1
Q
List the different types of capacitors you know
A
- Aluminium electrolytic
- Ceramic capacitors
- Metallized film capacitors
- Tantalum capacitors
- Supercapacitors
- Power capacitors
2
Q
Classify the types of capacitors in terms of voltage and capacitance (cosmos chart)
A
- Aluminium electrolytic capacitors: medium to high capacitance (1uF-1F), low to medium voltage (5V-800V).
- Ceramic capacitors: low capacitance (1pF-1uF), medium voltage (15V-100V).
- Film capacitors: low to medium capacitance (2pF-100uF), low to high voltage (50V-1000V).
- Tantalum capacitors: low to high capacitance (1uF-1000uF), low voltage (1V-60V).
- Supercapacitors: very high capacitance (1F-1000F), low to medium voltage (1V-100V)
- Power capacitors: medium to high capacitance (100nF-1F), medium to high voltage (100V-above 1000V)
3
Q
General applications (cosmos chart) of different types of capacitors
A
- Ceramic C: coupling and RF, power supply, filtering.
- Film C: coupling and RF, power supply, snubber, power electronics.
- Tantalum C: power supply.
- Aluminium electrolytic C: power supply, power storage, power electronics.
- Power C: power electronics, power supply.
- Ultracapacitors: power storage.
4
Q
Fundamental definition of capacitor
A
- Parallel plates with area A separated by distance d.
- Equal charge q with different polarity is accumulated on the plates.
- Charges attract each other and distribute uniformly on the inner surfaces A of the plates with a charge density rho.
- Electric field is established between the plates proportional to charge density and dielectric constant of inner material.
5
Q
9 Fundamental equations of capacitors
A
- Divergence of E field = Electric charge density/Permittivity
- Electric field = charge density/permittivity
- Charge density = Electric charge/Area
- Electric field = 0 (outside the plates)
- Voltage = Electric field * distance = Electric charge * distance /(permittivity*Area)
- Electric charge on each plate = Capacitance*Voltage
- Capacitance (no dielectric) = permittivity*Area/distance
- Capacitance (dielectric) = permitivitty of material*Area/distance
- Energy stored = Capacitance*Voltage^2/2
- I = C*dv/dt
- Xc = 1/jwC
- W = (I^2)*ESR
6
Q
Explain the effect of introducing a dielectric between the plates of a capacitor
A
- Atoms inside the dielectric are polarized by the accumulated charges in each plate. Positive charge is induced on one surface and negative charge is induced on the other surface of the dielectric.
- This polarization creates an E field inside the dielectric opposite to the original E field without dielectric.
- The field inside the plates is therefore reduced.
- Reduction of E field implies a reduction of the voltage, but as the charge remains the same the capacitance must increase, as shown by the equation. In other words, it effectively reduces distance between the plates and therefore increases capacitance (in case of placing a conducto).
7
Q
Relative permittivity of paper, polypropelene, polyethylen, glass, AlO, BaTiO3
A
- Glass: 6-8
- Polypropylene: 2.1
- paper: 1-4
- Aluminium oxide: 9
- Barium titanate: 103-104
- Polyethylen: 2.4
8
Q
Main challenges of applications where capacitors are used
A
- Lifetime/reliability
- Ripple current
- Humidity
- Corrosion
- Recycling
- Cost
- Mechanical vibration
- Temperature
- Size and weight
- Breakdown voltage
- Parital discharge
9
Q
List different types of housing
A
- Can, box: Aluminium, steel, polymers.
- Polymer film, epoxy or PU resin.
- No housing
- Deck.
10
Q
List different 5 types of terminals for capacitors
A
- Wire
- Snap in
- Screw terminal
- SMD
- Solder lugs
11
Q
What is a general behaviour of capacitors when frequency increases
Very simple and basic answer
A
They start behaving as inductors
12
Q
Comparison between aluminium electrolytic and film C (2 ideas at least)
A
- Film capacitors are more reliable and can withstand higher voltages
- AL provide higher capacitance
13
Q
Why polypropylene has low dielectric constant
A
Because it lacks of polarization due the chemical/atomic structure (carbon atoms)
14
Q
Why do humidity reduces dielectric breakdown of air?
A
- Because water molecules can be easily polarized/ionized.
- Water is a dipole
15
Q
1 Advantage and 1 disadvantage of tantalum capacitors
A
- Expensive
- More reliable
16
Q
Principl by which E fields can be added
A
principle of superposition of fields
17
Q
Describe atomic dipoles effect
A
- An electric field in any material causes a displacement of electrons and protons relative to each other.
- In an insulator electron do not move a lot, but they shift a little bit. Although an atom or molecule remains neutral, there is a small separation of its positive and negative charges and it becomes a microscopic dipole.
18
Q
Formula of dipole moment of two charges
A
dipole moment = electric charge*separation
19
Q
Describe some properties of water in terms of electric dipoles and compare with other substances
A
- Water molecule is neutral but charges are not all at one point.
- It has a strong dipole moment due to assymetry of charges
- CO2 molecules for example have no dipole moment due to symmetry of the molecule
20
Q
what is the problem of sharp points on a conductor
A
- The E field is larger than in other regions because small amounts of charge on sharp regions can provide a large surface density and therefore high E field around this region.
- It is a problem since it will increase the probability of air/dielectric breakdown.
21
Q
5 List general properties of capacitors
A
- Operating principle based in E fields
- Opposes to change in voltage
- Stores energy when constant voltage is applied
- Keeps the energy stored and voltage after removal of external voltage (it makes capacitors dangerous/should be discharged)
- Acts as open circuit for DC current, current leads the voltage by 90° in AC circuits.
- As energy stores, mechanical force is produced between the conductors.
22
Q
Give a definition of permittivity
A
- Measure of resistance of a medium to form an E field.
- Permittivity is determined by the ability of a material to polarize in response to E field, and hence reduce the total E field inside the material.
23
Q
Is the capacitance constant against the voltage? if so write the new equation of C
A
- No, i.e. when the capacitor charges it affects its mechanical properties (force), causing the capacitance to change.
- In this case capacitance can be defined in terms of incremental changes of charge: C = dq/dv
24
Q
Formula of capacitance in series and parallel
A
- Series: 1/C=1/C1+1/C2…
- Parallel: C = C1+C2+…
25
Explain general details of charging of capacitor
- At t=0, capacitor voltage is zero and current is maximum limited only by series R.
- The current exponentially decays with time and charge of capacitor exponentially increases.
- Eventually current through capacitor is zero.
26
general definition of dielectric
- Insulator material that can be polarized by an applied E field.
- Electric charges do not flow inside the dielectric, but shift slightly from their equilibrium position (dielectric polarization).
- This creates an E field that reduces overall field in the dielectric.
- Weakly bonded molecules in the dielectric reorient to align with the E field.
- Atoms consists of a cloud of electrons surrounding a positive charge at the centre. The cloud is distorted by the E field and forms a dipole.
- When E field is removed atoms reorient to original state. The time required to do so is called relaxation time.
- Dielectric supports high E field while dissipating minimal energy as heat.
27
Compare high and low dielectric constants materials based on capacitance, voltage and size
- High dielectric constant materials make possible to produce high-value capacitors in small sizes, but they cannot withstand high E fields (due to material properties and distance between electrodes)
- Low dielectric can withstand high E fields but provide low capacitance.
28
Is dielectric breakdown reversible?
- Yes for components like gases or liquids when the voltage decreases below a critical point.
- Not for solid dielectrics.
29
Is permittivity of dielectrics constant?
No, it tends to vary with T and sometimes with f and V (class II ceramics)
30
5 factors that affect dielectric strength
1. Temperature
2. Frequency
3. Voltage
4. Shape of electrodes (sharp regions increase E field)
5. Impregnants (can fill air void in the dielectric and increase breakdown voltage)
31
Some factors that affect capacitance are
1. Temperature.
2. Voltage (in ceramics for example, the E field applied changes the dielectric constant)
32
What is the usual degradation of lifetime due to temperature rise
Each 10°C rise of temperature causes a 50% reduction of capacitor life (Arrhenius equation).
33
Main characteristics of electrolytic capacitors
- High values of capacitance
- Unsuitable for filters, due to wide tolerance, polarized and frequency dependent capacitance.
34
Are all electrolytic capacitors polarized
No, there are bipolar electrolytic where no polarizing voltage is needed.
Capacitors
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