1.01 D.C. transients 1.


The study of d.c. transients focuses on the “short term” changes in voltage and current, that occur in some circuits, just after they are switched on or off. After this transient period has passed, the voltage and current settle down to their final values. These transient effects are caused by two electrical properties, called capacitance and inductance. In this section we will only consider transients in capacitive circuits. (Transients in inductive circuits will be considered after electromagnetism and inductance have been covered.)

Transients in capacitive circuits

This first section contains a fairly comprehensive, qualitative, description of the principles involved. As well as defining what capacitance is, it also explains how the physical construction of a capacitor, determines its capacitance value. Throughout this section, we will make extensive use of an analogy with an air pump, to help to explain the basic concepts. (This analogy does not assume any formal study of the behaviour of gases and will be described using generic descriptive terms, such as “suction” and “back pressure” etc.)

We will first consider an imaginary scenario, of being able to start up the chemical reactions in a previously dormant, isolated battery. This would create an internal current flow that will quickly decay to zero as the charge accumulates at the battery terminals. We will then see how connecting conductive plates to the battery terminals enables more charge to be transferred, with the same battery voltage. Finally we will see that by changing the physical parameters of the conductive plates, the amount of charge stored can be increased further. True electron flow will be used in the descriptions throughout this section.

Once the basic principles of capacitors have been established, we will use a simpler analogy to help develop the electrical equations that are used to calculate voltages and currents, at different times during the transient period. In that section we will use conventional current flow.