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
① 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
① 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