Current Limiting
Ballasts limit the current through an electrical load. These are most often used when a load presents a negative (differential) resistance to the supply. If such a device were connected to a constant-voltage power supply, it would draw an increasing amount of current until it was destroyed or caused the power supply to fail. To prevent this, a ballast provides a positive resistance or reactance that limits the current. The ballast provides for the proper operation of the negative-resistance device by limiting current.
An example of a negative-resistance device is a gas-discharge lamp, where after lamp ignition, increasing arc current reduces the voltage drop.
Ballasts can also be used simply to deliberately reduce the current in an ordinary, positive-resistance circuit.
Prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system.
Series resistors ballasts are used to control the current through the LED.
Read more about this topic: Electrical Ballast
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