Korean, Edit

Chapter 4. Capacitors and Coils

Recommended Article : Circuit Theory


1. Capacitors

2. Coils


1. Capacitors

⑴ Definition : Component that accumulates charge to store energy

① Capacitor can be considered as two metal plates with an area almost infinite compared to the distance between them.

② Capacitor is represented by two wires of equal length.


image

Figure 1. Symbol of Capacitor


⑵ Principle : Accumulation of charge


2-2_2

Figure 2. Accumulation of charge


① Before charging : No charge is accumulated on A and B.

② During charging : Electrons move from A to B.

③ After charging

○ When the voltage between A and B reaches V, no further charge transfer occurs.

○ Capacitor retains energy even when the switch is open.

⑶ Terminal characteristics

① Mathematical representation

Figure 3. Terminal characteristics and sign of capacitor

② Proof : Proof for Q = CΔV for two conductors

Assumption 1: Two conductors A and B with voltages VA (t) and VB (t) are connected by a wire.

Assumption 2: Initial charges on A and B and VA (t) - VB (t) are both 0.

○ (Note) Since any two conductors always start in Assumption 2, this assumption is reasonable.

○ Current flows from B to A through the wire, causing displacement current from A to B : Displacement current is described by Maxwell’s equations.

○ Applying integration by parts gives

○ C : = g(t) ⇒ Q = CΔV

○ (Note) In reality, ∫ f ‘(t) (VA - VB) ∝ VA - VB does not hold, so Q = CΔV doesn’t hold for real capacitors.

○ (Note) Knowing Q > 0 and ΔV > 0 implies C > 0.

③ Proof : Proof for Q = CΔV for n conductors

Assumption 1: n+1 conductors with voltages vp for each conductor with respect to a reference conductor.

Assumption 2: All voltages except vp and vq are 0.

○ Can use conclusion of Case 1.

○ If voltage between conductors is electrically linear, it can be expressed as follows.

④ Proof : Terminal characteristics for two conductors with wide plates facing each other

○ General equation

Case 1: When the distance of the capacitor keeps changing : Used for creating speed sensors based on C variation.

Case 2: Capacitors treated in circuit theory have fixed electrodes, so the second term is always 0.

⑷ Stored energy

⑸ Capacitance in different scenarios

① Series combination

② Parallel combination

③ Capacitance of a spherical capacitor

Figure 4. Capacitance of a spherical capacitor

⑹ Relative permittivity and breakdown voltage

① Relative permittivity (Dielectric Constant)

Table. 1. Relative permittivity

② Breakdown voltage (Dielectric Strength)

○ Capacitor experiences a stress proportional to 0.5εE2

○ Dielectrics have a limit to the mechanical stress they can endure under electrical stress.

○ Note: 1 mil = 1/1000 inch = 0.0254 mm

Table. 2. Examples of breakdown voltage

⑺ Real capacitors : Leakage current exists.

① Assume no free electron flow until breakdown voltage is reached.

② In reality, impurities in dielectric or forces within dielectric cause free electron flow.

③ Leakage current is explained using the following circuit model

Figure 5. Leakage current model

⒜ represents charging process, ⒝ represents discharging process

④ Leakage current is very small, but considerable in Electrolytic type Capacitors

⑻ Types of capacitors

Type 1: Fixed Capacitor

Type 1-1: Mica

○ Resistant to temperature changes and suitable for high-voltage applications.

○ Very small leakage current (Rleakage is approximately 1000 MΩ)

○ Used from a few pF to 200 pF, voltage around 100 V

○ Temperature coefficient : -20 ppm/℃ ~ +100 ppm/℃

Figure 6. Leakage current model

Type 1-2: Ceramic Capacitor

○ Available in types with Silver electrodes and Metal electrodes.

○ Very small leakage current (Rleakage is approximately 1000 MΩ)

○ Used in both DC and AC circuits

○ Used from a few pF to 2,000 pF, voltage around 5,000 V

Figure 7. Leakage current model

Type 1-3: Electrolytic Capacitor

○ Symbol

Figure 8. Symbol of Electrolytic Capacitor

○ Most commonly used in applications ranging from mF to thousands of mF

○ Insulating in one direction, conducting in the other

○ Used in DC and short-term AC applications

○ Large leakage current (Rleakage is approximately 1 MΩ), low breakdown voltage

○ Typically used from μF to thousands of μF, operating voltage around 500 V

Figure 9. Leakage current model

Type 1-4: Tantalum Capacitor

○ Available in solid type and wet-slug type

○ 1st. High-purity tantalum powder is compacted into rectangular or cylindrical shapes

○ 2nd. Anode lead wires are inserted into the structure

○ 3rd. Structure is baked in a high-temperature vacuum to create a porous structure

○ 4th. Porous structure increases surface area per unit volume

○ 5th. Thin manganese dioxide (MnO2) layer forms on porous material when dipped in acid solution

○ 6th. Electrolyte is added between MnO2 layer and cathode to create Solid tantalum Capacitor

○ 7th. Adding wet acid turns it into a wet-slug tantalum Capacitor

Figure 10. Tantalum Capacitor

Type 1-5: Polyester-film Capacitor

○ Two metal films are separated by insulation (e.g., Mylar)

○ Large ones have printed data for static capacitance and operating voltage on the surface

○ Small ones use color coding

○ Black band is printed on the outer metal film near the lead connection

○ Lead near this band must be connected to low voltage

○ Very small leakage current (Rleakage is approximately 1000 MΩ), used in DC and AC

○ Axial lead type : Used from 0.1 μF to 18 μF, operating voltage up to 630 V

○ Radial lead type : Used from 0.01 μF to 10 μF, operating voltage up to 1,000 V

Figure 11. Polyester-film Capacitor

Type 2: Variable Capacitor

○ Symbol

Figure 12. Symbol of Variable Capacitor

Type 1: Different electrode area structure

Figure 13. Structure of Variable Capacitor

○ Consists of a semicircular fixed metal plate and a rotatable metal plate

○ Principle : Dial rotation → Changing area of opposing plates → Change in capacitance

Type 2: Different electrode spacing structure

○ Feature : Insulator is air. Typically below 300 pF.

⑼ Marking Schemes

① Color Coding

○ Add -00 after the voltage value.

○ 8 : 0.01, 9 : 0.1 signify.

Figure 14. Structure of a Variable Capacitor

② Standard values are used like resistors.

③ Provide the values of capacitance, allowable tolerance, and if necessary, the maximum operating voltage.

④ The size of the capacitor represents the capacitance value (small units in pF, large ones in μF).

⑤ M : ±20 %, K : ±10 %, J : ±5 %, F : ±1 %

⑽ Utilization of Capacitors

① Computer Keyboard

Figure 15. Structure of a Computer Keyboard

○ 1st. Pressing the keyswitch.

○ 2nd. Decreased gap between capacitor’s metal plates increases electrical capacitance.

○ 3rd. Since charge is constant, increased electrical capacitance leads to higher voltage.

○ 4th. Computer recognizes voltage increase, resulting in character input.

② Touch Screen

Figure 16. Structure of a Touch Screen

○ 1st. Applying voltage to the glass charges the surface.

○ 2nd. Touching the top glass surface with a finger attracts stored electrons to the contact point, changing surface charge.

○ 3rd. Since electrical capacitance is constant, changes in charge result in voltage variation.

○ 4th. Sensor detects voltage changes.

③ Condenser Microphone

Figure 17. Structure of a Condenser Microphone

○ 1st. Vibrating metal plate due to sound.

○ 2nd. Changing gap between two metal plates alters electrical capacitance.

○ 3rd. Since charge is constant, capacitance change leads to voltage variation.

○ 4th. Voltage change is converted into an electrical signal.

Camera Flash

○ 1st. Capacitor charged before taking a photo.

○ 2nd. Pressing the flash switch discharges ignition capacitor.

○ 3rd. Current flows in flash tube, generating light.

⑤ Automated Defibrillator

○ 1st. Powering on the automated defibrillator charges the capacitor.

○ 2nd. Pressing the switch discharges briefly, sending a strong current to stimulate the heart.



2. Coil (Inductor)

⑴ Definition : Component that stores energy through a magnetic field.

Figure 18. Structure of a Coil

⑵ Terminal Characteristics

① Mathematical Expression

Figure 19. Terminal Characteristics of a Coil

② Proof : Self-inductance

○ First, consider the situation below.

Figure 20. Situation for Proof of Self-inductance

○ By Maxwell’s Equations and Green’s Theorem, the following equation holds.

○ Green’s Theorem applies to arbitrary surfaces, considering a circular plane like the one in the figure.

○ n vector is defined outward from the ground.

○ Naturally, the orientation of the surface’s boundary is counterclockwise as shown.

○ (Note) The direction in Green’s Theorem is important. Let’s use the right hand often!

○ The coil is a solenoid.

○ N : Number of turns

○ ℓ : Solenoid length

○ L : Proportionality constant between magnetic flux ΦB and current i.

Case 1: Assume the coil is right-handed according to the current direction.

○ View the situation from the direction where the current flows out.

Figure 21. Assuming the Coil is Right-Handed according to Current Direction

○ Magnetic field and surface normal are parallel.

Case 2: Assume the coil is left-handed according to the current direction.

○ View the situation from the direction where the current flows in.

Figure 22. Assuming the Coil is Left-Handed according to Current Direction

○ Magnetic field and surface normal are parallel.

Conclusion: The forms of the equation are the same whether it’s right-handed or left-handed.

Extension of Conclusion: Now, consider the coil with N layers and measure the potential difference between both ends.

③ Proof : Extension of ② Proof

○ General Expression: You can write the following equation for any conductor.

Case 1: If L varies with time:

○ The 2nd term arises from the coil’s movement, relevant in electromechanical applications.

○ Can also be used as a sensor.

Case 2: If L is constant: Generally, coils dealt with in circuit theory are fixed, so the 2nd term is assumed to be 0.

④ Understanding the Signs

Case 1: If i increases, induced current flows in the direction to hinder i’s increase due to inertia.

Figure 23. Scenario for Increasing i

Case 2: If i decreases, induced current flows in the direction to hinder i’s decrease due to inertia.

Figure 24. Scenario for Decreasing i

⑶ Stored Energy

⑷ Various Cases of Inductive Coefficients

① Series Composition

② Parallel Composition

⑸ Characteristics of a Coil

① Notation of a Coil

Figure 25. Notation of a Coil

② Standard values for coils (5, 10%) are like resistors.

③ Types of Coils

Figure 26. Types of Coils

⑹ Real Coils

① Model actual coils as shapes containing resistors and capacitors.

② Ignore capacitors in the model for practical applications.

③ Rl is around several Ω to hundreds of Ω; if the coil conductor is thin and long, resistance increases.

Figure 27. Modeling of Actual Coil



Input: 2016.01.12 11:17

Modified: 2018.12.13 18:08

results matching ""

    No results matching ""