Capacitance and capacitors

Question 1

What is the construction of a capacitor?

Answer 1

They usually consist of two metal plates, or metal foils that are insulated from each other by a material called the dielectric.

Question 2

What is the dielectric?

Answer 2

The dielectric uses materials that increase the strength of the electric field between the plates, which increases the amount of charge that can be stored at a given voltage. The effectiveness of the dielectric material is measured by a quantity called the “dielectric constant”.

Question 3

When choosing a capacitor, what voltage rating should you generally choose?

Answer 3

It is best to choose a capacitor with a voltage rating at least twice that of the expected working value required in the circuit.

Question 4

Which type of capacitor can only be used with DC?

Answer 4

Electrolytic

Question 5

On axial capacitors, the positive terminal always has a?

Answer 5

Crimped end

Question 6

How do capacitors work?

Answer 6

When a voltage is applied across the two opposite plates of a capacitor, a current flows due to the electric field generated between the plates as the charge accumulates. Eventually the current stops due to this accumulation of charge.

Question 7

What determines the amount of charge the plates can hold?

Answer 7

The amount of charge held by the plates depends on the following:

Plate area
The distance between the plates
The applied voltage

Question 8

What determines the amount of charge stored?

Answer 8

The applied voltage

Question 9

What is the unit for capacitance?

Answer 9

The farad

Question 10

What is a farad?

Answer 10

A capacitor has a capacitance of 1 farad if a current of 1 amp flowing for 1 second produces an increase in Potential Difference (PD) of 1 volt across the capacitor.

Question 11

The capacitance depends on what quanties?

Answer 11

Area of overlap of the plates
Distance between the plates
Dielectric material used

Question 12

What capacitors can be used with AC or DC?

Answer 12

Paper or film capacitors

Question 13

What happens to capacitance when connected in series?

Answer 13

Capacitance is reduced and the sum of the capacitances is smaller than that of the smallest individual capacitor.

Question 14

When capacitors are connected in parallel, what happens to the total capacitance?

Answer 14

Their capacitances must be added together to find the total capacitance.

Question 15

What governs the charge and discharge time of a capacitor?

Answer 15

The law of exponential decay.

Question 16

What occurs when you charge a discharged capacitor?

Answer 16

When connecting a completely discharged capacitor to a battery, the initial current will be extremely high, as the capacitor initially has no PD across it.

To limit this initial current, we always use capacitors with a resistor.

Question 17

What dictates charging time?

Answer 17

The higher the capacitance, and the resistance, the longer the charging time will be.

Question 18

What constant describes charging and discharging of capacitors?

Answer 18

The time constant

Question 19

What makes up the time constant?

Answer 19

Resistance and capacitance.

Question 20

What will a good capacitor show on a meter?

Answer 20

A good capacitor that can store a charge with less leakage will show an infinite resistance reading. This means there will be no current flow after it is fully charged.

Question 21

What will a bad capacitor show on a meter?

Answer 21

A shorted capacitor (fault) will show a zero or near-zero reading and will never swing to infinity.

Question 22

What type of meter is required for small capacitors?

Answer 22

An LCR tester

Question 23

What does an LCR tester measure?

Answer 23

Inductance, capacitance and resistance

Question 24

What are the two main types of capacitor failure?

Answer 24

Catastrophic Failures: Generally caused by a short circuit through dielectric breakdown or open circuits caused by connection failures.
Degradation Failures: Usually caused by a gradual decrease in leakage resistance, resulting in an increase in leakage current, equivalent series resistance, or dielectric absorption.

Question 25

What failure only occurs in AC circuits?

Answer 25

Another possible failure is excessive equivalent series resistance. This can be caused by a defective lead to plate contact, resistive leads, or resistive plates; it only occurs in AC circuits.

Question 26

What fault occurs mainly in electrolytic capacitors?

Answer 26

Dielectric absorption, which makes up 25% of defects, occurs mainly in electrolytic capacitors when they do not completely discharge during use and retain a residual charge. This can be dangerous, in that a capacitor that appears to have been discharged can still hold a large charge when the leads are disconnected from each other after shorting.