Selecting Capacitors
Article : Andy Collinson
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Introduction
There are many different types of capacitors in use in electronics. Although all perform the same basic function, factors such as the type of construction, tolerance, working voltage and temperature coefficients need to be taken into account when selecting a capacitor. This tutorial is a practical guide to selecting the right capacitor for any circuit.

Value
Capacitors are available in sizes from 1pF (1 x10-12 F) to 100,000mF. Depending on the type of capacitor, it may not be available in every value.

Working Voltage
The general rule is always 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%.

Polarisation
Capacitors that are polarised, have marked positive and negative terminals. They must never be connected the wrong way around or used in a circuit where the voltage may reverse polarity. Some electrolytics may explode if connected incorrectly.

Tolerance
Some circuits e.g. timing or oscillators may require a high precision capacitor. 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.e. their value increases or decreases with temperature or NPO (Negative Positive Offset). 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). Working voltages between 50V and 1500V and are available in 5%, 10%, 15% tolerance. Often used in decoupling circuits.

Polycarbonate
These capacitors are available from 100pF to 10uF, working voltages up to 400V and tolerance 5% or 10%. They have a temperature coefficient of about 100 ppm / °C. Often used in filtering and timing circuits.

Ceramic Disc
Ceramic disc are available from 1pF to 220nF, working voltages up to 100V and available in wide tolerance ranges. They are used for many purposes including decoupling circuits and have high capacitance for their small size.

Electrolytic
Electrolytics are available from 1uF to 47000uF, working voltages up to 600V and available in wide tolerance ranges. They are used extensively in power supplies, audio amplifiers and decoupling work. Care must be taken to connect with the correct polarity, otherwise gas can form and case explode.

Variable and Trimmer
Variable capacitors are used extensively in radio circuits, RF oscillators and transmitters. Often available as single gang or multi gang and are available from 25p to 1000pF. The trimmer capacitor is the smaller brother and once adjusted is left fixed. Trimmers are available from 5p to 100pF.