Direct current (DC), unlike alternating current has two poles – positive and negative with the difference in electric potential, i.e. the voltage. By convention, the charges flow from the positive to negative pole, however, the reverse is true. Direct current waveform is a straight line (Fig. 1).
Fig. 1. DC waveform
The main sources of direct current are power supplies and voltaic cells (i.e. batteries and rechargeable batteries). Most electronic devices are supplied with direct current. It can easily be provided by power supplies and rectifiers directly from the AC wall socket.
Both the devices supplying direct current (Fig. 2) and the devices supplied with direct current have a DC logo. Also, a direct current symbol is often used (Fig. 3).
Fig. 2. DC power supply rating plate
Fig. 3. Direct current symbol
In case of devices supplied with direct current, it is critical to correctly connect the positive and negative pole. Otherwise, it may permanently and irreversibly damage the device. The cables with direct current are usually red for positive and black for negative. If both cables are black, negative is marked with dotted line. The power supplies often use a 2.1/5.5 (Fig. 4) plug (barrel connector), where the outer barrel is negative, and the inner barrel is positive.
Fig. 4. Standard polarity of power supplies with a 2.1/5.5 mm plug
Power supplies are electronic devices supplying direct current widely used in devices with integrated circuits. Since many electrical and electronic devices are supplied directly from the wall socket, i.e. alternating current, the current must be converted into direct current by the power supply.