D.C. Transients. & Capacitor-Resistor Series Circuit (Charging)

Chapter 8 D.C. Transients.

Fundamental Electrical and Electronic Principles Third Edition Book
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Summary of Contents

Fundamental Electrical and Electronic Principles Third Edition Book

  • D.C. Transients Chapter 8 Learning Outcomes This chapter explains the response of capacitor-resistor, and inductor-resistor circuits, when they are connected to and disconnected from, a d.c. supply. On completion of this chapter you should be able to: 1 Show how the current and capacitor voltage in a series C-R circuit varies with time, when connected to/disconnected from a d.c. supply. 2 Show how the current through, and p.d. across an inductor in a series L-R circuit varies with time, when connected to/disconnected from a d.c. supply. 3 Defi ne the term time constant for both types of above circuits. 249 8.1 Capacitor-Resistor Series Circuit (Charging) Before dealing with the charging process for a C-R circuit, let us fi rstly consider an analogous situation. Imagine that you need to infl ate a ‘ fl at ’ tyre with a foot pump. Initially it is fairly easy to pump air into the tyre. However, as the air pressure inside the tyre builds up, it becomes progressively more diffi cult to force more air in. Also, as the internal pressure builds up, the rate at which air can be pumped in decreases. Comparing the two situations, the capacitor (which is to be charged) is analogous to the tyre; the d.c. supply behaves like the pump; the charging current compares to the air fl ow rate; and the p.d. developed between the plates of the capacitor has the same effect as the tyre pressure. From these comparisons we can conclude that as the capacitor voltage builds up, it reacts against the emf of the supply, so slowing down the charging rate. Thus, the capacitor will charge at a non-uniform rate, and will continue to charge until the p.d. between its plates is equal to the supply emf. This last point would also apply to tyre infl ation, when the tyre pressure reaches the maximum pressure available from the pump. At this point the air fl ow into the tyre would