B - A - 3.9 - Capacitance Capacitor PDF

Module 3: Electrical Fundaments Topic 3.9: Capacitance/Capacitor  INTRODUCTION On completion of this topic you should be able to: 3.9.1 Describe the operation and function of a capacitor. 3.9.2 Describe how the following factors affect c...

Module 3: Electrical Fundaments Topic 3.9: Capacitance/Capacitor  INTRODUCTION On completion of this topic you should be able to: 3.9.1 Describe the operation and function of a capacitor. 3.9.2 Describe how the following factors affect capacitance: Area of plates Distance between plates Number of plates Dielectric and dielectric constant Working voltage Voltage rating 3.9.3 Identify various capacitor types and describe their construction and function. 3.9.4 Describe capacitor colour coding 3.9.5 Perform calculations of capacitance and voltage in series and parallel circuits. 3.9.6 Describe exponential charge and discharge of a capacitor and time constants. 3.9.7 Describe the testing of capacitors. 30-04-2024 Slide No. 2  REVISION – ATOMS AND CHARGES Normal atom – equal number of electrons and protons – electrically neutral. If it loses an electron – becomes positively charged If it gains an electron – becomes negatively charged 30-04-2024 Slide No. 3  REVISION – ATOMS AND CHARGES A conductor can be given a charge in a similar way. If electrons can be forcibly removed or added – static charge can be developed on it. Static electricity: Generated by friction Can reach a significantly high value (1,000s of volts) Energy gained in static charge is stored in an electric field as shown 30-04-2024 Slide No. 4  CHARGES Any conductor can be made to hold a charge. Charge can be varied by increasing the voltage applied to conductor. However, on a single conductor, even a very high voltage only stores a small charge. To increase charge – plates used instead of single conductors – greater surface area. 30-04-2024 Slide No. 5  BASIC CAPACITOR 2 plates are placed close together. Plates separated by an insulator or dielectric. Voltage is applied to plates. Electrons move from one plate and deposit onto other plate – charging each plate. Very strong electric field created between the 2 plates. Charge held by this combination for a given voltage can be quite large. Basic 30-04-2024 function of a capacitor is to store electrical charge. Slide No. 6  BASIC CAPACITOR OPERATION At instant voltage is connected to plates – rush of current. Current flows and builds potential difference (pd) across plates. When pd across plates is equal to applied voltage – current flow ceases. This plate voltage governs any further current flow, into or out of plates. Except for small charging and discharging currents – charged capacitor blocks DC. 30-04-2024 Slide No. 7  BASIC CAPACITOR OPERATION Fully charged capacitor. If battery is now reversed – will be a rush of current through battery. Plates first discharge themselves and then re-charge in opposite direction. Capacitor again charges to battery voltage (opposite direction/polarity). All current flow again ceases – charged capacitor blocks DC. 30-04-2024 Slide No. 8  BASIC CAPACITOR OPERATION Alternating current applied to a capacitor. It would charge first in 1 direction and then in other– always following supply voltage. Therefore, AC to capacitor– current always flowing in wires to capacitor plates. Even though current flows in wires – NO electrons transfer across capacitor dielectric. Since 30-04-2024 there is always this current flow in wires – capacitor is said to pass AC. Slide No. 9  BASIC CAPACITOR OPERATION 30-04-2024 Slide No. 10  CAPACITANCE VALUES 10 µF capacitor As learned in an earlier learning subject – electric charge is measured in coulombs. 1 coulomb exists when plates have a deficiency or excess of 6.25 x 1018 electrons. If charge creates a pd of 1 volt across plates, said to have a capacity of one farad (F). This unit (1F) is however an extremely large unit and not easy to achieve. Usual size unit of capacitance values in practical use: Microfarad (µF) – (10-6 x 1 farad) Picofarad (pF) – (10-12 x 1 farad) 30-04-2024 Slide No. 11  CAPACITANCE ANALOGY 1 psi supply output 10 ft3 @ 1 psi Capacity = 10 ft3 If a tank with a capacity of 10 ft3 is filled from an air supply at 1 psi, it will provide an output of 10 ft3 at 1 psi. If input pressure is 20 psi, capacity will still be 10 ft3 but at 20 psi. Remember – EMF (or voltage) is like electrical pressure. 30-04-2024 Slide No. 12  FACTORS AFFECTING CAPACITANCE Charge Voltage Capacitance - same Capacitance of any given combination of capacitor plates is constant. If a higher voltage is applied – charge will increase – capacitance remains same. Capacitance – ratio of charge/voltage (C = Q / V). Amount of gas in a gas cylinder depends on charging pressure. Charge in capacitor depends on electrical pressure or voltage applied. Capacitance depends on physical shapes and sizes of capacitor - such factors: Area of plates Distance between plates 30-04-2024 Dielectric material Slide No. 13  AREA OF PLATES NUMBER OF PLATES Small plate area Large plate area Small capacitance Large capacitance Capacitance increases as total area of plates increases. Greater plate area can hold a greater charge. Connecting capacitors in parallel does same thing (in effect) – enlarging plate area. For capacitors in parallel, simply add up their values: Ctotal = C1 + C2 + C3 ….. 30-04-2024 Slide No. 14  DISTANCE BETWEEN PLATES NUMBER OF PLATES Small distance Large distance Large capacitance Small capacitance Capacitance increases as distance between plates decreases. Moving plates closer together creates a stronger electric field. Moving plates apart weakens the field – giving a lower capacitance. Connecting in series has effect of increasing distance between first and last plate. Inside plates form an intermediate plate – which forces outside plates further apart. For capacitors in series, formula is: 30-04-2024 Slide No. 15  DIELECTRIC Polyethylene dielectric A dielectric material is a substance that is: A poor conductor of electricity, but An efficient supporter of electrostatic fields An insulator with a function similar to that of permeability in magnetic materials. Minimises current flow between plates – supports electrostatic lines of flux. 30-04-2024 Slide No. 16  DIELECTRIC CONSTANT Small K Large K Small capacitance Large capacitance Different materials have different dielectric strengths – measured for given thickness. Measured by potential difference required before spark can jump through material. Dielectric materials compared to air as a standard - dielectric constant (K) found. Dielectric constant – figure defines ability of material to improve capacitor capacity. Examples: Air (K = 1) Tantalum oxide (K = 11) Mica – most common (K = 6.5 - 8.7) 30-04-2024 Slide No. 17  DIELECTRIC CONSTANT Dielectric strength (breakdown voltage) is the maximum voltage difference a certain thickness of a dielectric can sustain without electrical breakdown. Usually expressed in volts per mil (V/.001") or volts per centimeter (V/cm). 30-04-2024 Slide No. 18  WORKING VOLTAGE Capacitors are usually marked with their safe working voltage. Max. DC voltage which can be safely applied without risk of dielectric breakdown. If value exceeded – increased electric field between plates may cause flashover. 30-04-2024 Slide No. 19  VOLTAGE RATING To enable capacitors to stand up to higher voltages – thicker dielectric is required. Thicker dielectric will move plates apart – lowers capacitance. Larger plates needed to compensate for change – makes capacitor physically larger. Occasionally marked with Peak Voltage Rating – surge voltage limit of capacitor. Voltage rating of capacitor should be at least 50% greater than highest voltage applied. 30-04-2024 Slide No. 20  PAPER CAPACITORS Possibly the simplest and longest enduring capacitor. Now largely superseded by polystyrene type – method of manufacture is same. 2 long rolls of aluminium foil conductor separated by a thin layer of insulation. 30-04-2024 Slide No. 21  PAPER CAPACITORS Insulation – waxed tissue paper in older type – polystyrene in newer type. Conductor and insulation are rolled tightly into cylinder and leads connected to foils. Complete assembly is sealed into another cylinder of cardboard or plastic. Usual range – 300 pF to 4 µF (approx) – working voltage rarely exceeds 600 volts. 30-04-2024 Slide No. 22  MICA CAPACITORS High quality capacitor. Made of thin layers of metal – interleaved with sheets of mica. Mica is excellent dielectric – withstands higher voltage than paper of same thickness. Alternate sheets of metal are connected together and brought out to leads. Assembly placed inside a moulded plastic case. Usual range – 50 pF to 0.2 µF (approx) – working voltages up to 2,000 volts. When used as trimmer capacitor – mounted so trimmer screw can compress stack 30-04-2024 Slide No. 23  CERAMIC CAPACITORS Contains a ceramic dielectric. 1 type uses a hollow ceramic cylinder as both: The form on which to construct the capacitor, and As the dielectric material Consist of thin films of metal deposited onto ceramic cylinder. 2nd type is manufactured in shape of a disk. After leads are attached – capacitor covered with insulating moisture-proof coating. Usual range – 1 pF to 0.01 µF (approx) – working voltages up to 30,000 volts. Very stable capacitor and often used for temperature correction purposes. 30-04-2024 Slide No. 24  ELECTROLYTIC CAPACITORS Symbols Provide very large values of capacitance in small size case. Manufactured with very thin film of dielectric impregnated gauze between plates. In most cases – cylindrical aluminum container acts as –ve terminal. POLARISED – must be connected with correct polarity otherwise may explode. Polarity of terminals is normally marked on case – typically indicate –ve terminal. Since it is polarity sensitive – use is ordinarily restricted to a DC circuit. Special electrolytics available for certain AC applications (motor starting, 30-04-2024 coupling). Slide No. 25  TANTALUM CAPACITORS Newer type of electrolytic capacitor – cost is greater. As with electrolytic capacitor – ensure polarity is correct. Tantalum capacitors have +ve terminal marked. Some tantalums use normal numbering system for capacitance values. Others have values printed directly on covering. Some older types use a colour coding system for values. 30-04-2024 Slide No. 26  TANTALUM CAPACITOR VALUE COLOUR CODES Value in microfarads (µF) Multiplier Color 1st Digit 2nd Digit Voltage Dot Black 0 0 x1 10V Brown 1 1 x 10 - Red 2 2 x 100 - Orange 3 3 - - Yellow 4 4 - 6.3V Green 5 5 - 16V Blue 6 6 - 20V Violet 7 7 - - Value equals? Grey 8 8 x 0.01 25V 6.8 µF 6.3 volts White 9 9 x 0.1 3V Pink - - - 35V Black beetles running on your garden bring very good weather 0 1 2 3 4 5 6 7 8 9 30-04-2024 Slide No. 27  DETERMINATION OF CAPACITOR VALUES PRINTED BODIES Value equals? Value equals? 2.2 µF +/- 10% Value in picofarads (pF) 0.1 µF +/- 20% TOLERANCE TOLERANCE Number Multiply by: LETTER 10pF or LESS OVER 10pF 0 1 B +/- 0.1pF 1 10 C +/-0.25pF 2 100 D +/- 0.5pF 3 1000 F +/- 1.0pF +/- 1% 4 10,000 G +/- 2.0pF +/- 2% 5 100,000 H +/- 3% 6 1,000,000 J +/- 5% 8 0.01 K +/- 10% 9 0.1 M +/- 20% 30-04-2024 Slide No. 28  VARIABLE CAPACITOR Value of capacitance can be varied – easiest way – vary the plate area. Variable capacitors (adjustable capacitor) 2 sets of metal plates intermesh on rotary shaft – air is dielectric Used for tuning most radio receivers Another type of variable capacitor – trimmer capacitor – small adjustments Consists of 2 plates separated by a sheet of mica or other dielectric A screw adjustment varies distance between plates – varies capacitance 30-04-2024 Slide No. 29  RC CIRCUITS EXPONENTIAL CAPACITOR CHARGE When power applied to capacitive circuit – current immediately begins to flow. However, voltage across plates rises exponentially as plates become charged. Charge time – dependant on resistance and capacitance values in circuit. Time constant (1): Time required for voltage across capacitor to reach 63.2% of source voltage Determined by multiplying a circuit’s capacitance by it’s resistance TC = Time constant in seconds R = Resistance in ohms TC = R x C C = Capacitance in farads 30-04-2024 Slide No. 30  RC CIRCUITS 10 kΩ 100 VDC Vin 100 µF Timing circuits are often made using a capacitor and a resistor in series. E.g. 10 kΩ resistor is series with a 100 µF capacitor across a 100 volt power source. Current begins to flow when circuit is closed – current flow limited by resistance. Time constant in this circuit equals? 1 second (TC = R x C : 10,000 Ω x 0.0001 F = 1 second) Therefore, in 1 second, voltage across capacitor rises to 63.2 volts. In 5 seconds, voltage across capacitor will equal source voltage – current flow stops. 1 time constant (TC) to reach 63.2% and 5 TCs to reach 100% of source voltage. 30-04-2024 Slide No. 31  EXPONENTIAL CAPACITOR DISCHARGE 10 kΩ 100 VDC Vin 100 µF 4 kΩ Same time constant (TC) applies when discharging a capacitor in an RC circuit. After discharge begins – it takes 1 TC to discharge capacitor by 63.2% (to 36.8%). In 5 time constants (TCs) – voltage across capacitor is 0 – completely discharged. Time constant in discharge circuit equals? 0.4 seconds (TC = R x C : 4,000 Ω x 0.0001 F = 0.4 seconds) Therefore, in 0.4 seconds, voltage across capacitor drops to 36.8 volts. In 2 seconds, voltage across capacitor will equal zero – complete discharge. 1 TC to drop by 63.2% and 5 TCs to complete discharge. 30-04-2024 Slide No. 32  RC CIRCUIT CALCULATIONS 10 kΩ 2 kΩ 28 VDC 12 VDC Vin Vin 47 µF 100 µF 40 kΩ 12 kΩ Complete charge = 2.35 secs. Complete charge = 1 sec. Complete discharge = 9.4 secs. Complete discharge = 6 secs. Vc (after 1 TC) charging = 17.7 V Vc (after 1 TC) charging = 7.58 V Vc (after 1 TC) discharge = 10.3 V Vc (after 1 TC) discharge = 4.42 V Calculate the following for the above (Note different charge and discharge paths): Time required for complete charge Time required for complete discharge Voltage across capacitor on charging after 1 TC Voltage across capacitor on discharging after 1 TC 30-04-2024 Slide No. 33  CAPACITORS IN PARALLEL CAPACITANCE AND VOLTAGE DROP Formula for calculating total capacitance of capacitors in parallel. Ctotal = C1 + C2 + C3 ….. Voltage drop across capacitors in parallel – All equal regardless of capacitance. All capacitors in parallel are subject to same voltage source. 30-04-2024 Slide No. 34  CAPACITORS IN SERIES CAPACITANCE AND VOLTAGE DROP Formula for capacitors in series. Voltage drop across capacitors in series – inversely proportional to capacitance. Eg. 30 VDC source – 5 µF and 10 µF capacitors connected in series. 5 µF 10 µF 20 V 10 V Sum of voltage drops across each capacitor adds up to total applied voltage. 30-04-2024 Slide No. 35  TESTING CAPACITORS Note bulging capacitor Testing Capacitors – initially perform visual Inspection for swollen capacitors. Inspect closely to see if capacitor is swollen or bulged on sides or top. If so – remove and replace with new capacitor. Take note of polarity connection when removing to enable correct polarity for new. WARNING: Ensure capacitor is fully discharged prior to handling or removal. 30-04-2024 Slide No. 36  TESTING CAPACITORS Verify correct capacitance Performed with: Capacitance meter DMM with capacitance function Analogue multimeter (VOM) DC power supply and series resistor WARNING: Ensure capacitor is fully discharged prior to handling or removal This is both for your safety and the continued health of your meter. Disconnect capacitors from circuit before testing – prevents false readings. Polarity of test leads is important if checking a polarised capacitor. For polarised – ensure red test lead (+ve) is connected to +ve capacitor terminal. Using DC power supply and series resistor – capacitance calculated using TC = RC. Measure rise time to 63.2% of power supply voltage (TC=RC) and calculate. 30-04-2024 Slide No. 37  TESTING CAPACITORS WITH ANALOGUE OHMMETER WARNING: Ensure capacitor is fully discharged prior to handling or removal Set ohmmeter to highest resistance scale and connect leads to capacitor terminals. For polarised – ensure red test lead (+ve) is connected to +ve capacitor terminal. If resistance starts low and gradually increases – capacitor is good. If resistance starts low and doesn’t increase – capacitor is shorted. If resistance is high and remains relatively the same – capacitor is open. 30-04-2024 Slide No. 38  CONCLUSION Now that you have completed this topic, you should be able to: 3.9.1 Describe the operation and function of a capacitor. 3.9.2 Describe how the following factors affect capacitance: Area of plates Distance between plates Number of plates Dielectric and dielectric constant Working voltage Voltage rating 3.9.3 Identify various capacitor types and describe their construction and function. 3.9.4 Describe capacitor colour coding 3.9.5 Perform calculations of capacitance and voltage in series and parallel circuits. 3.9.6 Describe exponential charge and discharge of a capacitor and time constants. 3.9.7 Describe the testing of capacitors. 30-04-2024 Slide No. 39  This concludes: Module 3: Electrical Fundaments Topic 3.9: Capacitance/Capacitor