to make a module is not an easy balancing act, so if you need the higher voltage ratings of a module, it’s probably better to have it made for you.

Conclusion

Like just about everything else in electronics, technology improvements are lowering costs and improving the performance of capacitors. Whether you are beginning a new design project or gathering components for an existing one, it’s best to review your technology and sourcing options for capacitors. The results can be well worth the effort, potentially saving your company money and producing a better product.

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