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1. Elektronika AgusSetyo Budi, Dr. M.Sc Sesion #09 JurusanFisika FakultasMatematikadanIlmuPengetahuanAlam

2. Outline Alternating Current in a Capacitive Circuit The Amount of XC Equals 1/(2πf C) Series or Parallel Capacitive Reactances Ohm's Law Applied to XC Applications of Capacitive Reactance Sine Wave Charge and Discharge Current © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 2 07/01/2011

3. Capacitive Reactance 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 3

4. 1 XC = 2πf C 17-1: Alternating Current in a Capacitive Circuit Capacitive reactance is the opposition a capacitor offers to the flow of sinusoidal current. Symbol: XC Units: Ohms Formula (this applies only to sine wave circuits): 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 4

5. 17-1: Alternating Current in a Capacitive Circuit Current flows with ac voltage applied to a series connected capacitor and light bulb. There is NO current through the capacitor’s dielectric. While the capacitor is being charged by increasing applied voltage, the charging current flows in one direction in the conductors to the plates. While the capacitor is discharging, when the applied voltage decreases, the discharge current flows in the reverse direction. With alternating voltage applied, the capacitor alternately charges and discharges. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 5

6. 17-1: Alternating Current in a Capacitive Circuit With a 4-μF capacitor, the bulb lights brightly. The capacitor blocks dc and the bulb cannot light. The smaller capacitor has more opposition to ac and the bulb is dim. Fig. 17-1: Current in a capacitive circuit. (a) The 4-μF capacitor allows enough current I to light the bulb brightly. (b) Less current with smaller capacitor causes dim light. (c) Bulb cannot light with dc voltage applied because a capacitor blocks the direct current. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 6

7. 17-1: Alternating Current in a Capacitive Circuit Summary: Alternating current flows in a capacitive circuit with ac voltage applied. A smaller capacitance allows less current, which means more XC with more ohms of opposition. Lower frequencies for the applied voltage result in less current and more XC. With a steady dc voltage source (zero frequency), the capacitor’s opposition is infinite and there is no current. In this case the capacitor is effectively an open circuit. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 7

8. 17-1: Alternating Current in a Capacitive Circuit Summary, cont. XC depends on the frequency of the applied voltage and the amount of capacitance. XC is less for more capacitance. XC is less for higher frequencies. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 8

9. 17-2: The Amount of XCEquals 1/(2πf C) Factors Affecting XC The value of XC is inversely proportional to the value of capacitance: Increasing C decreases XC Decreasing C increases XC The value of XC is inversely proportional to the frequency: Increasing f decreases XC Decreasing f increases XC 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 9

10. 1 1 XC= C= 1 2πf C 2πf XC f = 2πC XC 17-2: The Amount of XCEquals 1/(2π f C) Summary of the XC Formulas: When f and C are known: When XC and f are known: When XC and C are known: 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 10

11. 17-3: Series or Parallel Capacitive Reactances Capacitive reactance is an opposition to ohms, so series or parallel reactances are combined in the same way as resistances. Combining capacitive reactances is opposite to the way capacitances are combined. The two procedures are compatible because of the inverse relationship between XC and C. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 11

12. 17-3: Series or Parallel Capacitive Reactances Series Capacitive Reactance: Total reactance is the sum of the individual reactances. XCT = XC1 + XC2 + XC3+ ... + etc. All reactances have the same current. The voltage across each reactance equals current times reactance. VC1 = I ×XC1 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 12

13. 1 1 1 1 = + + + ... + etc. XCT XC1 XC2 XC3 17-3: Series or Parallel Capacitive Reactances Parallel Capacitive Reactances Total reactance is found by the reciprocal formula: All reactances have the same voltage. The current through each reactance equals voltage divided by reactance. IC = VC / XC 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 13

14. 17-4: Ohm's Law Applied to XC Current in an ac circuit with XC alone is equal to the applied voltage divided by the ohms of XC. I = V/XCT = 1/3 A I = V/XC = 1 A IT = I1 + I2 = 1 ½ A Fig. 17-6: Example of circuit calculations with XC. (a) With a single XC, the I = V/XC. (b) Sum of series voltage drops equals the applied voltage VT. (c) Sum of parallel branch currents equals total line current IT. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 14

15. 17-5: Applications of Capacitive Reactance The general use of XC is to block direct current but provide low reactance for alternating current. Ohms of R remain the same for dc or ac circuits, but XC depends on frequency. The required C becomes smaller for higher frequencies. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 15

16. 17-5: Applications of Capacitive Reactance 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 16

19. 17-6: Sine Wave Charge and Discharge Current VA is positive and increasing, charging C. VC decreases by discharging. VA increases but in the negative direction. C charges but in reverse polarity. Negative VA decreases and C discharges. Fig. 17-7: Capacitive charge and discharge currents. (a) Voltage VA increases positive to charge C. (b) The C discharges as VA decreases. (c) Voltage VA increases negative to charge C in opposite polarity. (d) The C discharges as reversed VA decreases. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 19

20. dv ic = C dt 17-6: Sine Wave Charge and Discharge Current Calculating the Values of iC. The greater the voltage change, the greater the amount of capacitive current. Capacitive current is calculated i is in amperes C is in farads dv/dt = volts per second. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 20

21. 17-6: Sine Wave Charge and Discharge Current 90° Phase Angle iC leads vC by 90°. The difference results from the fact that iC depends on the dv/dt rate of change, not v itself. The ratio of vC/ iC specifies the capacitive reactance in ohms. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 21

22. dv/dt Voltage 0  Sine wave Vinst. = Vmax x cos  17-6: Sine Wave Charge and Discharge Current dv/dt for Sinusoidal Voltage is a Cosine Wave 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 22

23. 07/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 23 TerimaKasih