Direct Current (DC) is the unidirectional electric current. Uni meaning is one. And the flow of charge in same direction. We know that conventional current flow from positive terminal of battery to negative terminal of battery and electrons flow from negative terminal of battery to positive terminal of battery. The positive terminal of battery is the area of low electron density and negative terminal of battery is the area of high electron density. So the electrons flow from high electron density to low electron density area. The time is taken on x-axis and current I is taken on y-axis. In case of Direct Current the magnitude and direction is constant. We get Direct Current from solar cell, cell, etc. We can convert Direct Current (DC) to Alternating Current (AC) using inverter. We can also convert AC to DC using rectifier. Inverter converts DC to AC and rectifier converts AC to DC.
Use of transformer is not possible in DC. Transformer cannot work on DC. Transformer works on electromagnetic induction principle. Electromagnetic induction is when flux linking with coil changes an emf induced in it. DC will set constant magnetic field. On left side primary winding of transformer is present and on right side secondary winding of transformer is present. If we give DC to transformer there is no induced emf in primary winding of transformer. So the electric power cannot be transformed. And also there is no induced emf in secondary winding of transformer. If the rated DC voltage is applied high current will be drawn due to high current more heat will be generated. Due to more heat winding insulation will burn. DC is use in HVDC system, DC machine, electroplating, battery charging etc. The frequency of pure DC is zero. But may have high frequency ripple. Ripple is a periodic non-sinusoidal waveform. DC maybe of triangular, trapezoidal, or square (maybe called pulsating DC). The magnitude of DC may constant or vary but direction do not change with respect to time.