Korean, Edit

Lesson 2. Power Supply

Recommended Article: [Circuit Theory] Circuit Theory Index


1. Power Supply

2. Voltage Source

3. Current Source


1. Power Supply

⑴ Power Supply : Interpreting circuits by approximating actual power sources as ideal power sources



2. Voltage Source

⑴ Overview

① Definition : A power source that provides a constant voltage under specific conditions

② Lower internal resistance is better for a voltage source

⑵ Classification

① Independent Voltage Source : A voltage source that always provides a constant voltage

② Dependent Voltage Source : A voltage source that provides voltage according to external circuit conditions

○ Example : Some transistors

○ VCVS (Voltage-Controlled Voltage Source) : Left

○ CCVS (Current-Controlled Voltage Source) : Right

⑶ Practical Voltage Source, Non-Ideal Voltage Source

Figure 1. Current-Voltage Characteristics of a Practical Voltage Source

⑷ Battery

① Definition : A power source that provides a constant voltage using the potential of a chemical reaction

② Components, capacity, electrochemical theory

③ Mercury Battery

○ Reaction

(-) Zn-Hg Amalgam HgO + KOH HgO, E0 = 1.3 V

(+) : HgO + H2O + 2e- → Hg + 2OH-

(-) : Zn + 2OH- → Zn(OH)2 + 2e-

○ Features : Button-shaped, very small size, constant voltage, primary cell

○ Drawback : Mercury pollution due to mercury formation

④ Voltaic Cell

○ Reaction

(-) Zn H2SO4(aq) Cu(+), E0 = 1.10 V

(+) : Cu2+(aq) + 2e- → Cu(s)↓, E0 = 0.34 V

(+) : 2H+(aq) + 2e- → H2(g)↑

(-) : Zn(s) → Zn2+ + 2e-, E0 = -0.76V

○ Polarization Phenomenon : Hydrogen gas generated on the copper plate surrounds the copper plate, interfering with the reduction reaction, causing a rapid drop in electromotive force

○ Depolarizer : An oxidizing agent used to remove polarization phenomenon, oxidizing hydrogen gas to water

○ Example : H2O2, MnO2, KMnO4, K2Cr2O7

⑤ Daniell Cell

○ Reaction

(-) Zn ZnSO4(aq) CuSO4(aq) Cu(+), E0 = 1.10 V

(+) : Cu2+(aq) + 2e- → Cu(s)↓, E0 = 0.34 V

(-) : Zn(s) → Zn2+(aq) + 2e-, E0 = -0.76 V

○ No gas generation, so no polarization phenomenon occurs

○ Salt Bridge

○ A salt bridge is essential for the Daniell Cell

○ Prepared by dissolving KCl or KNO3 in water, boiling the solution, and cooling it to create a U-shaped tube

○ Acts as a pathway for ion movement, balancing the charges in the overall circuit

⑥ Manganese-Zinc Battery

○ Reaction

(-) Zn Saturated NH4Cl Solution MnO2, E0 = 1.5 V

(+) Carbon Rod : 2MnO2 + 2NH4+ + 2e- → Mn2O3 + 2NH3 + H2O

(-) Zinc Plate : Zn → Zn2+ + 2e-

○ Features : No need for a salt bridge, short lifespan due to weak acid electrolyte, produces around 1.5 V

⑦ Alkaline-MnO2 Cell : Primary cell, most common

○ Reaction

(-) Zn KOH MnO2, C(+), E0 = 1.43 V

(+) Zinc Plate : 2MnO2(s) + H2O(l) + 2e- → Mn2O3(s)↓ + 2OH-(aq), E0 = 0.15 V

(-) Carbon Rod : Zn(s) + 2OH-(aq) → ZnO(s)↓ + H2O(l) + 2e-, E0 = -1.28 V

○ Initially produces a constant voltage due to all the products being solids

○ Longer lifespan compared to the manganese-zinc dry cell

○ Voltage : Depends on the purity of MnO2, typically around 1.50 ~ 1.65 V for new alkaline cells

○ Nominal Voltage : 1.2 V/cell

○ Nominal Capacity : 5 Ah

○ Voltage drops to about 0.90 ~ 1.0 V when the battery is fully discharged

○ Structure and types of energizers, availability range

Figure 2. Structure, Types, and Availability Range of Energizers

⑧ Lead-Acid Cell : Secondary cell, used in cars, submarines

○ Reaction

(-) Pb H2SO4 PbO2(+), E0 = 2.05 V

(+) Lead Dioxide Plate : PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e- → PbSO4(s)↓ + 2H2O(l), E0 = 1.685 V

(-) Lead Plate : Pb(s) + HSO4-(aq) → PbSO4(s) + H+(aq) + 2e-, E0 = -0.365 V

○ Voltage : Various cells due to acidity, generally around 2.10 V, discharges at around 1.95 V

○ Nominal Voltage : 2.0 V/cell

○ Nominal Capacity : 10 Ah

○ Multiple cells connected in series to create 12 V, 24 V systems

○ SG of 1.30 indicates normal, around 1.1 indicates a need for charging

○ Charging a lead-acid cell to complete depletion might render it unchargeable

○ Requires DC power for charging, both voltage and current sources are possible

○ During discharge, the mass of both electrodes gradually increases, and the concentration of sulfuric acid solution decreases

○ Charging : Applying current from an external DC power source initiates the reverse reaction, diluting the sulfuric acid and restoring electromotive force

○ Charging should be done before the battery voltage drops below 1.8 V

○ The PbO2 at the positive electrode acts as a depolarizer

○ Structure of lead-acid cell

Figure 3. Structure of a Lead-Acid Cell

⑨ General Battery

○ Battery : Stores DC power converted by rectifiers

○ Components of battery system : Battery, charging device, security device, control device

○ Battery Capacity Formula : C = K × I / L

○ C : Battery capacity (Ah), L : Capacity maintenance factor (capacity loss rate), K : Capacity conversion time coefficient, I : Discharge current (A)

○ Charging Methods

○ Initial Charging : Charging performed for the first time by injecting electrolyte into an uncharged battery

○ Standard Charging : Periodic charging at a fixed rate when needed

○ Fast Charging : Charging at 2 to 3 times the standard current for a relatively short period

○ Float Charging : Balancing discharge of battery - applying constant load, load power supply - temporary heavy current load, etc.

○ Improved methods of float charging include trickle charging and equalizing charging

○ Charging Current (A) = Battery Capacity (Ah) / Rated Discharge Time (h) + Constant Load Capacity (VA) / Standard Voltage (V)

Figure 4. Float Charging

○ Recovery Charging

○ Sulfation Phenomenon : Applicable to lead-acid batteries

○ Cause : Prolonged storage in discharged state, high discharge current, insufficient repeated charging

○ Phenomenon : Plate turns white and bends, electrolyte temperature rises during charging, specific gravity decreases, gas generation increases

○ Characteristics of Alkaline Batteries

○ Advantages : Longer lifespan, resistant to vibration and shock, good charge/discharge characteristics, stable discharge voltage, wider operating temperature range

○ Disadvantages : Lower nominal voltage compared to lead-acid batteries, higher cost

⑩ Ni-Cad Battery (Nickel-Cadmium Cell) : Secondary battery, not commonly used nowadays due to toxicity of heavy metals

○ Reaction Equations

(-) Cd KOH NiO(OH) (+), E0 = 1.40 V

(+) : NiO(OH)( s ) + H2O( l ) + e- → Ni(OH)2( s ) + OH-( aq ), E0 = 0.52 V

(-) : Cd( s ) + 2OH-( aq ) → Cd(OH)2( s )↓ + 2e-, E0 = -0.88 V

○ Voltage : Maintains a constant voltage of 1.2 V, lower than the 1.5 V of alkaline batteries

○ Becomes unusable after 4000 charge/discharge cycles

○ Memory Effect : Due to nickel, capacity decreases when not fully discharged before charging

○ Ni-Cad batteries should be charged with a constant current source

○ Types of Ni-Cad Batteries

Figure. 5. Types of Ni-Cad Batteries

⑪ Fuel Cells

○ Overall Reaction : H2( g ) + 1/2 O2( g ) → H2O( l ), E0 = 1.23 V

(+) : 1/2 O2( g ) + H2O( l ) + 2e- → 2OH-( aq )

(-) : H2( g ) + 2OH-( aq ) → 2H2O( l ) + 2e-

○ Generates electricity and heat during the electrochemical reaction, increasing overall efficiency to over 80%

○ Thermal efficiency typically around 25-30%

○ Low noise, environmentally friendly

⑫ Lithium-Ion Battery

○ Overall Reaction : LiCoO2( s ) + 6C( s ) → CoO2( s ) + LiC6( s )

(+) : Li+( aq ) + 6C( s ) + e- → LiC6( s )

(-) : LiCoO2( s ) → CoO2( s ) + Li+( aq ) + e-

○ Requires electrolyte and separator for Li+ ion movement

Figure. 6. Components of Lithium-Ion Battery

○ No need for salt bridge, unlike conventional chemical cells

○ Types of Cathode Materials

Type 1. LCO

○ Material : LiCoO2

○ Type : Cobalt-based

○ Advantages : Capacity

○ Disadvantages : Output/safety

○ Applications : IT

Type 2. NCM

○ Material : Li[NiCoMn]O2

○ Type : Ternary

○ Advantages : Capacity

○ Disadvantages : Output/safety

○ Applications : EV, ESS

Type 3. NCA

○ Material : Li[NiCoAl]O2

○ Type : Ternary

○ Advantages : Capacity/output

○ Disadvantages : Safety

○ Applications : Non-IT, EV

○ High Nickel : Aiming to replace costly cobalt, NCA with over 90% nickel content in cathode

Type 4. LMO

○ Material : LiMn2O

○ Type : Manganese-based

○ Advantages : Output

○ Disadvantages : Capacity

○ Applications : Non-IT, ESS

Type 5. LFP

○ Material : LiFePO4

○ Type : Iron phosphate-based

○ Advantages : Safety

○ Disadvantages : Capacity

○ Applications : Non-IT, EV

⑬ Other Types of Batteries

○ Lithium-MnO2 Cell : Primary battery, used for memory backup

○ Zinc Air Cell : Primary battery, uses air as electrolyte, used in hearing aids, medical monitoring devices

○ Silver Oxide Cell : Primary battery, used in wristwatches

○ Lithium-Iodine Cell : Primary battery, provides long-term power for circuits, used in pacemakers for artificial hearts

Figure. 7. Lithium-Iodine Cell

○ Nickel-Metal Hydride Cell : Secondary battery, used in portable devices (laptops, mobile phones, etc.)

○ Lithium-Ion & LiPo Cell : Secondary battery, electrode materials (lithium-carbon), used in electric vehicles, mobile phones, drones

⑸ Solar Cells (Photovoltaic Cells) : Utilize the photovoltaic effect

① When P-type and N-type semiconductors are joined, an electric field is generated around the PN junction from N to P when voltage is applied

② Wire connections are established for electron flow within the semiconductor

③ Shining light onto the N-type semiconductor side causes electrons to move from P to N at the PN junction

④ Inverter converts direct current to alternating current

Figure. 8. Solar Cell

⑹ Direct Current Generator : Utilizes Faraday’s law of electromagnetic induction

Figure. 9. Direct Current Generator

Figure. 10. Direct Current Generator

⑺ Electronic Power Supplies by Rectification

① AC → DC

② Structure of Power Supply : (Right) from top to bottom: Voltage adjusting screw, Current Limit Checker, switch arrangement

Figure. 11. Electronic Power Supplies by Rectification

② Types

of Terminals

③ - Terminal connected to Ground Terminal : Voltage is set at 10 V

④ + Terminal connected to Ground Terminal : Voltage is set at 15 V

⑤ Floating Supply : Danger of electrical shock if ground terminal is not connected to any terminal

○ Due to airborne charges, 10 V could become 200 V/210 V

⑥ Uninterruptible Power Supply (UPS)

○ UPS : Provides power to the load normally when abnormalities occur in input power

○ Block Diagram

Figure. 12. Uninterruptible Power Supply

○ Converter : Converts AC to DC

○ Inverter : Converts DC to AC with standard frequency

⑻ Thermocouples

Seebeck Effect (Thermal Phenomenon) : Also known as the Thermo Electric effect

○ Definition : When the ends of two different conductors or semiconductors are joined, applying a temperature difference generates electromotive force

○ Also known as Thermoelectricity

○ Discovered by Thomas S. Seebeck in 1821

② Often used with one end immersed in ice water (0°C) as a temperature sensor

○ Magnitude and polarity of thermoelectric power do not depend on wire thickness or length

Thermoelectric Coefficient: Thermoelectric power per 1°C temperature difference

③ Types of thermocouples include those made from materials like Bi and Sb thin films, single-crystal silicon, etc.

④ Thermocouples have a wider range of applications compared to thermistors

⑤ Thermocouples are weaker for practical power applications

Figure. 13. Example of Thermocouples

⑥ Thermocouples can be used for a wider range of applications compared to thermistors

Figure. 14. Comparison between Thermocouples and Thermistors

⑼ Piezoelectric Sensors

Piezoelectric Effect

Figure. 15. Piezoelectric Effect 1

○ Definition : Deformation occurs in certain crystals when pressure is applied, creating a polarized voltage

○ Also known as Piezoelectricity

○ Examples : Quartz, ceramic, cadmium sulfide, gallium arsenide compounds

○ Applications : Gas stoves, load cells, microphones, speakers, etc.

② Used in pressure sensors, force sensors, accelerometers, ultrasonic devices, etc.

③ Sauerbrey equation

○ f0 : resonant frequency (Hz)

○ Δf : frequency change (Hz)

○ Δm : loaded mass (g)

○ A : piezoelectrically active area (electrode area) of the crystal (cm²)

○ ρq : density of quartz (2.648 g/cm³)

○ μq : shear modulus of AT-cut quartz crystal (2.947 × 1011 g/cm·s²)

○ vq : transverse wave velocity in quartz (m/s)

④ (Note) Piezo resistive phenomena

○ Resistance changes due to pressure or stress

○ Usually, resistance increases with applied pressure



3. Current Sources

⑴ Overview

① Definition : A power supply that provides a constant current under specific conditions

② A higher internal resistance is better for a current source to maintain a stable current despite external resistances

⑵ Classification

① Independent Current Source : Always provides a constant current

② Dependent Current Source : Provides current based on external circuit conditions

○ CCCS (Current-Controlled Current Source) : Left

○ VCCS (Voltage-Controlled Current Source) : Right

⑶ Practical Current Sources (Non-Ideal Current Sources)

Figure. 16. Practical Current Sources

⑷ Transistors

① Some transistors function as current sources above a certain voltage

② Voltage-Current Graph

Figure. 17. Voltage-Current Graph

③ Equivalent Circuit

Figure. 18. Equivalent Circuit of Current Source

④ Applications : Constant-current battery chargers

○ Converts AC to DC using a rectifier circuit

Advantages 1. Charging time is greatly reduced in constant voltage mode

Advantages 2. In mobile phone batteries, fluctuating current can cause damage



Input: 2015.12.29 18:57

Modified: 2022.09.11 16:14

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