All About Capacitors
What do resistors do?
Capacitors and batteries both store electrical energy although batteries work in very different ways a capacitor is like a battery as a capacitor stores up electrons ready for use in the circuit.
Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric. The assembly is commonly consists of many small plates in parallel for each terminal, each separated from the other by a thin plastic film.
The table below shows the two main types of Capacitors and their different symbols used in circuits.
 | None Polarised |
 | Electrolytic EU |
 | Electrolytic US |
Capacitors are rated in Farads, and the standard symbol is "C" or "F", depending upon the context. A Farad is so big that capacitors are most commonly rated in micro-Farads (uF). Below are the units used to rate a capacitor.
- uF: Micro-Farad, 1x10-6 Farad (1,000,000th of a Farad)
- nF: Nano-Farad, 1x10-9 Farad (1,000,000,000th of a Farad)
- pF: Pico-Farad, 1x10-12 Farad (1,000,000,000,000th of a Farad)
A capacitor with a value of 100nF may also be written as 0.1uF. A capacitor marked as 2n2 has a value of 2.2nF. It may also be marked as 2,200pF.
In the case of a capacitor, the impedance is called Capacitive Reactance generally shown as Xc. The formula for calculating Xc is shown below
Xc = 1 / 2 ∏ F C
Capacitor Markings
Unlike resistors, capacitors are colour coded. The type of marking depends on the type of capacitor.
Ceramic Capacitors: These capacitors are usually used when extremely low values are needed. Ceramic capacitors typically range in value from 1pF up to 100nF, but in some cases and styles this will vary. They are commonly marked in pF (such as 100p), or a multiplier is used (such as 101, meaning 100pF - 10 plus one zero).
Plastic Film Capacitors: These are available in many different materials. Polyester is one of the most popular capacitor types, and these combine predictable size and good performance. If we take a 47nF (0.047uF) MKT capacitor as an example, this could be marked as 473k, 473k63, or 47n. A 4.7nF capacitor can be marked 472k, 472k63, or 4n7. The third digit is a multiplier, and indicates the number of zeros to give the value in pF. 63 means that the working voltage is 63V, and this must not be exceeded.
Electrolytic Capacitors: Used where large values are needed, these capacitors are always marked directly with the value in uF and the maximum voltage. Sometimes the maximum temperature is also indicated, but if not, 85 °C should be assumed. Electrolytic capacitors are Normally polarised, and the negative leg is marked clearly on the case. If no negative leg is clearly marked on the electrolytic capacitor then this will be a none-polarised capacitor and either leg can be used at the negative or positive.
Tantalum Bead Capacitors: Another form of polarised capacitor. Theoretically unaffected by zero bias voltage.
Capacitor Tolerance
The quoted tolerance of most polyester (or other plastic film types) capacitors is typically 10%, but it is usually better than that. Close tolerance types for example 1% are available, but they are usually much more expensive.
Electrolytic capacitors have a typical tolerance of +50/-20%, but this varies from one manufacturer to the next. Electrolytics are also affected by age, and as they get older, the capacitance falls. Modern electrolytics are better than the old ones, but they are still potentially unreliable at elevated temperatures or with significant current flow.
