A 400 Watt Ballast suitable to run a 400 Watt High Pressure Sodium. Ballasts are most commonly needed when an electrical circuit or device presents a negative resistance to the supply. If such a device were connected to a constant-voltage power supply, it would draw an ever-increasing amount of current until it was destroyed or caused the power supply to fail. To prevent this, the ballast will provide a positive resistance or reactance that limits the ultimate flow of current to an appropriate level.
