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Analogue Electronic ETEC 4824

Analogue Electronic ETEC 4824. Capacitor. Learning Outcomes. At the end of the lesson, students should be able to : Explain the functions of capacitors Identify various type of capacitors Calculate time constant of a charging circuit Calculate time constant of a discharging circuit

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Analogue Electronic ETEC 4824

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  1. Analogue ElectronicETEC 4824 Capacitor

  2. Learning Outcomes At the end of the lesson, students should be able to : • Explain the functions of capacitors • Identify various type of capacitors • Calculate time constant of a charging circuit • Calculate time constant of a discharging circuit • List the applications of capacitors • Sketch the capacitor charging and discharging graphs

  3. The Simple Capacitor • A capacitor is a passive electronic component that stores energy in the form of an electrostatic field • A simple capacitor consists of two metal plates • Separated by an insulating material called a dielectric

  4. The Simple Capacitor • The capacitance is directly proportional to the surface areas of the plates, • Inversely proportional to the separation between the plates. • Capacitance also depends on the dielectric constant of the substance separating the plates.

  5. The Simple Capacitor

  6. The Simple Capacitor • Function • Capacitors store electric charge. • Also as a filter, passing alternating current (AC), and blocking direct current (DC). • They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge.

  7. The Simple Capacitor • They are used to smooth varying DC supplies by acting as a reservoir of charge. • They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals

  8. The Simple Capacitor • Capacitance is a measure of the ability of a capacitor to hold an electric charge. • The unit of capacitance is the Farad (F), and the symbol is C.

  9. The Simple Capacitor A capacitance of one Farad is defined as The capacitance of a capacitor between the plates of which there appears a potential difference of 1 volt when it has been charged by 1coulomb of electricity

  10. capacitance • 1F is very large, • prefixes are used to show the smaller values. • Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

  11. capacitance • µ means 10-6 (millionth), so 1000000µF = 1F • n means 10-9 (thousand-millionth), so 1000nF = 1µF • p means 10-12 (million-millionth), so 1000pF = 1nF

  12. Polarised capacitors (large values, 1µF +) • Examples:       • Circuit symbol: • they must be connected the correct way round.

  13. Unpolarised capacitors (small values, up to 1µF) • Examples: • Circuit symbol: 

  14. Capacitor Charging

  15. Capacitor Charging • The rate at which a capacitor charges (fills up) depends on its size and the circuit resistance • The small capacitor will charge (fill) faster than the larger one • The higher the circuit resistance the slower the rate at which a capacitor charges.

  16. Time Constant & Characteristic Curves

  17. Time Constant & Characteristic Curves • Time Constant : The time (in seconds) that it takes to charge the capacitor is determined by the value of capacitance (in Farads) by the value of resistance within the charging circuit.

  18. Time Constant & Characteristic Curves • t = RC (in seconds) • eg. 10F x 100k = 1Second • 10 x 10-6 x 100 x 103 = 1

  19. Time Constant & Characteristic Curves • One time constant (from 0 to t1) the capacitor charges up by 63.2% of the maximum charge.

  20. Time Constant & Characteristic Curves

  21. Time Constant & Characteristic Curves • In the next time constant, the capacitor charges up 63.2% • 63.2% of 36.8% = 23.26%. • 36.8% = 100% - 63.2% • Making a Total charge of 86.4% on the capacitor. • The process of charging is an exponential shape (curve).

  22. Time Constant & Characteristic Curves • it is considered that a capacitor will have reached full charge (and for that matter, full Discharge) after 5 time constants have elapsed

  23. The current flow is Maximum at the start • Imax = V/R • At the end of t1 the current has reduced by 63.2% of Imax. • At the end of 5 time constants that no current flows within the circuit.

  24. Graph showing the voltage for a capacitor charging time constant = RC

  25. Graph showing the current for a capacitor charging time constant = RC

  26. Graph showing the voltage for a capacitor discharging time constant = RC

  27. Graph showing the current for a capacitor discharging time constant = RC

  28. http://www.wisc-online.com/Objects/ViewObject.aspx?ID=DCE9604http://www.wisc-online.com/Objects/ViewObject.aspx?ID=DCE9604

  29. Questions • 1. State the function of capacitor • 2. Calculate the time constant of the circuit Given : R = 10 kΩ C = 47 µF

  30. Questions • 3. Sketch the Vc vs time graph of the circuit Given : Vs = 12 V R = 10 kΩ C = 47 µF Calculate the Imax Flow through the circuit

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