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Understanding Capacitance and RC Circuits

This chapter explains capacitance and capacitors, their behavior in DC and AC circuits, and the transient response of a capacitor. It also discusses different types of capacitors and the calculation of capacitance.

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Understanding Capacitance and RC Circuits

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  1. Chapter 15 Capacitance and RC Circuits

  2. Objectives • Define capacitance and capacitor. • Identify the many different types of capacitors. • Describe the transient response of a capacitor. Permission granted to reproduce for educational use only.

  3. Objectives • Explain how a capacitor behaves in a dc circuit. • Discuss the effect of capacitance on an ac circuit. • Describe the results of combining capacitance and resistance in a circuit. Permission granted to reproduce for educational use only.

  4. Capacitance and the Capacitor Permission granted to reproduce for educational use only.

  5. Capacitance and the Capacitor (Cont.) • Dielectrics • Conductive plates Permission granted to reproduce for educational use only.

  6. Capacitance and the Capacitor (Cont.) • Electrons from negative terminal of source flow to one plate of capacitor • Capacitor blocks direct current • Dielectric constants • Working voltage (WV) Permission granted to reproduce for educational use only.

  7. Calculating Capacitance • Measured in farads (F) • Determined by • Dielectric material • Area of plates • Distance between plates Permission granted to reproduce for educational use only.

  8. Calculating Capacitance (Cont.) • Capacity increases with area of plates or dielectric constant • Capacity decreases when distance between plates increases Permission granted to reproduce for educational use only.

  9. Types of Capacitors • Fixed paper capacitors • Rectangular oil filled capacitors • Can type electrolytic capacitors • Tubular electrolytic capacitors • Ceramic capacitors Permission granted to reproduce for educational use only.

  10. Types of Capacitors (Cont.) • Mica capacitors • Variable capacitors • Stators • Rotors • Trimmer capacitors • Adjusted with alignment tools • Tantalum capacitors Permission granted to reproduce for educational use only.

  11. Review What is capacitance? A property that opposes any change in voltage Permission granted to reproduce for educational use only.

  12. Review What is a device that temporarily stores an electric charge? A capacitor Permission granted to reproduce for educational use only.

  13. Review What is the insulation separating the two plates in a capacitor? Dielectric Permission granted to reproduce for educational use only.

  14. Review What are the three factors that determine capacitance? Material used as a dielectric, area of the plates, and distance between the plates Permission granted to reproduce for educational use only.

  15. Transient Response of the Capacitor • Response of current and voltage immediately after change in applied voltage • RC circuits Permission granted to reproduce for educational use only.

  16. Transient Response of the Capacitor (Cont.) • Voltage builds up gradually with closed switch • Current falls as capacitor charges • Capacitor remains charged when switch opens Permission granted to reproduce for educational use only.

  17. Transient Response Example • Short circuit in this RC circuit when switch closes Permission granted to reproduce for educational use only.

  18. Transient Response Example (Cont.) • Discharge current rises to maximum and falls off Permission granted to reproduce for educational use only.

  19. RC Time Constant • Time between charge and discharge of voltage and current in RC circuit • Time constant •  = R× C • Five time constant periods needed for complete charge or discharge Permission granted to reproduce for educational use only.

  20. RC Time Constant (Cont.) • At ignition, or firing voltage, lamp glows • Repetitive cycle, causes neon to flash Permission granted to reproduce for educational use only.

  21. Capacitors in Series Permission granted to reproduce for educational use only.

  22. Capacitors in Series (Cont.) Permission granted to reproduce for educational use only.

  23. Capacitors in Parallel • CT = C1 + C2 + C3… + CN Permission granted to reproduce for educational use only.

  24. Review What does transient response refer to? The response of current and voltage in a circuit immediately after a change in applied voltage Permission granted to reproduce for educational use only.

  25. Review What is the amount of time needed for the capacitor to charge or discharge 63.2 percent? The time constant of the circuit Permission granted to reproduce for educational use only.

  26. Capacitance in AC Circuits • Power in capacitive circuits • Resistance and capacitance in AC circuits • Parallel RC circuits Permission granted to reproduce for educational use only.

  27. Capacitance in AC Circuits (Cont.) • AC voltage causes plates to charge and discharge repeatedly Permission granted to reproduce for educational use only.

  28. Capacitance in AC Circuits (Cont.) • Current begins at maximum • Current leads voltage by 90° in capacitive circuit Permission granted to reproduce for educational use only.

  29. Capacitance in AC Circuits (Cont.) • Current depends on rate of capacitor’s charge and discharge • As ac frequency increases, current increases • Capacitive reactance Permission granted to reproduce for educational use only.

  30. Capacitance Example • 10 µF capacitor • 120-hertz frequency Permission granted to reproduce for educational use only.

  31. Capacitance Example (Cont.) Permission granted to reproduce for educational use only.

  32. Power in Capacitive Circuits • Wattless power • Apparent power = 100 V × 10 A = 1000 VA Permission granted to reproduce for educational use only.

  33. Resistance and Capacitance in AC Circuits • Capacitive reactance causes phase displacement • Total resistance is vector sum of XC and R • Impedance Permission granted to reproduce for educational use only.

  34. Resistance and Capacitance Example Z = 500 Ω Permission granted to reproduce for educational use only.

  35. Resistance and Capacitance Example (Cont.) • Angle between Z and R is phase displacement between current and voltage cos = 0.6  = 53.1° Permission granted to reproduce for educational use only.

  36. Resistance and Capacitance Example (Cont.) • Apparent power = I× E • Apparent power = 0.2 A × 100 V Apparent power = 20 VA • True power = I × E × cos  • True power = 0.2 A × 100 V × cos 53.1° True power = 12 W Permission granted to reproduce for educational use only.

  37. Parallel RC Circuits • No phase difference among voltages • Phase difference among total and branch currents • Current in phase with resistive branch’s voltage Permission granted to reproduce for educational use only.

  38. Parallel RC Circuit Formulas Permission granted to reproduce for educational use only.

  39. Parallel RC Circuit Example • Capacitive reactance XC 80 Ω • Branch currents Permission granted to reproduce for educational use only.

  40. Parallel RC Circuit Example (Cont.) • Phase angle arctan 1.25 = 51.3° • Total circuit current • Impedance Permission granted to reproduce for educational use only.

  41. Review What is capacitive reactance? The resistance resulting from capacitance Permission granted to reproduce for educational use only.

  42. Review What is power called in a capacitive circuit? Wattless power Permission granted to reproduce for educational use only.

  43. Review What is the ratio of true power to apparent power in an ac circuit called? Power factor (PF) Permission granted to reproduce for educational use only.

  44. For Discussion • Research different types of capacitors, focusing on construction and various applications. • Discuss the reasons a capacitor blocks dc while passing ac. Permission granted to reproduce for educational use only.

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