Direct current and alternating current

One of the most important foundations for automation technology is electrical engineering, since most technical systems need electrical energy both to drive them and to process incoming signals. For that reason an overview of the most important fundamentals of electrical engineering is provided below.

A simple electrical circuit consists of a voltage supply, a consuming device and the connecting cables for transferring the electrical energy. Every electrical circuit is subject to the following simple rule: "from the generator to the consuming device and back". In physical terms, within the electrical circuit are negatively charged particles, the electrons that move from the negative terminal of the voltage supply to the positive terminal via the electrical conductor. This movement of the charged particles is referred to as electrical current. An electrical current can only flow when the circuit is closed.

A distinction is made between direct and alternating currents:

• If voltage always flows in one direction in the circuit, this produces a current that also always flows in one direction. This is a direct current (DC) or a DC circuit.

• With alternating current (AC) or in an AC circuit, the voltage and current change direction and intensity at specific intervals.

Figure 3.2 shows a simple DC circuit consisting of a voltage supply, electrical cables, a control switch and a consuming device (a bulb in the example).

Technical direction of current

When the control switch is closed, a current I flows through the consuming device. The electrons move from the negative terminal to the positive terminal of the voltage supply. Before the existence of electrons was discovered, the direction of current was defined as flowing from "positive" to "negative". This definition is still valid in practice today and is called the technical direction of current.