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Capacitor experiment


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Capacitor experiment

  1. 1. THE KENYA POLYTECHNIC UNIVERSITY COLLEGE DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING CERTIFICATE IN COMPUTER SERVICING AND MAINTENANCE EXPERIMENT: CHARGING AND DISCHARGING OF A CAPACITOR1.0 CapacitanceCapacitance (symbol C) is a measure of a capacitors ability to store charge. A large capacitancemeans that more charge can be stored. Capacitance is measured in farads, symbol F. However1F is very large, so prefixes (multipliers) are used to show the smaller values:  µ (micro) means 10-6 (millionth), so 1000000µF = 1F  n (nano) means 10-9 (thousand-millionth), so 1000nF = 1µF  p (pico) means 10-12 (million-millionth), so 1000pF = 1n2.0 PARTS AND MATERIALS  6 volt battery/ DC Power Supply  Two large electrolytic capacitors, 1000 µF minimum  Two 47KΩ resistors or any other value close to this.  One toggle switch, SPST ("Single-Pole, Single-Throw")  Voltmeter  Stop WatchNote: Be warned that most large capacitors are of the "electrolytic" type, and they are polaritysensitive! One terminal of each capacitor should be marked with a definite polarity sign. Usuallycapacitors of the size specified have a negative (-) marking or series of negative markingspointing toward the negative terminal. Very large capacitors are often polarity-labeled by apositive (+) marking next to one terminal. Failure to heed proper polarity will almost surelyresult in capacitor failure, even with a source voltage as low as 6 volts. When electrolyticcapacitors fail, they typically explode, spewing caustic chemicals and emitting foul odors.Please, try to avoid this!3.0 LEARNING OBJECTIVES  Capacitor charging action  Capacitor discharging action  Time constant calculation  Series and parallel capacitance 1
  2. 2. 4.0 SCHEMATIC DIAGRAM Connect the circuit as shown below. 5.0 CHARGE AND ENERGY STORED The amount of charge (symbol Q) stored by a capacitor is given by: Q = charge in coulombs (C) Charge, Q = C × V where: C = capacitance in farads (F) V = voltage in volts (V) When they store charge, capacitors are also storing energy: Energy, E = ½QV = ½CV² where E = energy in joules (J). Note that capacitors return their stored energy to the circuit. They do not use up electrical energy by converting it to heat as a resistor does. The energy stored by a capacitor is much smaller than the energy stored by a battery so they cannot be used as a practical source of energy for most purposes. 6.0 CAPACTITOR CHARGING Build the "charging" circuit and measure voltage across the capacitor when the switch is closed. Notice how it increases slowly over time, rather than suddenly as would be the case with a resistor. TIME CONSTANT (RC) 2
  3. 3. The "time constant" (τ) of a resistor capacitor circuit is calculated by taking the circuit resistanceand multiplying it by the circuit capacitance. For a 1 kΩ resistor and a 1000 µF capacitor, thetime constant should be 1 second. This is the amount of time it takes for the capacitor voltage toincrease approximately 63.2% from its present value to its final value: the voltage of thebattery.PROCEDURE:Plot a graph of Voltage across the capacitor versus the Time constant (RC) for the followingvalues of RC on a graph paper.Time Constant (seconds) Voltage across the Capacitor (Vc)0RC1RC2RC3RC4RC5RCVoltage RC7.0 CAPACITOR DISCHARGINGBuild the "charging" circuit and measure voltage across the capacitor.Plot a graph of Voltageacross the capacitor versus the Time constant (RC) for the following values of RC on a graphpaper.Time Constant (seconds) Voltage across the Capacitor (Vc)0RC1RC2RC3RC4RC5RC 3