Everything About Capacitors

Table of Contents

What Are Capacitors?

Basic Description

Capacitors are electronic components that can store charge, acting very similarly to batteries. However, unlike batteries which can hold large amounts of charge, capacitors are designed to hold a small amount of charge for very quick tasks, such as stabilising power supplies or filtering signals. Furthermore, unlike batteries, capacitors can charge and discharge extremely quickly, which makes them ideal for all kinds of circuits, ranging from radio frequency amplifiers to signal conditioning.

Capacitors come in all kinds of shapes and variations

Schematic Symbol

The schematic symbol for capacitors depends on the type of capacitor being used, but generally, there are two main symbols; one for polarised, and one for unpolarised capacitors. Polarised capacitors are those that have a specific voltage polarisation, with one pin being used as the positive, while the other being used as the negative. Non-polarised capacitors, however, do not that this polarisation, meaning that they can be inserted in any orientation.

The vast majority of capacitors have two pins, and are available in many different packages including through-hole (with leads), and surface mount. Capacitors used in MitchElectronics kits are typically either ceramic disc or aluminium electrolytic, which each have their own pros and cons, but more on this later.

How Capacitors Work

Construction

A basic capacitor consists of two plates separated by some distance with a dielectric material in between. Because of this construction, current cannot flow from one plate to the other as the dielectric is an insulator, but this does mean that charge can be stored on either side of the capacitor. 

For example, is a capacitor is connected to a battery, one side of the capacitor will become positively charged, while the other side will become negatively charged. If the battery is removed and some device connected to the capacitor, for a brief moment (depending on how much energy the capacitor can store), the device will be powered by the capacitor.

The most basic capacitor looks identical to its symbol!

Real Capacitors

Common capacitors used in electronic circuits are more advanced than just two plates separated by air, due to the need for small sizes, large capacitances, and suitable operating voltages. 

For example, surface mount ceramic capacitors consist of a specialised ceramic dielectric and many layers of plates, while aluminium electrolytic devices have a long length of two sheets of metal separated by a paper dielectric soaked in an electrolyte. 

The inside of an electrolytic capacitor is complex

Capacitor Storage

The capacity of a capacitor (how much charge it can store) is measured in Farads, with one farad being the ability for a capacitor to provide a potential difference of one volt when delivering one coulomb of charge. This definition of a Farad isn’t very helpful because capacitors have all kinds of different voltage and current ratings, and that one Farad is a very large amount of energy. 

A better way of looking at Farads is that one Farad is equal to ½ a joule of energy if the voltage across the capacitor is 1V. Considering that ½ a joule is a lot of energy for small circuits, most capacitors used in electronic circuits are generally between 1000 microfarads and 1 picofarads.

How Are Capacitors Used?

Basic Application - Decoupling

By far the most common application of capacitors is in power soothing, also known as decoupling. While power supplies are essential in providing energy to a circuit, the operation of some components, such as switch mode power supplies and integrated circuits, can induce electrical noise into the power rails which can then interfere with other circuit elements.

The capacitors in this circuit are being used as decouplers, smoothing out the power rails

The reason why this electrical noise forms is that when these devices switch, the power supply providing power either cannot respond quick enough to the change in demand, or the demand for power is so great that the power supply simply struggles.

As such, placing capacitors in parallel with power supplies can help eliminate such noise. While capacitors cannot generally provide much energy (over a period of a few milliseconds), their ability to charge and discharge quickly means that they can provide energy rapidly. This is why decoupling capacitors are always found around integrated circuits, power supplies, and any device that switches either rapidly, switches a large amount of energy, or both.

Basic Application - Coupling

The second most common application of capacitors is separating AC signals from DC signals, also known as coupling. In some applications involving analogue or digital signals, a DC component to a signal can be present which can upset certain amplifier circuits, and such signals are commonly found in audio and radio systems.

The capacitor C2 is being used to filter out the AC signal from the microphone MK1

To get rid of this DC component, a capacitor can be put in series with a signal. While capacitors cannot conduct electricity, the charging / discharging effect does actually result in a current flow on both sides of a capacitor. So, for example, if the voltage on one plate suddenly rises, it causes the voltage on the other plate to also rise as capacitors like to try and maintain the same voltage across their plates. At the same time, if the voltage on one plate suddenly drops, then the other plate will also fall by around the same voltage. 

Thus, using this effect, AC signals can essentially “pass through” a capacitor, while DC signals cannot. This effect can also be used to create negative voltages, as is done in the MitchElectronics Simple Power Supply kit. 

Basic Application - Filtering

Capacitors are also massively helpful in filters, which are used to condition signals, whether it is removing a specific signal, or only allowing a specific signal to pass. These filters can be found in many different electronic circuits, including radio frequency amplifiers, receivers, transmitters, power supplies, and microcontroller circuits. 

While the concept behind these circuits is far too complex to discuss here, capacitors in filters are often combined with resistors and inductors to allow for extreme selectivity.

Types Of Capacitors

Ceramic Disc Capacitors (Through Hole)

Ceramic disc capacitors are extremely common throughout MitchElectronics kits, and used to be the most common type in the electronics industry (before being replaced by surface mount ceramic multilayer capacitors). These capacitors are made up of individual plates sandwiched next to each other separated by a ceramic material, and two leads coming out of the disc are used for securing to circuit boards.

These capacitors have capacitance values typically ranging from 1pF to 100nF, meaning that they are good for small decoupling applications, and their voltage rating tends to be on the higher side (around 30V to 50V). Furthermore, these capacitors tend to have very low series resistance, which makes them ideal for reacting to fast changing signals, can be used for AC signal coupling, and are unpolarised.

However, while these capacitors are incredibly cheap and easy to work with, they are also temperature dependent, meaning that their capacitance can vary greatly depending on the ambient temperature. Furthermore, these capacitors also have bad tolerances, being as large as ±50% (meaning that a 100nF capacitor could be anywhere from 150nF to 50nF). 

Aluminium Electrolytic Capacitors (Through Hole)

Aluminium electrolytic capacitors are also extremely common amongst MitchElectronics kits as well as other commercial electronic circuits. As the name suggests, these capacitors are made from two sheets of aluminium foil separated by a paper electrode and soaked in an electrolyte. This sheet is then roll tightly into a cylinder and then housed in a metal can with protective sheath and plug.

Unlike ceramic capacitors, aluminium electrolytic capacitors are polarised, meaning that they have to be used in a specific orientation. If an electrolytic capacitor is inserted incorrectly, it is possible for the capacitor to generate gas internally, swell, and explode. 

Such capacitors have much larger capacitances than ceramic capacitors, with values of 10,000 microfarads being possible, however, most MitchElectronics aluminium electrolytic capacitors vary from 10 microfarads to 470 microfarads. The voltage rating of such capacitors is also often limited, with 16V to 50V being common.

As such, these capacitors are ideal for use in power supplies where a large amount of instantaneous power may be needed (such as motor systems). However, as these capacitors also have larger series resistances than ceramic capacitors, they are not ideal for dealing with high-frequency noise. This is why electrolytic and ceramic capacitors are used together, providing the advantage of each. 

The tolerance of these capacitors can vary greatly, but common tolerances are around ±20%.

Multilayer Ceramic Capacitors (SMD)

Multilayer SMD ceramic capacitors are, as the name suggests, made from ceramic and multiple plate layers. However, unlike ceramic disc capacitors, these are surface mount devices, which means that they don’t have legs that go through a PCB with underside soldering.

Instead, these capacitors have two solder tabs that are directly soldered to exposed PCB pads. As such, these capacitors can be made extremely small, with some being as small as 0.6mm x 0.3mm. However, to make soldering easier, MitchElectronics SMD trainer kits use the 0805 size (2mm x 1mm). 

Multilayer SMD capacitors (often shorted to MLCC), are unpolarised meaning that they can be inserted in any orientation, and this makes them exceptionally easy to use in standard mass production environments such as pick and place. Furthermore, these capacitors can easily survive the reflow process, meaning that they are easy to solder using large ovens that blow hot air across a board. 

With regard to tolerances, they are typically around ±20% of their stated value, meaning that they should not be used in precision circuits if possible. However, special variations of MLCCs do exist, with different tolerances, temperature ratings, and dielectrics, that are all ideal for different applications. For example, a special dielectric called C0G is used in microwave applications involving extremely high frequencies.

The capacitance and voltage range of these capacitors also varies, but generally speaking, they operate up to around 50V and have capacitances up to 10 microfarads. Thus, these capacitors are ideal for basic filtering applications, coupling signals, and decoupling power rails.

Aluminium Electrolytic (SMD)

These capacitors are identical in their construction to aluminium electrolytic capacitors, except that instead of having a pair of legs that go through a circuit board, they have a pair of tabs that are soldered directly to surface pads (hence, making them surface mount devices). However, these capacitors are often much smaller than their through-hole counterparts, making them ideal for small devices.

As these capacitors are polarised, they must be inserted correctly, and have a marking on their can that indicates which side the positive and negative tab is located. If a reverse voltage is applied, then, as with through-hole aluminium electrolytic capacitors, they will swell, emit steam, and may even explode. 

Again, these capacitors are ideal for power applications thanks to their larger capacitances compared to ceramic capacitors (between 10 microfarads and 1000 microfarads). However, due to their reduced size, they often have reduced voltage ratings, with 6.3V to 16V being common. Of course, larger voltage variations are available, but this will either make them significantly more expensive, or physically larger.

Tantalum Capacitors

Tantalum capacitors are an odd type in the field of electronics, but often play an important role. By far their most famous trait is that when they fail, they fail spectacularly, in that they catch fire and burn everything else around them to a crips. So, you may be wondering why they are used, but there is a very good reason for their existence.

Compared to other capacitor technologies, tantalum capacitors have some of the highest energy densities, allowing them to be made incredibly small. What would require a large bulky aluminium electrolytic can be replaced with a much smaller tantalum variation, allowing circuits to be made incredibly small and lightweight.

Furthermore, as they are polarised parts, they can easily replace electrolytic capacitors with no need for any changes to a circuit. However, this also means that they need extra care when being installed, as a reversed-biased tantalum can fail rapidly.

Interestingly, tantalum capacitors are often found in high-end automotive and aerospace applications where weight and size are everything. Even though they can be used in commercial applications, it is often best to avoid such parts due to their damage capabilities.

Example Circuits Using Capacitors

Power Decoupling

Capacitors are commonly found in decoupling applications so that noise in power lines can be smoothed out. In some cases, it may be required to have different types of capacitors and values in parallel so that each capacitor can deal with different types of noise, whether it is a sudden drop of voltage over a short period, or a larger draw on the power rails.

Signal Coupling

When removing DC signals from an AC signal, a capacitor in series with the signal can be used. Because capacitors cannot conduct electricity across them, DC voltages cannot cross the capacitor, but constantly changing AC signals can pull and push charges on the opposing plate of the capacitor (like how a magnet under a table can move a magnet on the top side of the table). 

Soldering Guide For Capacitors

PCB Ident

On PCBs, capacitors have the ident of “C” followed by their schematic reference number. For example, C1 would refer to the first capacitor in a schematic, while C23 would refer to the 23rd capacitor. There is no difference in this naming convention for different capacitors, but their PCB symbol and shape will be.

Non-Polarised vs. Polarised Capacitors

In the case of non-polarised ceramic disc capacitors and MLCCs, there is no particular orientation that they need to be inserted in. However, polarised components such as aluminium electrolytics and tantalums have to be inserted in a very specific orientation, otherwise they will fail when in use.

Generally speaking, the positive pin on a PCB can be identified by a square pad, while the negative pin can be identified with a round pad. In some cases, the negative side may be completely shaded in as well, and a plus symbol may also be placed next to the positive pad. 

When it comes to SMD parts, the positive pin can be identified in a number of different ways. One such way is that the shape of the outline will match the outline of the capacitor (with one side having cut-outs). Another way that the positive pad can be identified is that a thick line is drawn very close to it (which incidentally is the opposite to diodes, where a thick line would indicate the cathode).

Heat Sensitivity

Thankfully for engineers, most capacitors are far from being heat sensitive, meaning that no special procedures need to be followed when soldering them. Of course, that doesn’t mean capacitors should be kept at a high temperature for an extended time, as they can still melt (such as the plastic skirt that goes around SMD aluminium electrolytic capacitors).

Static Sensitivity

Again, just like with heat sensitivity, capacitors are not sensitive to static shock (also called ESD). This means that special practices of discharging oneself are not needed before handling and using capacitors.