Electrical principles - Properties

Capacitance

Capacitance is the ability of a device to store an electrical charge.

The unit of capacitance is the farad.

Quantity

Symbol

Unit

Abbreviation

Meaning

capacitance

C

farad

F

ability to store a charge

 

The capacitor

A capacitor ia a device capable of storing an electrical charge. This is done by separating conductive plates with an insulating material. The insulator material is called a dielectric.

 

Capacitor parts

Internal view of a capacitor

Factors affecting the capacitance value of a capacitor:

  • area of the plates (bigger plates, more capacitance)
  • distance between the plates (bigger distance, lower capacitance)
  • material used as dielectric (higher insulative qualities, more capacitance).

 

Types of capacitors

Various capacitor types are used for specific purposes.

 

Electrolytic capacitor

Electrolytic capacitor

Electrolytic capacitor

These capacitors have aluminium or tantalum and the dielectric rolled together then enclosed. They are polarity sensitive and if connected incorrectly will explode.

Ceramic capacitor

Ceramic capacitor

Ceramic capacitor

A ceramic disc is silver plated on both sides. They are often used where a capacitor of high values is required. Ceramic capacitors are not polarity sensitive.

 

Variable capacitor


A variable capacitor is made up so that one plate can be moved in or out to change the capacitance.

 

Stacked plate capacitor


Stacked plate capacitors have the plates and dielectric stacked alternately. Each alternate plate is connected together to form one plate. These are often used where it is important not to have any self-inductance.

 

Dangers of capacitors

Capacitors will hold their charge until they are discharged. If a capacitor is not discharged before it is touched, it can cause electric shock.

Polarity sensitive capacitors can explode if they are not connected correctly.

 

Connection of capacitors

Series

Capacitors, when connected in a series, work the opposite to resistors and the overall value of the capacitance will reduce.

Here is the formula to calculate total capacitance in a series circuit.

Total capacitance formula for capacitors in series

Formula for total capacitance in a series circuit

Parallel

When capacitors are connected in parallel, the result is the sum of all the capacitors.

Here is the formula to calculate total capacitance in a parallel circuit.

Total capacitance formula for capacitors in parallel

Formula for total capacitance in a parallel circuit

 

Time constants

When voltage is applied to a capacitor it does not charge instantly. The capacitor is charged up over five time periods. These periods are called time constants. In the first time constant, a capacitor will always achieve 63.2% of total charge. The actual time that it will take can range from milliseconds to hours.

Select the switch to see the capacitor charge.

Text alternative

Charging of capacitor

The time it takes to charge up, depends on the size of the capacitor and the resistance in the circuit.

Now select the switch to see the capacitor discharge.

Text alternative

Discharging of capacitor

 

Calculations

The time constant depends on the resistance and capacitance (RC) in a circuit. The time constant can be calculated using the equation:

t = RC

Where:

t  =  time in seconds
R  =  resistance in ohms
C  =  capacitance in farads.

After five time constants a capacitor will be fully charged, therefore the charging time of a capacitor equals five times the time constant.