A ballast is a device used in electrical circuits to limit the amount of current in the circuit. It is commonly used in fluorescent lamps to limit the current through the tube, which would otherwise rise to a destructive level due to the negative differential resistance of the tubes voltage-current characteristic. Ballasts can be as simple as a resistor, inductor, or capacitor wired in series with the lamp, or as complex as the electronic ballasts used in compact fluorescent lamps (CFLs). The main functions of a ballast are to start the lamp and to control its operation. Ballasts interact with the lighting mechanism to control, regulate, and ultimately stabilize the light output of the lamp. They help control and regulate the supplied current during different operating phases and voltages for certain bulb types. Ballasts are used with electric-discharge lamps to obtain the necessary circuit conditions (voltage, current, and waveform) for starting and operation. All fluorescent and HID light sources require a ballast for proper operation. There are many different types of ballasts, including magnetic and electronic ballasts. Ballasts are not used in LED lights.