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Capacitor Guide

Author: Apogeeweb
Date: 30 Aug 2018
 5340
capacitor definition

Introduction

As a passive component used in circuits, capacitors such as aluminum electrolytic capacitors, filter capacitors, tantalum capacitors, and chip ceramic capacitors are often seen. Due to the features of each capacitor, the corresponding application is different. So this article first introduces the basic knowledge of capacitors, whose differences and characteristics were compared, then summarizes the techniques for selecting capacitors in practice.

Capacitor Guide: What is a capacitor?

Catalog

Introduction

Catalog

I Classification and Function of Capacitors

II Capacitor Symbols

III Unit of Capacitance

IV Capacitor Withstand Voltage Unit: V 

V Types of Capacitors

VI Selection Tips of Capacitors

Ⅶ FAQ

Details

Basic knowledge of capacitors

I Classification and Function of Capacitors

A capacitor consists of two metal poles with an insulating material (medium) sandwiched between them. According to the structure, capacitor can be divided into fixed capacitor, variable capacitor, and adjustable capacitor. And according to the dielectric material, capacitor can be divided into gas-dielectric capacitor, liquid dielectric capacitor, inorganic solid dielectric capacitor, organic solid dielectric capacitor and electrolytic capacitor. Capacitor is also divided into polar capacitor and non-polar capacitor based on polarity. And electrolytic capacitor is the most common one. The capacitor has the function of blocking DC power and passing AC power in the circuit

Capacitors

Figure1. Capacitors

II Capacitor Symbols

There are domestic capacitor symbols and international electronic symbol notation and both are similar. The only difference is in polar capacitance, the domestic use an empty box under a horizontal line to represent the positive pole, while the international is an ordinary capacitance symbol plus a "+" symbol. In the circuit diagram, capacitors are generally identified by the symbol C .

Schematic Symbols of Capacitors

Figure2. Schematic Symbols of Capacitors

III Unit of Capacitance

The basic unit of capacitance:  F,μF, nF and pF. The latter three are more common than the first in reality, and the exact conversion between them is as follows:

1F=1000000μF

1μF=1000nF=1000000pF

 

IV Capacitor Withstand Voltage Unit: V  

Each capacitor has it withstand voltage value, which is one of the important parameters of it. The nominal withstand voltage values of common non-polar capacitors are 63V, 100V, 160V, 250V, 400V, 600V, 1000V, etc. The withstand voltage of a polar capacitor is relatively lower than that of a non-polar capacitor, including 4V, 6.3V, 10V, 16V, 25V, 35V, 50V, 63V, 80V, 100V, 220V, 400V, ect.

 

V Types of Capacitors

There are many kinds of capacitors, they can be divided into non-polar variable capacitors, non-polar fixed capacitors and polar capacitors according to working principle. Based on the material, they can be divided into CBB capacitors (polyethylene), polyester capacitors, ceramic capacitors, mica capacitors, solenoid capacitors, electrolytic capacitors, tantalum capacitors and so on.

Different Types of Capacitors

Figure3. Different Types of Capacitors

VI Selection Tips of Capacitors 

With the basic knowledge of capacitors above, let's compare the differences and characteristics of several capacitors, and summarize some techniques for selecting capacitors in actual circuits and here is a video explaining how to do it.

How to choose the right capacitor - comparing material, temperature, voltage and capacitance

(1) Aluminum electrolytic capacitors

The main components are aluminum foil and electrolyte. And its production process is to roll the aluminum foil into a column shape, injects a liquid electrolyte, and then leads out the positive and negative terminals, finally seals the core material of the capacitor in the metal casing. There is a certain proportion of water in the liquid electrolyte,which can be decomposed into hydrogen and oxygen when there is leakage current flowing through the capacitor. Oxygen can form a new oxide film with the anode through oxidation reaction and hydrogen is discharged through the rubber plug of the capacitor, which can prevent capacitor from damage. The simple producing process and the low cost are advantages of the aluminum electrolytic capacitor. In addition,its other characteristics are as follows:

Aluminum electrolytic capacitor.jpg

Figure4. Aluminum Electrolytic Capacitor

  • Since the sealed case is not completely sealed, the electrolyte will dry easily, so the life of the aluminum electrolytic capacitor is limited.

  • The presence of water in the electrolyte affects the performance of the aluminum electrolytic capacitor in high temperature and low temperature environment. 

  • Due to process characteristics, ESR and ESL of aluminum electrolytic capacitor is more difficult to be small, so its self-resonance frequency is usually relatively low, roughly in the range of dozens of KHz to several MHz.

  • The capacity of the aluminum electrolytic capacitor is positively related to the size of the aluminum foil, and the capacity can be made large. The larger the capacity, the larger the capacitance size.

According to the above characteristics, aluminum electrolytic capacitors are widely used in low-frequency filtering applications, especially in the environment of dozens of KHz to several MHz. For example, the output filtering of the power supply often applies aluminum electrolytic capacitor. When using aluminum electrolytic capacitors, the capacitor withstand voltage should meet the circuit requirements.

 

In addition, in other cases where the requirements are not strict, the capacity can be selected as large as possible. The larger the capacity, the smaller the ESR, which is easier to meet the requirement of target impedance of the circuit. Small-volume and small-capacity aluminum electrolytic capacitors should be avoided in some high-temperature environments to prevent the evaporation of the electrolyte, which will cause the capacitor failure to affect the operation of the entire circuit. 


(2) Filter Capacitors

Electrolytic capacitors must be used since filter capacitor is large after rectification. When the filter capacitor is used in a power amplifier, the value should be 10000uF or more, and when used in a preamplifier, the capacity can be about 1000uF. When the power supply filter circuit is directly supplied to the amplifier, the larger the capacity, the better the sound quality. However, a large capacity capacitor will cause the impedance to rise from around 10 kHz. To avoid this, several small capacitors should be connected in parallel to form a large capacitor and several thin film capacitors should be connected in parallel next to the large capacitor. The characteristics of filter capacitance include the following aspects:

Filter Capacitor

Figure5. Filter Capacitor

  • The harmonic filter loop consists of a capacitor series reactor. The lowest impedance is formed at a certain harmonic order to absorb a large amount of harmonic current, and the quality of the capacitor affects the stable absorption effect of the harmonic filter. The service life of the capacitor has a great relationship with temperature: the higher the temperature, the lower the life. Filter full-film capacitor has the characteristics of low temperature rise, which can guarantee its service life.

  • Low loss, dielectric loss tangent (tgδ): ≤ 0.0003

  • Complying with GB and IEC standards, its internal unit capacitor is equipped with protection devices.

  • Small size and light weight, easy to carry and install

We can understand from the above characteristics that the filter capacitor is a kind of energy storage device that connects in parallel at the output end of the rectifier power supply circuit, and plays a role to reduce the AC ripple ripple coefficient and smooth the DC output. In an electronic circuit that converts AC to DC power, the filter capacitor not only stabilizes the DC output of the power supply, but also reduces the influence from alternating pulsating ripple to the electronic circuit. It also absorbs current fluctuations generated during the electronic circuit and the interference via AC supply, which makes the working performance of the electronic circuit more stable.


(3) Tantalum capacitors 

Tantalum capacitorsare another widely used capacitors. Tantalum capacitors are also an electrolytic capacitor like aluminum electrolytic capacitors. Its main process is to press and form the tantalum powder into a porous solid block, which is anodized to form an oxide film, then covered with a solid electrolyte, coated with a layer of graphite and lead-tin coating, and finally encapsulated with resin to be solid tantalum capacitor. And tantalum capacitors have the following characteristics:

Tantalum Capacitor

Figure6. Tantalum Capacitor

  • Unlike aluminum electrolytic capacitors, the electrolyte of tantalum capacitors is solid, so there is no problem of electrolyte to be dry and the life will be longer.

  • Since the capacity temperature characteristics of the solid electrolyte are relatively stable, the temperature has little effect on the capacitance capacity, so its performance is superior to the aluminum electrolytic capacitor no matter the temperature is high or low.

  • Tantalum electrolytic capacitors can achieve smaller packages with larger capacity, so ESR and ESL can be controlled to be smaller, and its self-resonance frequency is higher than aluminum electrolytic capacitors.

  • It is more complicated than aluminum electrolytic capacitors in process, and the cost is higher.

Compared to aluminum electrolytic capacitors, tantalum capacitors have many unique advantages, and tantalum capacitors can be a good substitute for aluminum electrolytic capacitors in some filtering circuits. However, there are several points that need attention: Due to the structure of the tantalum capacitor, the withstand voltage is generally not high, so attention should be paid to the withstand voltage requirement in the actual circuit, and a certain margin should be left. Tantalum capacitors are not as good as aluminum electrolytic capacitors in dealing with power supplies, for example, instantaneous large current surges and large voltage transients. In practice, we can ignore the effect of temperature as it has little effect on tantalum capacitors.


(4) Ceramic capacitors

Being most used in practice and having a relatively simple structure, ceramic capacitors are alternately stacked and sintered together by ceramic sheets. Its main features are:

Ceramic Capacitor

Figure7. Ceramic Capacitor

  • Small size. The ceramic capacitor has a simple structure and can be made small in size, and ceramic capacitor of 0402 or even 0201 package is widely used in applications such as mobile phones whose size is strictly required.

  • Stable electrical performance and not sensitive to temperature.

  • Low ESR, ESL values, high self-resonance frequency, and can satisfied the filtering requirements of several MHz to 1 GHz on PCB.

  • The structural characteristics and the packaging of the multilayer cause it perform terrible in resisting bending, and the bending deformation of the PCB may cause capacitance cracking and failure.

According to the above characteristics, including the filtering scenarios, ceramic capacitors are also widely used in various applications such as blocking, coupling, and bypass. Its operating frequency is greatly improved compared to electrolytic capacitors, and can meet the applied requirement of several MHz to 1 GHz.

 


Ⅶ FAQ

1. How do I know which capacitor to use?

The general rule is always to use a capacitor with a higher working voltage than the circuit it is used in. This is of particular importance in power supply circuits with high-value electrolytic capacitors. The working voltage should always exceed the peak working voltage of the circuit by a minimum of 20%.

 

2. What size capacitor is dangerous?

At 100 to 400 volts range up to 100 Joules at slow discharge is considered safe (low probability of fibrillation). At high voltage, the energy between 1 and 10 Jules is considered dangerous. Over 15 kV the safety limit is 350 million Joules.

 

3. What does 50 uF mean on a capacitor?

It's a symbol that means micro so 50 μF means 50 microfarads or 000050 Farads. The farad is such a large unit that the microfarad is the practical unit for capacitance.

 

4. Is it safe to touch a capacitor?

A charged capacitor can be very dangerous, so it's important that you avoid coming into contact with the terminals at all times. Never touch the capacitor anywhere but on the sides of its body. If you touch the two posts or accidentally connect them with a tool, you could get badly shocked or burned.

 

5. How do you know if a capacitor is working?

For a good capacitor, the resistance will be low in the beginning and will gradually increase. If the resistance is low at all times, the capacitor is a Shorted Capacitor and we have to replace it. If there is no movement of the needle or the resistance always shows a higher value, the capacitor is an Open Capacitor.

 

6. What is the most common type of non-polarized capacitor?

Ceramic capacitors are the most common type of non-polarized capacitors. They are a well-tested technology and the cheapest kind of capacitor.

 

7. How long can a capacitor power LED?

The capacitor stores 1/2 CV^2 joules of energy: 300J. That would suggest 300/0.04 = 7500 seconds or about 2 hours. However, in practice, you won't get all of the energy out because the voltage will drop fairly quickly below a level that will output light.

 

8. What is the working principle of a capacitor?

A capacitor works on the principle that the capacitance of a conductor increases appreciably when an earthed conductor is brought near it. Hence, a capacitor has two plates separated by a distance having equal and opposite charges.

 

9. Does the physical size of the capacitor matter?

Generally, no, the physical size of an electrolytic capacitor is not a factor as long as the capacitance and voltage ratings are the same. A possible exception is, if it's a switching power supply that uses low ESR capacitors, then the sizes might be different.

 

10. What happens if you bypass a capacitor?

These unwanted perturbations (if not controlled) can couple directly into the circuit and cause instability or damage. In this case, the bypass capacitor is the first line of defense. It eliminates voltage droops on the power supply by storing electric charges to be released upon the occurrence of a voltage spike.

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