Capacitors are used to “Store electrical energy”, along with Resistors they make up two of the fundamental components used in electronic circuits.

## Capacitors – Use

Capacitors are used in several different ways in electronic circuits:

- Used to store charge for high-speed use or as a back-up supply in case of short power-cuts
- Capacitors can also help to eliminate supply voltage ripples.
- A capacitor can block DC voltage. Any alternating current (AC) signal flows through a capacitor unimpeded.
- To filter out fast voltage changes and high-frequency signals.
- To link high-frequency signals between two circuits at different D.C. voltages.
- With a resistor to provide a time delay.
- In oscillator circuits as a timing element.

## Charge and Voltage

A capacitor is an electrical component used to store energy electrostatically in an electric field between its plates. The capacitance is dictated by the surface area of the plates and the distance (and the material) between them, the closer and larger the plates the higher the capacitance.

The higher the density of charge the higher the potential difference the capacitor has across the plates.

Unlike a resistor, an ideal capacitor does not dissipate energy.

Charge *Q* is measured in Coulombs. 1C (=As) is the amount of charge held by 6.25×10^{18} electrons (electron charge: 1.6×10^{-19}C).

Current *I* of 1A represents the flow of a charge *Q* of 1C of electrons for 1 second, so *I=Q/t*, and *Q=It*.

A 1 Farad (F=As/V) capacitor has 1V at its terminals when charged with 1As.

The Farad is named after English physicist Michael Faraday.

To work out the capacitance of a particle capacitor you can use the following equation: C = ∈_{o}∈_{r} A/d where C is the Capacitance, ∈_{o} is the permittivity of free space (8.854187817 × 10^{−12} F/m). ∈_{r} is the dielectric constants of the material between the plates, A is the area of the plates, and d is the distance between the plates.

Most capacitors have values that are very small parts of a Farad; pF, nF, and μF, are common units of measurements for capacitors, 1F is 1,000,000 μF, and as you can see from the table to the right n and p are even smaller.

Until fairly recently 1F capacitors were very uncommon outside specialist applications and Car audio systems.

There are a large number of different types of capacitors, each of which has a number of different performance criterium, fundamentally they are made up of two, or multiples of two sheets of metal in parallel with each other separated by a dielectric (I will cover different construction in a later post).

However, the most common ones are small SMD ceramic (look a lot like SMD resistors), Disk Capacitors, Metal film capacitors, Tantalum capacitors, and Super Capacitors.

## Charging and Discharging

To charge a capacitor the Voltage curve can be resolved using the equation

V_{C} = V_{0} (1 – e^{((-t)/RC))}

The Current curve can be resolved using the equation

I = V_{0} / R e^{((-t)/RC)}

τ=RC is the time constant of an RC-circuit

The discharge curve is the inverse.

I will cover this topic in more detail in a later post, as I suspect it is important to also cover Inductors before we go much further.

## 7 thoughts on “Capacitors”

I hope you have a slide about why they’re one of the only parts that has not shrunk in the last 50 years.

Thanks for the idea 🙂

Hi! Great work!

I would however maybe add the reference of “electrostatically stored energy” much early in the page, at the first instance you mention storing charge and add a counter to that statement “as opposed to inductors which electromagnetically store/hold charge”

That will define the category of energy storage very early on and give a comparison.

Second… the tables look backwards to me in the layout?

When looking at 1 metre broken down in nits we often see 1 m = 100 cm = 1000 mm = 1000000um = 1000000000 nm Reading from left to right.

The relationship 1 nm has to 1 m is the same as 1 nF to 1 F. Both 1 nm and 1 nF is 1e-9. 1 billionth of 1 m or 1 F

Using a scale/rule that compares the units getting smaller will help the reader visualise why a 6 digit number of capacitance may actually be a lower capacitance than a 1 digit number and most people already understand metres, centimetres and millimeters. So it can be used to visually aim the viewer.

Other than that, very interesting and good.

^{-3}) of a Farad) = 0.001 F = 1,000 μF = 1,000,000 nF^{-6}) of a Farad) = 0.000,001 F = 1,000 nF = 1,000,000 pF^{-9}) of a Farad) = 0.000,000,001 F = 0.001 μF = 1000 pF^{-12}) of a Farad) = 0.000,000,0000,001 F = 0.001 nFTrying to provide constructive feedback, as per the resistor presentation you do not provide clear explanations of the terms used in the formulae. For example there is no mention that C represents capacitance until the section calculating the capacitance.

Overall there is a lot of good information in the article, in particular the chart depicting the type and usage.

There is an old adage, “do not sacrifice clarity for brevity”. For a non technical audience a little more explanation of the formulae is required.

Nice concise description.

Might be worth expanding “store electrical energy” to explain why a capacitor is different to a battery. It’s more of an “energy reservoir” to me, than “energy storage”.

“an ideal capacitor does not dissipate energy”… but a real capacitor has a tiny resistance (that can usually be ignored).

Some people might like a sentence about super-capacitors, perhaps?

Thanks for reminding me, I will add this to posts at some point soon, but until then, I will put them here: