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What does it do?

Capacitors store electrical charge. They are often used for reducing ‘noise’ on a voltage or for controlling timing operations. Very large value capacitors (‘supercapacitors) can be used in place of rechargeable batteries.

How does it operate?

A capacitor contains a pair of conducting metal plates, separated by a thin layer of insulating material. When a current is fed into one of the plates it becomes electrically charged, and the voltage between the plates increases. So a capacitor acts as a storage system for charge and keeps voltage stable.

Capacitor Values

The capacitance (electronic ‘size’ or storage capacity) of capacitors is expressed in Farads (F). Most capacitors are very much smaller than a Farad in capacity.

The smallest capacitors are measured in pico Farads (pF). A pico Farad is a thousandth of a thousandth of a thousandth of a thousandth of a Farad; 10^-12F.

The next largest size of capacitors is measured in nano Farads (nF). A nano Farad is a thousandth of a thousandth of a thousandth of a Farad; 10^-9F. So, 1 nF = 1,000 pF.

Larger still capacitors are measured in micro Farads (mF). A micro Farad is a thousandth of a thousandth of a Farad; 10^-6F. So, 1 mF = 1,000 nF = 1,000,000 pF.

The ‘p’ and ‘n’ are often used in place of a decimal place. So, 2n2 is frequently used in place of 2.2nF. Capacitors come in preferred values, such as 1n, 2n2, 3n3, 4n7, 6n8, 10n, 22n etc.

Types of capacitor

A wide range of types of capacitors are available. The names given to capacitors are based on the insulating material used in them. The broad divisions are: non-electrolytic capacitors – generally low capacitance capacitors; electrolytic capacitors – higher values; and supercapacitors – very large vales.

Circuit diagram symbol for a non-electrolytic capacitor	Non-electrolytic capacitors This type of capacitor typically has a capacitance in the range between 1pF and 1mF. They are ‘unpolarised’ – meaning that they can be inserted into a circuit either way round. A wide range of insulating layers is used: ceramic, mica, mylar, polycarbonate, polystyrene, polypropylene, and polyester. In practice there is no important difference between these types, except in timing applications (see below).
Ceramic Capacitor	Metallised polyester capacitor
European circuit diagram symbol for an electrolytic capacitor	Electrolytic capacitors This type of capacitor typically has a capacitance in the range between 1mF and 10,000mF. They are ‘polarised’ – meaning that they can only be inserted into a circuit one way round. The negative lead is marked on the body of the capacitor. Electrolytic capacitors should not be operated above their rated voltage.
Radial electrolytic capacitor	Axial electrolytic capacitor

They are available in a ‘radial’ shape (these take up less space on the PCB but are taller), and an ‘axial’ shape (which lies flat on the PCB, like a resistor).

Electrolytic capacitors have a small internal ‘leakage current’ which means they gradually discharge.

The insulating layer in most electrolytic capacitors is aluminium oxide. Tantalum capacitors are smaller, have a lower leakage current but are more expensive.

Supercapacitors
These are a special type of electrolytic capacitor, sometimes called memory backup capacitors. They have much larger capacitance, in the range 0.1F to 50F.

Because of their very large capacitance, they can act as small electrical power sources that can be recharged in a few seconds – like a rapidly rechargeable battery. They can provide enough power for an electronic system that only consumes a low current. For example, a high efficiency LED can operate at 5mA. A low cost 0.22F supercapacitor can power this for a few minutes.

Supercapacitors do no hold their charge permanently because they have a small internal ‘leakage current’. If a new supercapacitor is charged up for a few minutes the leakage current will discharge it within a few hours. However, if it is kept on charge for about a day the leakage current is reduced and the charge is retained for several weeks.

Capacitors for Timing
Ceramic disc capacitors should not be used as timing capacitors. They are not sufficiently stable in capacitance to operate properly for timing. Suitable capacitor types are: silver mica, mylar, polycarbonate, polystyrene, tantalum, or similar types.

Capacitor Maths
The quantity of electrical charge stored in a capacitor is equal to the capacity (C) in Farads multiplied by the voltage (V) to which it has been charged. The quantity of charge (Q) is measured in Coulombs.

Expressed mathematically:

 

So, a 10F capacitor charged to 2.5V stores 25 Coulombs of charge because

 

As charge is a quantity of electricity, it corresponds to a flow of electrical current for a specified length of time. That is, charge (Q) is electric current (I) in Amperes multiplied by the time (t) in seconds for which it has been flowing i.e.

So, one Coulomb is the amount of charge in a current of one Amp current flowing for one second.

A LED drawing 5mA from a 0.22F capacitor charged up to a voltage of 5V would, theoretically, operate for:

 

 

 

 

 

In practice the voltage across the LED would fall as the capacitor discharged and there would come a point where the voltage would be too low to light the LED. So in practice the operating time would be shorter.

If a capacitor (C) is discharged through a resistor (R) the time (t) that the voltage takes to fall to half its original value is given by t = 0.7 x R x C.  So, for example, a 1F capacitor, discharging through a 10W resistor, would fall to half its starting voltage in about 7 seconds.

Investigating Capacitor Discharge

 

 

 

 

 

 

 

 

 

 

Click on the circuit diagram to download a Livewire file of the circuit showing how the capacitor C1 discharges through the resistor R1. Press switch SW1 to fully charge the capacitor and SW2 to discharge it. How long does the capacitor take to discharge through R1 to half its fully-charged voltage? Change the values of C1 or R1. Using the formula t = 0.7 x R x C, how long do you expect the capacitor to take to discharge to half its original voltage now? Check the result. Replace R1 with alternative Output subsystems. How long can they operate?

Possible applications

• Reducing electronic ‘noise’ from an electrical motor – the capacitor acts as a reservoir for charge, so the voltage across it cannot change rapidly
• Controlling the time period of a Pulse Unit
• Providing power to low current output devices (only supercapacitors)

Making

Before designing the PCB, check the package size and shape of the capacitor and the lead spacing. In the case of electrolytic capacitors and supercapacitors, make sure that they are inserted the right way round.

Fault finding

If there is a fault, check that:

• The capacitor value is correct
• The circuit voltage is not greater than the specified voltage for the capacitor
• The capacitor is the right way round

Alternatives

There are no alternatives to capacitors in most applications.

Web links

• Capacitor tutorial
• How capacitors work

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