Capacitance and capacitors Flashcards
(26 cards)
What is the construction of a capacitor?
They usually consist of two metal plates, or metal foils that are insulated from each other by a material called the dielectric.
What is the dielectric?
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”.
When choosing a capacitor, what voltage rating should you generally choose?
It is best to choose a capacitor with a voltage rating at least twice that of the expected working value required in the circuit.
Which type of capacitor can only be used with DC?
Electrolytic
On axial capacitors, the positive terminal always has a?
Crimped end
How do capacitors work?
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.
What determines the amount of charge the plates can hold?
The amount of charge held by the plates depends on the following:
Plate area
The distance between the plates
The applied voltage
What determines the amount of charge stored?
The applied voltage
What is the unit for capacitance?
The farad
What is a farad?
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.
The capacitance depends on what quanties?
Area of overlap of the plates
Distance between the plates
Dielectric material used
What capacitors can be used with AC or DC?
Paper or film capacitors
What happens to capacitance when connected in series?
Capacitance is reduced and the sum of the capacitances is smaller than that of the smallest individual capacitor.
When capacitors are connected in parallel, what happens to the total capacitance?
Their capacitances must be added together to find the total capacitance.
What governs the charge and discharge time of a capacitor?
The law of exponential decay.
What occurs when you charge a discharged capacitor?
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.
What dictates charging time?
The higher the capacitance, and the resistance, the longer the charging time will be.
What constant describes charging and discharging of capacitors?
The time constant
What makes up the time constant?
Resistance and capacitance.
What will a good capacitor show on a meter?
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.
What will a bad capacitor show on a meter?
A shorted capacitor (fault) will show a zero or near-zero reading and will never swing to infinity.
What type of meter is required for small capacitors?
An LCR tester
What does an LCR tester measure?
Inductance, capacitance and resistance
What are the two main types of capacitor failure?
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.
