3.9 Capacitance And Capacitors Flashcards
What are capacitors?
Components that store charge in a circuit for a short time
What is the general construction of a capacitor?
Two metal plates insulated from each other by a material called dielectric
What is the purpose of Dielectric in a capacitor?
To increase the strength of the electric field which increase the amount of charge that can be stored at a given voltage
The effectiveness of the dielectric material is ensured by a quantity called?
Dielectric constant
What is the abbreviation IEC?
International Electrotechnical Commission
What is the abbreviation ANSI?
American National Standards Institute
What is a polarised capacitor also know as?
Electrolytic capacitor
What’s the difference in symbols between a normal capacitor and a polarised capacitor?
It has a + icon next to the positive terminal
How can a variable capacitor symbol be identified?
It has an arrow going diagonally through the normal capacitor symbol
What are the two distinct hazards you must be aware of when working with capacitor?
-electric shock
-overheating due to excess voltage
What causes overheating in a capacitor?
Excessive or reverse voltage
Where can you find the maximum voltage value of a capacitor?
Marked or encoded on the body of the capacitor
When choosing or replacing a capacitor, what is best to do?
Choose a capacitor with a voltage rating at least twice that of expected working value required
Where is the only place a electrolytic capacitor can be used?
In a circuit with direct current and must be connected with the correct polarity
How can you identify the positive side of a electrolytic capacitor?
-it will be marked on the side with a ‘+’
-the positive lead (wire) will be longer
How can you identify the negative side of a electrolytic capacitor?
-it will be marked with a solid black line or a ‘-‘ on the side
-the negative lead (wire) will be shorter
How can you identify the positive end of an axial capacitor?
The positive end will always be at the crimped end of the cyclic dear
How does a capacitor give engineers electric shocks?
Because they store electric charge so will hand it out even if the circuit has no power
What voltage must engineers be carful of when working on capacitors?
65V
In a capacitor what determines the amount of charge it can hold?
-plate area
-the distance between plates
-applied voltage
What are the units for capacitance?
Farad
1 farad is a huge value of capacitance, therefore it is usually shown in?
-microfarads
-nanofarads
-picofarads
What does the capacitance of a capacitor depend on?
-area of overlap of the plates
-distance between the plates
-dielectric material used
What are two ways of getting use of both sides of the capacitor plates?
-rolling them up together
-have smaller but more plates placed in parallel (like a sandwich)
What benefits does using a dielectric material come with compared to just air?
-higher capacitance
-prevents sparking between plates
How is the dielectric constant used in fuel tanks?
Because fuel has a higher dielectric constant than air when it fills concentric tubes and a charge is passed through it will see the level depending on the capacitance of the sensors
How does having water in fuel tanks falsify the fuel level reading?
Because water has a dielectric constant of 80
What is special about the paper foil or plastic film capacitors?
-they are non-polar (either plate can be positive or negative)
-can be used with AC voltage
What values do paper foil or plastic film capacitors come in?
Any value you want
How are capacitance values and tolerances shown on a capacitor?
Using a colour code system (like resistors)
What are the colours painted on a capacitors body called?
Colour bands
What are the first and second colour bands on a capacitor indicating?
Representing the first and second digits of its capacitance
What is the third colour band representing on a capacitor?
The decimal multiplier
What does the fourth colour band indicate on a capacitor?
The tolerance
In the colour band system of capacitors what is the value given in?
Picofarads (10^-12F)
Sometimes on a capacitor a fourth band will be visible, what does this mean?
It is a type K capacitor meaning the band denotes the voltage rating of a capacitor
What happens when you connect capacitors in series?
The sum of capacitances is smaller than the smallest capacitor (The capacitance is reduced)
When capacitors are connected in series what happens to the sum of voltages?
They’re distributed in inverse proportions to capacitances (largest capacitor = smallest voltage)
If capacitors are connected in parallel what must you do?
Simply add them together to get your total
When connecting a completely discharged capacitor to a battery what happens to the initial current?
It will be extremely high as the capacitor initially has no
Potential difference across it
How do you prevent the initial current being really high on a new capacitor?
Use a resistor in series
What affects the charging time of a capacitor?
The capacitance and the resistance (higher means longer charge time)
What is known as the time constant?
It’s a estimate of the time taken to charge or discharge a capacitor which is in series with a resistor
When visually inspecting capacitor what are the two faults you can see?
Bulging of the casing and electrolyte leakage
What can be used to test a capacitor?
An ohmmeter
How do you firstly discharge a capacitor?
By touching the capacitors leads together
When testing a capacitor with an ohmmeter what will it read if its faulty?
It will show zero and never move
What is used to test capacitors with a very small capacitance?
An LCR tester (inductance L, capacitance C, resistance R)
What are the two categories that a capacitors goes into if it fails?
-catastrophic failure
-degradation failure
What is a catastrophic failure of a capacitor?
Caused by a short circuit through dielectric breakdown or open circuits caused by connection failure
What is a degradation failure of a capacitor?
Gradual decrease in leakage resistance, increase in current leakage, equivalent series resistance, dielectric absorption
What is excessive equivalent series resistance?
-Failure due to a defective lead to plate contact
-resistive leads
-resistive plates
What type of circuits does excessive equivalent series resistance occur in?
AC circuits
What percentage of all defective capacitors have excessive leakage current?
40%
What type of capacitors are particularly susceptible to excessive leakage current?
Electrolytic capacitors
Dielectric absorption makes up to what percentage of defects of capacitors?
25%
When does dielectric absorption occur?
When they aren’t completely discharged during use and retain residual charge
