Electrical ballast

An electrical ballast is a device placed in line with the load to limit the amount of current in an electrical circuit. It may be a fixed or variable resistor.

A familiar and widely used example is the inductive ballast 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 artifact in the tube's voltage-current characteristic.

Ballasts vary in complexity. They can be as simple as a series resistor or inductor, or a capacitor, or a combination of these. They may be as complex as the electronic ballasts used in fluorescent lamps and high-intensity discharge lamps.

An electrical ballast is a device which limits the current through an electrical load. These are most often used when a load (such as an arc discharge) has its terminal voltage decline when current through the load increases. If such a device were connected to a constant-voltage power supply, it would draw an increasing amount of current until it will be 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.

A gas-discharge lamp is an example of a device which, under certain conditions, has negative differential resistance. In such a situation (after lamp ignition), every little increase in the lamp current tends to reduce the voltage "dropped" across it (supposing the lamp to be connected in series with other circuit elements). Let ${\displaystyle dI}$ represent the change in current I, and ${\displaystyle dV}$ represent the change in voltage V. Each variation can be positive (or negative) if its variable increases (or decreases). The differential resistance is the ratio between ${\displaystyle \textstyle dV}$ and ${\displaystyle \textstyle dI}$, ${\displaystyle \textstyle {\frac {dV}{dI}}}$ and it can be either positive or negative (and sometimes even null). This is quite a different concept from the resistance, which is always considered positive. In the case of a gas-discharge lamp, the differential resistance (i.e., dV/dI) really becomes negative because the positive variation for the current (dI) causes a negative variation for the voltage (dV) across the lamp.

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