N scale locomotives are powered by DC motors which accept a maximum of 12 V DC. In traditional DC control, the speed of the train is determined by the amount of voltage supplied to the rails. The direction of the train is determined by the polarity of the power to the rails. Since the end of the 20th century, an increasing number of enthusiasts have started using digital train control systems to determine the speed and direction of their trains. This has in part been made possible by surface mount technology and new motors that draw very little current (typically 0.2 amps). The most popular digital control systems used in N scale model railways are NMRA-DCC and Selectrix.