Which capacitors to use

Send Email. Mon-Fri, 9am to 12pm and 1pm to 5pm U. Mountain Time:. A capacitor is a two-terminal, electrical component. Along with resistors and inductors, they are one of the most fundamental passive components we use. You would have to look very hard to find a circuit which didn't have a capacitor in it. What makes capacitors special is their ability to store energy ; they're like a fully charged electric battery.

Caps , as we usually refer to them, have all sorts of critical applications in circuits. Common applications include local energy storage, voltage spike suppression, and complex signal filtering. Some of the concepts in this tutorial build on previous electronics knowledge. Before jumping into this tutorial, consider reading at least skimming these first:. There are two common ways to draw a capacitor in a schematic. They always have two terminals, which go on to connect to the rest of the circuit.

The capacitors symbol consists of two parallel lines, which are either flat or curved; both lines should be parallel to each other, close, but not touching this is actually representative of how the capacitor is made. Hard to describe, easier to just show:. The symbol with the curved line 2 in the photo above indicates that the capacitor is polarized , meaning it's probably an electrolytic capacitor. More on that in the types of capacitors section of this tutorial. Each capacitor should be accompanied by a name -- C1, C2, etc..

The value should indicate the capacitance of the capacitor; how many farads it has. Speaking of farads Not all capacitors are created equal. Each capacitor is built to have a specific amount of capacitance. The capacitance of a capacitor tells you how much charge it can store , more capacitance means more capacity to store charge. The standard unit of capacitance is called the farad , which is abbreviated F. It turns out that a farad is a lot of capacitance, even 0.

Usually you'll see capacitors rated in the pico- 10 to microfarad 10 -6 range. When you get into the farad to kilofarad range of capacitance, you start talking about special caps called super or ultra -capacitors.

Note : The stuff on this page isn't completely critical for electronics beginners to understand We recommend reading the How a Capacitor is Made section, the others could probably be skipped if they give you a headache. The schematic symbol for a capacitor actually closely resembles how it's made.

A capacitor is created out of two metal plates and an insulating material called a dielectric. The metal plates are placed very close to each other, in parallel, but the dielectric sits between them to make sure they don't touch. Your standard capacitor sandwich: two metal plates separated by an insulating dielectric. The dielectric can be made out of all sorts of insulating materials: paper, glass, rubber, ceramic, plastic, or anything that will impede the flow of current.

The plates are made of a conductive material: aluminum, tantalum, silver, or other metals. They're each connected to a terminal wire, which is what eventually connects to the rest of the circuit.

The capacitance of a capacitor -- how many farads it has -- depends on how it's constructed. More capacitance requires a larger capacitor. Plates with more overlapping surface area provide more capacitance, while more distance between the plates means less capacitance.

The material of the dielectric even has an effect on how many farads a cap has. The total capacitance of a capacitor can be calculated with the equation:.

Electric current is the flow of electric charge , which is what electrical components harness to light up, or spin, or do whatever they do. When current flows into a capacitor, the charges get "stuck" on the plates because they can't get past the insulating dielectric. Electrons -- negatively charged particles -- are sucked into one of the plates, and it becomes overall negatively charged.

The large mass of negative charges on one plate pushes away like charges on the other plate, making it positively charged. The positive and negative charges on each of these plates attract each other, because that's what opposite charges do. But, with the dielectric sitting between them, as much as they want to come together, the charges will forever be stuck on the plate until they have somewhere else to go.

The stationary charges on these plates create an electric field , which influence electric potential energy and voltage. When charges group together on a capacitor like this, the cap is storing electric energy just as a battery might store chemical energy.

When positive and negative charges coalesce on the capacitor plates, the capacitor becomes charged. A capacitor can retain its electric field -- hold its charge -- because the positive and negative charges on each of the plates attract each other but never reach each other.

At some point the capacitor plates will be so full of charges that they just can't accept any more. There are enough negative charges on one plate that they can repel any others that try to join. This is where the capacitance farads of a capacitor comes into play, which tells you the maximum amount of charge the cap can store. If a path in the circuit is created, which allows the charges to find another path to each other, they'll leave the capacitor, and it will discharge.

For example, in the circuit below, a battery can be used to induce an electric potential across the capacitor. This will cause equal but opposite charges to build up on each of the plates, until they're so full they repel any more current from flowing.

An LED placed in series with the cap could provide a path for the current, and the energy stored in the capacitor could be used to briefly illuminate the LED. A capacitor's capacitance -- how many farads it has -- tells you how much charge it can store. How much charge a capacitor is currently storing depends on the potential difference voltage between its plates.

This relationship between charge, capacitance, and voltage can be modeled with this equation:. Charge Q stored in a capacitor is the product of its capacitance C and the voltage V applied to it. The capacitance of a capacitor should always be a constant, known value. So we can adjust voltage to increase or decrease the cap's charge. More voltage means more charge, less voltage That equation also gives us a good way to define the value of one farad. One farad F is the capacity to store one unit of energy coulombs per every one volt.

The gist of a capacitor's relationship to voltage and current is this: the amount of current through a capacitor depends on both the capacitance and how quickly the voltage is rising or falling. If the voltage across a capacitor swiftly rises, a large positive current will be induced through the capacitor. A slower rise in voltage across a capacitor equates to a smaller current through it. If the voltage across a capacitor is steady and unchanging, no current will go through it.

This is ugly, and gets into calculus. It's not all that necessary until you get into time-domain analysis, filter-design, and other gnarly stuff, so skip ahead to the next page if you're not comfortable with this equation. The equation for calculating current through a capacitor is:. The big takeaway from this equation is that if voltage is steady , the derivative is zero, which means current is also zero. This is why current cannot flow through a capacitor holding a steady, DC voltage.

There are all sorts of capacitor types out there, each with certain features and drawbacks which make it better for some applications than others. The most commonly used and produced capacitor out there is the ceramic capacitor.

The name comes from the material from which their dielectric is made. Ceramic capacitors are usually both physically and capacitance-wise small. A surface-mount ceramic cap is commonly found in a tiny 0.

While polymer capacitors are significantly higher priced than their liquid electrolyte counterparts, they are still far cheaper than an equivalent number of parallel ceramic capacitors. The low ESR of polymer capacitors makes them ideal for any high current ripple applications where large amounts of capacitance are required.

Aluminium polymer capacitors mainly offer very high capacitance density for their PCB footprint. The tall cylindrical aluminium capacitors allow you to provide exceptionally high capacitance by using high aspect ratio components, which are very tall relative to their footprint - if clearances allow.

Aluminium capacitors are known for their failure as the electrolyte either dries out or leaks. A leaking capacitor can destroy a circuit board that could otherwise be repaired by simply replacing the capacitor.

Due to the solid polymer electrolyte, no leakage is possible. As touched on earlier, polymer capacitors are excellent for high frequency applications in comparison to their liquid electrolyte counterparts. While not as good as a ceramic capacitor, they are very close and can offer high capacitance for a similar price and board footprint when compared to the ceramic capacitor option.

This makes the polymer capacitors excellent for power supplies and audio applications. While a polymer capacitor is typically more expensive than other alternatives, they can offer cost savings over ceramic capacitors due to the reduction in capacitance at the voltage in ceramics - requiring fewer polymer capacitors to do the same job.

Film capacitors, as the name suggests, use thin plastic film as a dielectric. These capacitors are cheap, very stable over time and have very low self-inductance and equivalent series resistance parameters. Some film capacitors can withstand extremely large reactive power surges. An extremely thin film is manufactured by a drawing process, which can then be metalized or left untreated depending on the properties required for the capacitor.

Electrodes are then added, and the assembly is mounted into a case which protects the capacitor from the environment. The relatively poor dielectric makes this type of capacitor very large in comparison to other types, giving it a very low capacitance per volume which lends it to significantly different applications to other options we have looked at.

Film capacitors are used in many applications where stability, low inductance and low cost is required. One interesting aspect of metalized film capacitors is that they are self-healing. Self-healing occurs when defects cause external voltage transients. Any arcing within the capacitor vaporizes the thin metalization of the film around the failure.

Polypropylene film capacitors are widely available and are useful in a wide range of applications. These capacitors can withstand extreme temperatures and ensure stable operation. However, these capacitors are relatively expensive and tend to be reserved for highly specialized applications. Polystyrene film is traditionally known as cheap general purpose capacitors with high stability and low dissipation and leakage.

Mica or silver mica capacitors are a type of capacitor that uses mica as a dielectric. Mica is a very electrically, chemically and mechanically stable material. Although it has the great characteristics of good electrical properties and high-temperature resistance, it has a high cost for raw materials. Mica is also resistant to most acids, water, oil and solvents. These capacitors are produced by sandwiching mica sheets with metal on both sides.

Silver mica capacitors are rare but still used when stable and reliable capacitors with very low values are required. They have very low loss, can be used for high frequencies and their values are incredibly stable change over time. Silicon capacitors, at least as a discrete component, are a relatively new type of capacitor. As an interesting note, the most common type of capacitor in the world by volume are silicone capacitors used in integrated circuits such as RAM and flash. This type of discrete capacitor is based on dielectrics such as silicon dioxide and silicon nitride, which are used to make high-density capacitors.

Such capacitors are highly applicable in situations when high stability, reliability and tolerance to high temperatures are required. The cost of silicon capacitors ensures they are only used in very specific applications.

Supercapacitors are another type of capacitor which cannot be compared with the others. This type of capacitor is used for a completely different purpose than those described above. Supercapacitors, in application at least, are more akin to batteries than the other types of capacitors we have discussed.

The main purpose of these capacitors is for energy storage with high current supply or memory backup applications such as RAM or GPS. There is a significant investment in research and development of supercapacitors currently, as an alternative to batteries for running electric vehicles.

The next decade is set to be very interesting with the rapid development of this technology. The capacitance range of supercapacitors starts from mF to several kilo farads, which is a considerable amount of energy. Their capacitance is thousands or millions of times higher than that of a typical capacitor you might use in a circuit design. While supercapacitors are often compared to lithium-ion batteries, they have substantially different properties.

In conclusion, every capacitor type has its place, even if that changes over time as new technologies and improvements to other capacitor types alter the market. Some types of capacitors can be superior to others. However, as we have seen, there are still many applications where one type of capacitor cannot be replaced for its ideal applications. Capacitors, just like every other type of component in electronics, are still advancing and progressing, being driven forwards by the demands of ever more advanced technology.

We often think of capacitors as a solved technology, but many capacitors we use today are significantly advanced from those available in recent history. MLCC applications are growing rapidly. They are the most popular capacitors and for a good reason. They are cheap, compact, generally and have good characteristics.

They offer the ideal tradeoffs of specifications and cost for most basic decoupling, filter, and bypassing applications. Tantalum capacitors have higher stability with temperature, DC bias and time.

Also, they are not impacted by the piezoelectric effect and are more stress-resistant. Unfortunately, they have high ESR, high price and a tendency to explode or turn into a little ball of fire if slightly mistreated. Aluminum electrolytic capacitors achieve very high capacitance and can have high maximum voltage ratings.

They are also much cheaper for the same capabilities as polymer capacitors. But they are large, have high ESR and dry out over time. Aluminium Polymer and Tantalum capacitors are superb and exciting new technology.

They have almost all the advantages of their traditional counterpart capacitors, with the addition of low ESR. However, currently, they are still relatively expensive and have fairly low maximum voltage ratings.

There are many types of film capacitor, each one being specific for a particular application. They are large and have low capacitance ratings, but are stable and have several other benefits. They have high tolerance, stability and precision but are relatively rare and expensive. Silicon capacitors are temperature stable and reliable, but are very expensive and have meagre capacitance ratings.

If the capacitor used does not have the same tolerance as stated in the parts list, then the circuit may not give the desired results or may not work. Temperature Coefficient This is the variation of capacitance with temperature.

Sometimes called Tempco and may be expressed as a percentage or as a variation in parts per million per degree Celsius. Capacitors may have positive or negative temperature coefficients; i. NPO capacitors are often used in radio and tuned circuits. Leakage Current In some capacitors, a leakage current flows through the dielectric and it may not hold its charge for long enough. Low leakage capacitors are available such as Tantalum Bead, and find use in applications like timing circuits.

Polyester Polyester capacitors are available from 1nF to 15uF. Sometimes packaged in colour bands matching the resistor colour code left and sometimes plain right.

Often used in decoupling circuits.